# setback thermostats and Harman Pellet stoves



## HarryBack

Ive noticed a few questions awhile ago about setback thermostats and pellet units. Harman just came out with a Technical Bulletin concerning them, and I thought Id take this time to communicate it before I forget all the particulars.

All Harman pellet units can utilize a setback thermostat. The thermostat must be:

1. Millivolt compatible
2. Have gold contacts
3. be low voltage
4. be low current


Harman reccommends the following Honeywell units for this use:

RTH230B
RTH6300B


If there is a jumper between Rh and Rc, it must be removed from the terminal block of the thermostat. The thermostat must be connected in series with the room sensing probe using the Rh and W terminals of the thermostat. Except with the P38, which has no room sensing probe, the set-back thermostat MUST be used in conjunction with the room sensing probe.

there are two modes of operation as well:

Occupied Mode: (someones home)
Set the thermostat to its max temp setting. The stove room-sensing probe will control room temperature, so set the desired room temp on the stoves temp dial.

Unoccupied Mode: (noones home)
Program the thermostat to the desired set-back temp. When the thermostat opens (is satisfied) the stove will go into a 4-blink status. If the stove is a auto-ignite model and the auto/manual switch is in auto, the stove will shut down. When the thermostat closes (calls for heat) the 4-blink status will reset and the stove will ignite.
If the stove is a manual ignition model, or an auto-igniting unit with the selector switch set to manual, the stove will go into the 4-blink status when the thermostat opens (is satisfied) and the stove will go to a minimum burn and stay there until the thermostat closes (calls for heat). When the thermostat closes, the 4 blink status will reset automatically.

Consult your owners' manual for a description of the 4-blink status. This error occurs when the contril board senses the room sensing probe isnt connected or has failed. This happens with the thermostat in series, since when the thermostat opens (is satisfied), there is a loss of continuity in the room-sensing circuit. The 4 blink status automatically resets when the board recognizes the room-sensing probe has been reconnected (ie: when the thermostat closes and calls for heat).


Anyhow, here's the gist of the tech bulletin. If it sheds some light, great.


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## allenfrme

It's always interesting to see how others use their stoves ! My 68 is just set to comfortable room temp most of the Winter, about 68, and left there. In the Fall and Spring, when it gets warm, it shuts off, and comes back on when the house cools. I just sorta forget about it except for the weekly 15 minute cleaning routine! 

I tried shutting it down, or lower in the evening, but found I was burning more pellets getting the house back up to temp. Plus, it's not like a furnace, where you have heat a couple minutes later, as we all know, it takes a few to get going again !

My only, only , real complaint about Harmon is the lack of communication from the company itself.  And I dearly wish they would find someone to design a better loop control for the thermostat ! The difference between 65 and 70 is a minor twitch of the fingers on mine ! 

Anyone heard about the new burn pot upgrade for the 68 ?


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## Hammerjoe

Sorry for bringing this old topic back to life.

Is anyone here using this setup?
How good does it work?

Any other compatible models besides those two honeywells?

The idea for me to use a t/stat is mainly to use it during fall/spring season and not so much in the winter due to the cold temps.
During fall and spring, temps are not that low that having the stove off when we are at work and have it turn on like half hour before we get home it shouldnt take too long to heat the home.
Any opinions on this?

Btw it would be really cool if someone that has a Harman with t/stat would post some pictures on how the wiring is done on the t/stat.
Thanks.


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## mkmh

I did this last night with a Honeywell RTH230B Thermostat. Sorry...no time for a pic right now as I have to head off to work. It took me about 5 minutes to set it up. Very simple just like the bulletin. I did have to remove the jumper from the thermostat as stated in the article. 
My plan is to just use the thermostat for turning the stove on at 5 AM and off at 8 or 9 PM...then maybe one cycle mid-day. So far it shut the stove down for me and started it up for me this AM with no trouble.


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## Hammerjoe

Thanks for the reply.

How did you program the t/stat?

I am concerned about the electronics of the stove by using a t/stat.
Will it warm in any way the stove if the t/stat opens and closes alot, ie suppose the stove is set up for a temp of 70f and we the t/stat is programmed for 65f.
The T/stat will close and the stove will start working trying to reach the 70f temp, but as soon it goes over 65 the t/stat opens and the stoves starts shutting dow, but as soon it goes lower 65F then it is told to start again and so on and this I would think very frequently.
Will this constant open and close not cause any problems to the stove??
I cant imagine it not doing any harm!


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## Tristan

Very exciting topic.  This is exactly what I am looking to do for my Harman XXV.  I totally agree with HammerJoe on the spring/fall thing.  I would like to be able to have my stove fire up to 75 deg at 5am to warm up the living spaces and the bathrooms down the hall (nothing like having to pee in the cold) and then having the stove settle down to about 65 during the day when no one is home, etc.  This t/stat discussion is exactly what i am looking for.  Can those with expertise and experience in this matter please chime in so that we may learn from your vast knowledge.  Pictures ... pictures of the wiring etc would really help.  Thank you.


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## Tristan

I just found the wiring diagram for the set back thermostat.  Interestingly, I currently have the exact honeywell t/stat that is recommended by Harman, its currently controlling my (now defunct) oil guzzling baseboards ..... hmmmm....


http://oatstoves.com/files/pellet_stove_tsta_tinfo.pdf

Follow the link above.


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## mkmh

I think your concerns are valid. These stoves are not meant to be run with a programmable thermostat, and the Tech Bulletin basically provides a way to trick the stove into "thinking" one of the leads to the room sensor probe has lost its connection...which causes the stove to go into shutdown mode. When the thermostat calls for heat again it completes the circuit and the stove picks up on the fact that the room sesning probe is in place again...causing a start-up cycle.
If you are looking for precision heating i'm not sure i'd recommend this arrangement. As people have mentioned, it could lead to many shutdowns and start-ups, which is ineffiecient...wastes electricity, taxes the igniter, and may "mess with" the electronices of the stove.
 What i've done is programmed teh thermostat to ensure that it only triggers one to 2 cycles per day.
Basically it starts up at 5 AM and will run until 9 AM or 90 degrees (which will never happen). At 9 AM until 4 PM it cycles down to 50 degrees (which ensures it will go into shutdown mode and stay off until 5 PM). At 5 PM it calls for heat again (90 degrees) until 8 PM....then from 8 PM - 5 AM it is back to 50 degrees ensuring that it stays off.
Probably still not fantastic for the electronics, but it IS a Harman bulletin, so this method is not entirely unsupported.

I'm doing something very similar with my Hastings St Croix, with good results. Basically you just have to keep in mind that your stove is not realy meant to be run the same way you'd run a forced hot water furnace. However, the auto on/auto off seems to be pretty low risk to me, and adds a huge measure of convenience. I'll post pics if I get a few minutes tonight, but the configuration is really very simple (just like in the bulletin).
Hoppe this helps


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## mkmh

One other note.  My stove settings are as follows.
---------------------------------
Room Temp Mode (Automatic) toward the middle of the dial
Temp setting around 85 degrees
Feed Rate 1
-----------------------------
This morning I found that the basement was getting too warm so I went in and turned the dial to the off position. Even though the thermostat was calling for heat, this initiated the shutdown as expected. I figure I will be doing a fair amount of this tweaking to the stove settings to get a comfortable temperature. For example, when it start to get a little colder I will most definitely kick up the feed rate to 2 or 3. 

Again, I am not expecting to achieve perfect stability in temperature...but the convenience of the auto starts and auto shutdowns is BIG for me. So far I am really liking my P61A. I've probably run through about 4 days of pellets and there is nearly zero ash build-up...and zero clinkers (NE hardwoods). I can tell that it is going to be a very low maintenece stove.


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## Tristan

When you say that there is nearly zero ash build up - where exactly is this ash build up expected to be?  Indeed my XXV has little ash build up also, but nonetheless there is ash at the tip of the burn pot (approx 1" wide from the tip extending back towards the fire itself) ... is that normal ash?  Or is that excessive?

Back to the original topic .... I am really excited about the potential of the t/stat automation.  The wiring diagram shows a Rh and a W terminal to be connected to the Honeywell t-stat, this then gets connected in a series to the room sensor ... my question is ... do i make this connection to the room sensor's red wire? (which room sensor's wire gets the series connection?)  Thank you.  

Also, is there a way to extend the original room sensor?

Thank you again.


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## Amaralluis

mkmh said:
			
		

> One other note.  My stove settings are as follows.
> ---------------------------------
> Room Temp Mode (Automatic) toward the middle of the dial
> Temp setting around 85 degrees
> Feed Rate 1
> -----------------------------


Feed rate of 1?? is that for the Harman?? That is strange, the advice on this board is to actually have it higher than four.
I have kept my accentra at a 4 but after reading a few topics here I decided to change to a 5.

To put it simply the t/stat can only be used as an on/off mechanism for the stove and not a temp controller.
And that is exactly what I need.


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## mkmh

Based on my limited expereince, I think your summary is about right Amaralluis.
Yeah, feed rate of 1 seems to work well for me. 2 when I want the flame a little higher. The one time I had it as high as 5 I was getting a really high flame out of it....it was overkill for my 700 square foot basement. I imagine as the temp continues to drop i'll have the feedrate back up in the neighborhood of 3...but I don't think i'll ever need to go higher than that.

I have not taken any precise measurements, but it seems like feedrate of 2, dist blower on high, temp set in the 85ish range my stove goes through about 1.25 pounds of pellets per hour.

My guess is that a feedrate of 5 would get me up there in the 50-60,000 BTU range.....which would definitely make the basement a bit too toasty


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## Hammerjoe

mkmh said:
			
		

> I think your concerns are valid. These stoves are not meant to be run with a programmable thermostat, and the Tech Bulletin basically provides a way to trick the stove into "thinking" one of the leads to the room sensor probe has lost its connection...which causes the stove to go into shutdown mode. When the thermostat calls for heat again it completes the circuit and the stove picks up on the fact that the room sesning probe is in place again...causing a start-up cycle.
> If you are looking for precision heating i'm not sure i'd recommend this arrangement. As people have mentioned, it could lead to many shutdowns and start-ups, which is ineffiecient...wastes electricity, taxes the igniter, and may "mess with" the electronices of the stove.
> What i've done is programmed teh thermostat to ensure that it only triggers one to 2 cycles per day.
> Basically it starts up at 5 AM and will run until 9 AM or 90 degrees (which will never happen). At 9 AM until 4 PM it cycles down to 50 degrees (which ensures it will go into shutdown mode and stay off until 5 PM). At 5 PM it calls for heat again (90 degrees) until 8 PM....then from 8 PM - 5 AM it is back to 50 degrees ensuring that it stays off.
> Probably still not fantastic for the electronics, but it IS a Harman bulletin, so this method is not entirely unsupported.
> 
> I'm doing something very similar with my Hastings St Croix, with good results. Basically you just have to keep in mind that your stove is not realy meant to be run the same way you'd run a forced hot water furnace. However, the auto on/auto off seems to be pretty low risk to me, and adds a huge measure of convenience. I'll post pics if I get a few minutes tonight, but the configuration is really very simple (just like in the bulletin).
> Hoppe this helps



I think this paragraph is important and it could actually allow the stove to give more control to the t/stat;

*If the stove is a manual ignite stove or an auto ignite stove with the auto-manual switch in manual mode, the
stove will go into a 4-blink status when the thermostat opens (is satisfied) and the stove will go to a
minimum burn and stay there until the thermostat closes (calls for heat). When the thermostat closes (calls
for heat) the 4-blink status will reset automatically*

This is how a furnace works imo, the stove wouldnt actually shut down, it keeps the fire going with a minimal consumption of pellets and the t/stat could be programmed to keep the temp in the room by having the probe maybe set to just shy higher than what the t/stat would be programmed for.
This would save the igniter, electronics I believe and would help pellet consumption.
Opinions on this?


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## mkmh

Yep, I think you are on the right track with that. However, for my situation my preference is to just have the stove off for extended periods of time. For the space i'm heating even running it on low will provide more heat then I need, and will consume more pellets than i'm willing to part with 

If you're comfortable with running the stove 24/7 then the programmable thermostat should be able to do a good job of maintaining an even temperature for you (assuming the stove is the right size for the space). Please let us know how it works out for you.


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## Hammerjoe

Truth is I havent made my decision yet.
I was actually today at Home Depot and was looking at getting one of these: http://www.ritetemp-thermostats.com/8085C.html
I know it is overkill but it looks fancy and it costs the same as the Honeywell RTH230b. 

I didnt buy it because there was nobody at the place that could get me some 24 gauge twisted wire. 


I am not sure what would be the best operation mode yet.
I still need to think about this more in dept. 
Why would I need to plug the stove to a T/stat?


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## Hammerjoe

I bought the ritetemp t/stat and installe dit last night.
It works great.

I programmed it for my needs, basically, from 8 to 11 put temp at 30 degrees celsius and from 11 till 8pm at 16.5 and from 8pm till 1am again at 30c.

I thought initially on having the stove on manual mode, from what I could find it burns 0.75lbs/hour in maintenance mode (I guess that what it is) but I am not sure how much it needs to ignite.

Anyway, it works perfectly except for the stove that is, the stove thing failed to ignite this morning... the pot was full of pellets but it didnt ignite.

And this is the major problem that I have with this stove, its ignition is not reliable and I am always afraid that I will going to a cold home, because the stove failed to ignite.


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## mkmh

Hi Hammerjoe,

How old is your stove? I picked mine up used and it is about 5 years old (only 2 years of real use from what I was told). When I purchased it I noticed that igniter was extremely weak and unreliable so I purchased a new one from the dealer for about 60 bucks. 
Here is the thread where a few of the experts helped me through that problem:
https://www.hearth.com/econtent/index.php/forums/viewthread/9189/

The first thing to check of course would be to make sure the area around the igniter is clean and free of ash.


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## Hammerjoe

The stove is three years old and had the ignitor replaced earlier this year.

That is the problem with this stove, I think the ignitor is not in the ideal position because of the ash amount and it defeats the purpose of an automatic ignition if every time the stove is off I need to clean the burn pot.

When the stove shuts off it burns all the pellets in the burn pot all the way down to the feeder. 
Of course the burn pot will be full of ash and no doubt carbon deposits and that is what makes it hard to start, so when new pellets come in, there isnt enough draft to blow all the ash and of course the carbon deposits cant be removed without a good scrape.

The way I want the stove to be set up is, basically burning when I am home, makes it hard for me to do any cleanup except on the weekend when I am all day home.
So how can I fix this?


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## pelletheat

Did you ever check the line voltage at the wall outlet the stove is plugged into? Is the outlet grounded? No less than 116 volts should be going to the stove from the outlet, not enough juice will not get the igniter hot enough to light the pellets. Is there anything drawing power from the same outlet? 
Just some things to check........


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## mkmh

When I replaced the igniter on mine I also removed the burnpot and drilled 4 extra holes an inch or so away from where the auger feeds. The dealer said that this would help with air flow (makes sense). So far no issues with it not starting up.  I know what mean though about the leftover ash. I get that too, but with the new igniter and those holes I don't have any issues. I have been scraping out the pot about once per week.

If you do elect to remove the burnpot and drill the holes, you'll need to replace the gasket behind it. It should be about an hour job.


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## Hammerjoe

Good advice.

The stove is plugged directly to the wall, no other appliances sucking juice.
As for drilling new holes, thats a bit too much for me.


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## Tristan

Please help.  I am trying to connect the setback t/stat to my Harman XXV.  I am using Honeywell RTH230B for the tstat, I am using 18 ga (24 was not avail) thermostat wire.  The bulletin states that 24 ga is minimum, since 24 ga is not avail, I though 18 would suffice.  The tstat and the room sensor is connect in a series as state in he Harman bulletin.  The tstat is program as directed in the bulletin.  Everything is connected as it should be.  However, when i turn the stove bAck on (room temp mode, with auto ignite) the blinking light (4 times) keeps blinking and does not reset as it should be.  Hence the stove does not turn on.  Please advise.  What am I missing?  Thank you.


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## mkmh

Well, the 4 blinks means something is amiss. Have you removed the jumper from the t-stat as mentioned in the bulletin?


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## GVA

Tristan said:
			
		

> Please help.  I am trying to connect the setback t/stat to my Harman XXV.  I am using Honeywell RTH230B for the tstat, I am using 18 ga (24 was not avail) thermostat wire.  The bulletin states that 24 ga is minimum, since 24 ga is not avail, I though 18 would suffice.  The tstat and the room sensor is connect in a series as state in he Harman bulletin.  The tstat is program as directed in the bulletin.  Everything is connected as it should be.  However, when i turn the stove bAck on (room temp mode, with auto ignite) the blinking light (4 times) keeps blinking and does not reset as it should be.  Hence the stove does not turn on.  Please advise.  What am I missing?  Thank you.


4 blink means your room sensor is open....
you have it in series with a external t-stat now which means that the t-stat is saying your room is at temp and opening the circuit....
Try to manually adjust the t-stat to turn on and then go from there...


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## Tristan

One word.  UNBELIEVABLE!!  In the past I have had a multitude of successes with a variety of do-it-yourself projects around my house.  I've finished a previously unfinished basement: framing, plumbing, electrical, heating, ventilation, drywall, cabinetry ... you name it, I've done it.  So when I set out to perform this task of setting up the setback t/stat for my Harman XXV (which i installed myself) I thought that i would once again be successful.  I was confident.  I followed all specifications.  To my dismay, alas, nothing.  Nothing.  Nothing but four blinking lights.  For 3 hours I tried all sort of wiring configuration.  Nothing.  Today's outdoor temp was averaging 20 deg with bright sunshine.  The house temp was dropping rapidly, down to 50.  My wife and my three young children are about to come home to a frigid home.  No heat.  Nothing.   Finally, something on the Honeywell t/stat blinked.  The battery indicator blinked  What?  Are you kidding me?  Ten minutes later, after a quick trip to a local quickymart and two new AA batteries.  I installed the new batteries.  HEAT   I have heat.  Batteries.  Fresh Batteries.  One word.  UNBELIEVABLE!


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## cantman

I installed my Harman XXV with a Honeywell 7400 thermostat and everything works great.
My only wish was that I had purchased the Honeywell 7500 because it has seven day programming.


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## Jim H.

I too just set mine up as well.....took a few tries,  seems as though the wires going to the t stat matter which way you put them.  the other thing that slowed me down was the Rh and Rc connections......I have no Rh so assumed it was the R.....works like a charm now!  time will tell as it gets cold how well it works.

   jim


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## slink

Amaralluis said:
			
		

> mkmh said:
> 
> 
> 
> 
> One other note.  My stove settings are as follows.
> ---------------------------------
> Room Temp Mode (Automatic) toward the middle of the dial
> Temp setting around 85 degrees
> Feed Rate 1
> -----------------------------
> 
> 
> 
> Feed rate of 1?? is that for the Harman?? That is strange, the advice on this board is to actually have it higher than four.
> I have kept my accentra at a 4 but after reading a few topics here I decided to change to a 5.
> 
> To put it simply the t/stat can only be used as an on/off mechanism for the stove and not a temp controller.
> And that is exactly what I need.
Click to expand...


I usually recommend 3.5 to 4 for the feed adjuster rate.  Lower settings take longer to get up to temo and higher settings can over feed the firepot as the stove tries to ramp up to temp.


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## mkmh

Jim H. said:
			
		

> I too just set mine up as well.....took a few tries,  seems as though the wires going to the t stat matter which way you put them.  the other thing that slowed me down was the Rh and Rc connections......I have no Rh so assumed it was the R.....works like a charm now!  time will tell as it gets cold how well it works.
> 
> jim



Nice job Jim! It took me a while to get mine working properly too. If I recall, I fumbled over some of the same stuff that you did. Frustrating when your going through it, but very satisfying when it all comes together.


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## Three3

Hi.  This is my first post here.  I'm quite a regular (and a member of the moderator crew) at a similar forum for Subaru Forester owners, so I've got a good idea of the amount of helpful assistance and great knowledge that can be found at places like this.  I'm also an engineer (retired, but once an engineer, always an engineer!), so I'm not afraid to dig into things, even things that are still under warranty!  (And I've already searched around to see what people might have already posted on the subject of this thread!)

OK, on with my question.  I just got a Harman Accentra pellet stove insert.  So far, so good, but I'm already starting to think of modifications, and the first one that comes to mind is nighttime setback.  I'm not keen on thermostat approaches that actually turn the stove on or off, for many of the reasons already cited here.  My thoughts are more in the direction of fooling the temperature sensor or altering the temperature setting.  Three possibilities come to mind:

- If indeed the temperature sensor is a thermistor as described in the parts list, then it should be possible to fool it by putting a resistor in series (to make it read low) or in parallel (to make it read high).  The resistor could then be taken in or out of the circuit with a timer-controlled switch or relay.

- Similar addition of a resistor at the stove's temperature control could alter the setting under control of a timer.

- A non-intrusive technique would be this one offered by the X-10 remote control people , which employs a small heating unit to artificially raise the temperature in the vicinity of the sensor, fooling the unit into thinking that the room is already hot enough.  My initial impression of this approach is that it might be harder to predict and control its actual impact on the sensor.

Has anybody looked into any of these or other similar approaches?  If so, what did you find in terms of resistance values, results, and the like?


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## 603BOB

The thermistor requires about 3K ohms in parallel to drop back 6 or 7 degrees.

I used a 5K Bourns trimpot and a time clock - set the pot for the set-back temp desired, with the stove set for normal temp desired.

Works slick, and no blinking lights.


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## Three3

Great!  Thanks.  I've already got all the parts in my junkbox.


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## 603BOB

The stove will work normally - if it is on Auto, it will go off and on.

If it is on Manual - it will go to maintenace burn.

I set the trimpot by watching what the temp dropped to on setback - then left it alone - that was three years ago !

My Advance is 7 years old - still on the original ignitor.

P.S. I have a Forester - just REPAIRED the knock sensor.


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## Three3

Come visit us at www.subaruforester.org!


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## GVA

Sounds like a great plan, I am only concerned about the klinker build up as they seem to build up more easily when in maintenance burn rather than full on burn.


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## jimx24

I have an Accentra Pellet Stove Insert and have been following with great interest the topic of adding some kind of setback thermostat.  I like the concept of fooling the temperature sensor or altering the temperature setting as described by three3 and bob603.  For those of us who are not heavy in circuit design/construction could you guys give a little more detail on what components you used (ie trimpot, clock/timer, PCB board etc), and how they were incorporated into the sensor probe circuit.  A schematic of the circuit would be very helpful.  Thanks in advance for any additional information you can provide.


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## 603BOB

I used a clock thermostat from Home depot - one that runs on 2 "AA" batteries.
One terminal of the thermostat goes to one wire of the sensor connected to the back of the stove. The sensor remains connected as it was. (I used some flat blade "Y" connectors from a auto parts store to allow 2 connections to the same point)
The other terminal of the thermostat goes through a 50K ohm trim pot to the other sensor wire at the back of the stove. The trim pot is connected as a rheostat - the center arm and one end only.(I mounted the trim pot in the back of the thermostat )
This results in the trim pot being connected in parallel with the sensor when the contacts close.

The themostat is then programmed BACKWARDS - i.e. - program it for 90 degrees when you want the LOWER setting - 50 degrees when you want the HIGHER setting. DO NOT program it for the night setback desired !! It is only acting as an ON-OFF switch.

Set the desired setback by adjusting the trim pot with the clock contacts CLOSED. This wil be a trial and error setting - start at about 30000 ohms.
Wire size is not critical - contact types and ratings are not critical - any device that will open and close a contact at the desired time will work fine. It is only a switch that places another resistance in PARALLEL with the thermistor sensor. DO NOT use a time clock designed to turn a lamp off and on - they provide 120 volt output. Use only "dry" contacts - like a thermostat.

Theory: The thermistor sensor has a negative temperature coefficient - when the temperature goes UP - the resistance goes DOWN.
             When you parallel resistors the result is a lower resistance.
              The stove sees the lower resistance as high room temperature, and reduces the fire to lower the temperature.
             The stove will maintain the lower temperature until the clock contacts open, which removes the shunt resistor, restoring operation to normal. (The stove setting will now determine room 
              temperature.)
Note:  The setback temperature will remain a constant difference when the stove setting is changed. If you adjust it to drop back 8 degrees - for example - at 70 degrees normal setting - it will drop 8 
           degrees if you raise the stove setting to 77 degrees.

The clock can be mounted in any convenient location - it is not sensing temperature. It can even be on the floor. I hung mine from a hole on the rear of the stove with a small "s" hook.

CAUTION; These connections go directly to static sensitve components on the control board. Static electricity can cause damage, and is a big problem in the winter. The stove should only be operated from a properly wired and grounded outlet. Touch the stove to discharge any static build-up before connecting and or adjusting anything involving the control circuit 


Set it and forget it - change the batteries once a year is the only thing i have to do - it is on the fourth year now with zero problems.


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## Three3

Dang, you're fast!  And you've changed your recommendations of resistance values from 3k to 30k.  I haven't done the mod yet, but the measurements I've been making on my thermistor do tend to suggest a value higher than 3k.

I just finished drafting my response offline.  In the face of yours, I'm going to go back and tweak it a bit. I shall return!


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## Three3

603BOB has already covered the theory of what's going on here.  My implementation, which isn't yet up and running, is a bit different from his, but they both do the same thing.

Circuitry is simple.  Start with a trimpot as suggested by 603BOB (a small variable resistor, about $3 on eBay including S&H) and set it somewhere in the middle of its range.

Then you need to open up the sensor cable.  If you don't mind leaving evidence, you can just cut it at a convenient location.  You'll find two wires inside.  Alternatively, you can pull the insert out of the fireplace, locate the twisted pair of blue wires that connects to the sensor cable, and connect a new two-wire cable in place of the old one, running it out of the stove at the same place where the original cable was located.  This could be a physical challenge, but it lets you return everything to stock in the event of warranty repair.  Use appropriate crimp connectors and insulate well.  Be sure that your wiring doesn't touch anything hot.

Now all you have to do is connect the cut wires back together the way they used to be (or marry the new cable up with the old one if you did it that way), and connect the trimpot between the two wires through a switch or relay controlled by a timer.  So when the switch or relay is open, it's just like the trimpot isn't there, and things will behave normally.  But when it closes, the trimpot is connected in parallel with the sensor, this lowers the resistance, this fools the stove into thinking that it's gotten too hot, and the stove cools down till it gets to the point where the resistance is what it's looking for. 

My initial plan was to start with a cheap timer (Harbor Freight 40148, $5.99 but often on sale cheaper in their stores), open it up, separate the switch from the electricity that powers the timer, and then use the switch to connect the trimpot into the circuit.  Unfortunately the connection between the switch and the AC was buried deep inside the mechanism and I just couldn't get to it from the outside.  Disassembling it farther (in the interest of science) resulted in a mass of gears on the floor (and then in the wastebasket).

This would still be a great approach if you could find a timer whose switch is easy to get at and to disconnect from the AC.  But I decided to move to Plan B, which isn't quite completed yet.  I've ordered a small relay on eBay for about $3.50 (including S&H) from the seller named loisn14rxh.  She deals in sewing goods and electrical stuff!  She carries lots of relays, most at that same price.  Just be sure that you get one with a 120 volt AC coil.  Current rating for the contacts doesn't matter in this application.

Then it will be a simple matter of mounting the relay in a regular electrical box, cutting off an extension cord and wiring it to the relay coil, and connecting the trimpot's center contact and one of its end contacts across the two sensor wires via the relay's Normally Open ('NO') contacts. Use crimp connectors if you need to preserve the stock wiring, or just solder everything directly.  Be sure that everything is well insulated.  Use shrink tubing on all connections.  You are mixing 120 volts AC and low voltage, sensitive control circuitry in the same box, so be careful!  

Then plug the cord into a timer like the above mentioned one from Harbor Freight, set the timer, and you're in business.  Play with the trimpot for final adjustment of the setback.


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## 603BOB

The 5K and 3K were typo's - SORRY.
I get confused when I type with BOTH fingers !!


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## Amaralluis

The only concern that I have is the wear and tear on the ignitors? I am using a programmable thermostat right now that basically will turn the stove on twice a day. Its not cold enough to have the stove on manual and burn one pound an hour.


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## jimx24

Hey three3 and 603bob thanks for the detailed explanations of your trimpot circuits.


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## Three3

I've got my nighttime setback device up and running and am happy with it.  After I sketched out the basic design (shown in an earlier post above), I realized that I could add a bit of versatility to it by including a switch (SPDT Center Off) that would allow me to:

  Operate it in the timed setback mode, or

  Take it out of the circuit completely so that the stove operates normally, or

  Run it fulltime at the setback temperature (like when I'm away all day).


Here are the resulting diagrams of the device and of the entire setup.











And here are a couple of pictures of the finished product.  If you're really sharp-eyed you can see that the actual wiring deviated just a tiny bit from that in the diagram, though they are electrically equivalent.  I drew the diagram for ease of comprehension, and wired it for ease of construction.













Quickly reviewing the design, in normal usage the stove is controlled by a thermistor, which is a resistor whose value changes with temperature, and in the opposite direction from the temperature change.  What this device does is place a resistor (I used a trimpot, which is simply a small, variable resistor) in parallel with the thermistor, under control of either the relay or the switch.  This lowers the resistance seen by the stove and fools it into believing that the room has suddenly gotten warmer.  So it cranks its heat back through its normal processes.  The resistance of the thermistor increases as the room temperature drops, until the combined parallel resistance reaches the value the stove is set to expect as normal.  Then the stove does its regular thing to keep the temperature at about that level.  Taking the resistor out of the circuit returns the thermistor to sole command, and the stove heats the room back up.  The stove never actually shuts itself off or exhibits any error indication; it just goes through its regular adjustment processes, though the adjustments it makes are greater than those it would usually encounter in normal operation.

Before connecting this to the stove I used my multimeter to set the trimpot to 30 kOhms as recommended by 603BOB.  I haven't run this long enough to get a reliable estimate of the actual setback that this accomplishes, but it seems to be a bit under ten degrees.  Theory says it should be in that ballpark when room temperature is around the normal comfort range.  I might tweak it a little bit, but I saw no reason to make the adjustment accessible from outside the box.  It will probably stay untouched forever after I've finished tweaking.  I think you could easily substitute a fixed resistor of about 30 kOhms and get quite satisfactory results.  And save a dollar or two.  A resistor of less than 30 kOhms will produce a greater setback; a greater resistance will produce a smaller one.  Wattage of the trimpot (or fixed resistor) is not critical.  This is a very low current situation, so most any wattage will work fine.

If you do build one of these and want to use a trimpot but don't have a meter to set it with, be sure to adjust the trimpot to somewhere around the middle of its range before plugging it in.  (Or buy a meter at Harbor Freight for $3.99.  Everybody should have one!)  You wouldn't want to take a chance on the setting being so low that it damages the control circuitry in the stove.  And adjust it in small steps, never letting it get down too close to zero.  My trimpot requires many turns to go through its full range, but I've seen others that have a very coarse adjustment and could easily get set too low.

The trimpot and the relay are secured in place with mounting foam.  Nothing moves around inside.

There are lots of ways this could be connected into the sensor cable.  I chose to use 3.5mm audio jacks and plugs since I already had them in stock and they presented a neat appearance.  I made a short adapter cable to connect the box to the existing crimp terminals on the sensor cable, since I wanted to leave it unmodified in the event of warranty repairs.  And I made up my own cable to connect the box back into the stove: crimp connectors on the stove end and audio plug on the other.  Polarity doesn't matter; either way is fine.  I would have done something like that anyway, because I wanted to mount the sensor farther away from the stove than its own cable would permit.  If ease of disconnecting isn't important to you, you could simply run one pair of wires out of the box and connect them to the existing cable.  It's not really necessary to run the sensor circuit through the box as I did.

Do be careful to ground yourself to discharge any static electricity (especially common during these winter months) before touching any part of the device that connects back into the stove.  You don't want to take a chance on zapping anything in there.  And keep the AC and the sensor circuitry well separated from each other.  Note the shrink tubing on the relay terminals, where the two are closest to each other.

I'm thinking about putting in a pilot light that will tell me when the box is getting AC from the timer, if I can find one small enough to fit and that's capable of still providing the necessary safe separation of the AC from the sensor circuitry.

I'll confess to one embarrassment.  The timer I used is one I had already set up for a day/night program, so I just left it as is.  That is, until I realized that the settings were backwards, and the stove was giving out regular heat when it should have dropped down to low.  That was an easy fix.

As always, measure, test, be careful, keep the AC well insulated and well separated from the control circuitry, know what you're doing, and proceed at your own risk.  Neither 603BOB nor I can guarantee that your stove is configured the same way that ours are or that something along the way won't go horribly wrong.


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## from France

Hi,

Here are my drawings, with on top : Stop the stove, and at the bottom : Go to "Eco temp" setpoint
http://bayimg.com/oAiMnAaCp

Picture of my "Eco temp" device :
http://bayimg.com/CajBhaACc

Potentiometer has been tuned for a 4°C decrease (20°C and 16°C temperature setpoints) :
keeps an acceptable temperature during working hours and nights,
allows a temperature rise of approx 2 hours,
program from 16°C to 20°C setpoint at 5:00, for a 7:00 breakfast,
program from 20°C to 16°C setpoint at 8:00, when leaving to work, etc.
one program per day of the week...

Tune of the potentiometer (50% of 100kOhm at the beginning) is easy using Control Led and the Setpoint Pot of the stove...

I can put Manual-ON or Manual-OFF on the clock to stay on one of the setpoints,
or simply unplug the relay to go to Normal temp (Harman) setpoint : easier for wife and kids !

 Be very CAREFUL using these information


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## Three3

Oui, à vos risques et perils!


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## craigsta1

I bought a Harman Accentra Insert last fall and hooked it up to a programable thermostat as described. It worked flawlessly for the entire heating season. I just fired it up for the first time this year and something strange is happening ... with the stove in room temp mode and on auto it should shut down when the thermostat open the circuit (stops calling for heat).  I get the 4 blink status, so I know the thermostat is working, but the stove goes into maintainance mode and will not shut down. I even tried turning the stove control to the off position AND it still continued to feed pellets and burn at the maintainance mode.  I finally turned the thermostat back up which eliminated the 4 blink status and the stove finally shut off ( the stove control knob was still in the off position).

Has anyone else had any issues like this? Any ideas how to fix it or is something wrong with the control board?


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## KGR1951

I have a somewhat different need.  My stove is in a vacation home and I want to control it remotely.  During the winter, I keep the primary heat on at about 40-45 degrees, and use the Harmon Advance to bring the heat up to normal when I'm going there.  It's a small house, so the Harmon can get it up to 68 degrees pretty fast.  Basically I want to be able to turn it on before I get there so the house will have warmed up.

I currently have the stove plugged into a X10 module and use the X10 telephone interface to turn it on.  Crude but it works.  The problem is that the X10 module can go to the "ON" condition if there has been a power outage when the power comes back on.  Not a good situation, it's a flaw in X10 that's been around forever.  Shutting it off would mean doing so without the benefit of a controlled shut down.  

My thought is to put an X10 controller on the temmperature sensor line.  X10 makes a dry contact module that is nothing more than a controllable relay.  So based on what I've read here, I could put a variable resistor in series with the sensor to raise the resistance to a level where the stove would not turn on.  Then I would place the relay in parallel with the resistor to create a short circuit around the resistor when I want the stove to go on.  This would also allow me to shut it down cleanly via the remote, and no confusing the controller by opening up the circuit.
Does that sound reasonable?


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## kcellwood

BOB603,  thanks for the info on the thermostat setback.  I just installed a P68 this year and was looking for a way to turn back the stove automatically.  I ended up setting up a system like you described and it works great.  I bought a Honeywell 7 day programmable to use as the clock.  I could not get a trimmer locally so ended up ordering one.  In the mean time I installed a 33k resistor in the thermostat.  Not sure yet how much the setback is with the 33k.   On the thermostat there is a HEAT/OFF/COOL switch. If I switch it to OFF it takes the resistor out of the circuit and runs off the thermistor.  Great little feature.  Again thanks for the info.


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## Three3

KGR1951 said:
			
		

> I could put a variable resistor in series with the sensor to raise the resistance to a level where the stove would not turn on.   ... Does that sound reasonable?


Sorry for late response.  I haven't been here for a while.

The resistor goes in *parallel *with the sensor to *lower *the resistance to a level where the stove would not turn on.  The resistance of thermistors goes in the opposite direction of temperature, and you want the stove to think that it's gotten much too hot.

But wouldn't you experience the same X-10 spurious turn on problem, one way or another?

There was a hack going around many years ago about disabling the specified performance of the X-10 device turning itself on when its load was turned on by its own switch.  It involved cutting a pin (7?) on the IC.  I wonder whether this would also inhibit its tendency to come on when power is restored.


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## mdoggm1

Can anyone provide a copy of that Harman tech bulletin that describes how to install this setback thermostat?
The one provided earlier at oatstoves.com is not valid any more and I can't find the blasted thing anywhere.

I've wired in the thermostat with the probe, guessing as to what "in series" means and which wire goes on what terminal.

The stove will turn on when the thermostat switches on.  When the thermostat clicks 'off', I get the 4-blink status light indicating an open circuit. If the stove is off at the time, this is good and the stove won't fire up.  But if the stove is running when the thermostat clicks 'off', the stove appears to keep on running. 
I know it takes some time for the stove to shut off.  But if my thermostat is 'off' why is the stove auger still cranking and feeding pellets to the fire?

Matt


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## silverfox103

Matt, what type of stove do you have?  Both my Harmon's are on thermostats and working. 

Tom


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## mdoggm1

It's a P43. Brand new last September.

Someone at Harman finally got back to me with the PDF of their tech note.  I am attaching it here for anyone else who might want it.

According to their diagram, it looks like I have it wired correctly. They don't indicate that it matters which wire (red/black) that you connect the t-stat to.  So maybe i just need to be patient and give the stove time to shut down.

What's weird is when testing. The stove is cold and set to room-temp mode and t-stat would be off.  I'd click the t-stat on. The stove would pick up the signal and begin startup.  Then I'd click the t-stat back off, but the stove would continue to run and feed pellets, even with the 4-blink status.  I didn't wait very long to see if it would eventually shut back off. I figured I had a problem and manually turned the stove back off.


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## Three3

There's nothing wrong with what they suggested here, but it does strike me as a bit unprofessional to rely on an error situation to produce the setback.  Though one could also call it a clever trick!  

It is important to follow the directions regarding setting the thermostat's normal mode temp to the max.  Otherwise there could be a continuing duel between it and the stove's own setting that leads to an unstable situation.

Choice of which wire is which would make no difference in any of the schemes we've discussed here.  Polarity isn't relevant in situations that simply involve on/off or changing resistance.


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## lbcynya

Three3 said:
			
		

> It is important to follow the directions regarding setting the thermostat's normal mode temp to the max.  Otherwise there could be a continuing duel between it and the stove's own setting that leads to an unstable situation.



No and yes.  See my sig for an alternative I've been running all winter with excellent results.


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## Bob221

So I'm bringing up an old thread, but I have another approach to this problem I'd like to throw out there. Three3, what about this:

Instead of creating a circuit that modifies the resistance of the stock temp probe (thermister), what about completely doing away with the stock probe and controlling the resistance yourself? This way you can create true thermostat control. Here is my theory:

1: take off the temperature probe. Put it in the fridge for a while, read the resistance, call it RL (Should be a high resistance)
2: put the probe somewhere hot, like near 100 degrees. Take a resistance reading, call it RHtotal (should be much lower)
3: find the Rh value. Should be Rh = (Rhtotal * RL)/(RL - RHtotal) (this gives you the actual resistor value that, when put in parallel with RL, will give you RHtotal, which should be a lower value than RL)

And then you just make a circuit that uses these. The extra relay in the ciruit is because, when a standard thermostat is above its setpoint (which means heat should turn off), it opens a switch. We basically need to reverse that, so I accomplished that by making the thermostat swtich control a second relay that is "normally closed" to reverse its actions. This means that when the thermostat says "turn the heat off", the relay in the circuit that matters will actually close instead of open.

Here's what happens:

You use a standard programmable thermostat that will close a circuit when the temp drops below its designated temp and open it back up when it warms above its setpoint. You create the circuit below. When the thermostat says it is above its setpoint, the thermostat switch will be open, which means the relay in the resistance circuit will be closed. This will reduce the total resistance to Rhtot, which will tell the stove it is 100 degrees in the room, so the stove will turn off.

When the thermostat calls for heat because the room temp is below the setpoint, it will close its circuit, which will open up the relay in the resistance circuit. This will send all the current through RL, which will tell the stove it's 40 degrees in the room, so it needs to turn on.

All of this hinges on the fact that the stove is set at a temp somewhere around 70 degrees... this will make it always turn on when it sees a resistance value equal to 40 degrees and always turn off when it sees a resistance value of 100 degrees.

What do you think?

Here is a circuit... I'm not good with the "real" symbols for stuff, but I think the point gets across:


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## 2004hdrkc

Is there a difference between a Trim Pot and a Potentiometer? I installed a 50k- Ohm Potentiometer today and it gave me a 4 flash error on the Harman when it went to cool down mode. I even took the programmable thermostat and hooked up the Pot. up directly in parallel with the thermister and still got the 4 flash error code.


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## 2004hdrkc

2004hdrkc said:


> Is there a difference between a Trim Pot and a Potentiometer? I installed a 50k- Ohm Potentiometer today and it gave me a 4 flash error on the Harman when it went to cool down mode. I even took the programmable thermostat and hooked up the Pot. up directly in parallel with the thermister and still got the 4 flash error code.


 

Got it!  I had to link the middle with one of the sides, now everything appears to be working correctly.


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## bmccue

I have had a Harmon P43 pellet stove for about 3 years now.  I have a setback wall thermostat installed next to the one I have for my "regular" forced hot air system (which is now locked at 50 degrees).

Hookup was easy.  I actually cut BOTH of the sensor leads coming out of the back and lengthened them so they could be routed through basement and back up to wall thermostat installed for pellet stove.  I then wired the room sensing probe in series with the thermostat itself and managed to place this probe inside of the thermostat housing itself.  I have my ROOM TEMP setting on the stove set to about 75.  When there's a call for heat, the stove is actvated and the stove attempts to reach a temperature of 75 at the room sensor probe (now in the thermostat housing).  However, the thermostat is set to 70 so it opens the series circuit before the stove hits that temperature.  At that point the status light flashes 4 times and the stove effectively goes into shut down mode.

I keep my "regular" furnace thermostat set to 50 in the event that the pellet stove runs out of pellets.  This is to keep the house from freezing (I live in NH).

System has worked quite well for 3 years and it provides 100% of the heat for my 2000 Square Foot two story house for about $600 year (propane was about $1,800).


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## grh372

is there any reason why you have to keep the the room sensor in the loop? most pellet stoves you just run the wire to the digital thermostat and that controls the whole setup.


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## railfanron

Yes there is. The temperature sensor is a varistor and it varies with the temperature in the room. This give the control board something to compare against its internal electronics and that determines whether the heat turns on or off. A thermostat is nothing more than a temperature controlled switch that either makes or breaks depending on the temperature it's set at. By putting it in series you're basically switching the varistor, when turned off, into an open situation and that triggers the error condition that shuts down the stove. When the thermostat calls for heat it makes the connection and the error goes away and the stove starts up. Very simple and it works very nicely. I've been doing it for 2 years. It works equally well in either room temp automatic or manual modes.
Ron


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## Stroud

I'm getting older and lazy.  I want my stove to start automatically say at 3 am will I'm still in bed, yes I said I was lazy.  So I trying to utilize a stat to start and stop my stove.


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## Stroud

railfanron said:


> Yes there is. The temperature sensor is a varistor and it varies with the temperature in the room. This give the control board something to compare against its internal electronics and that determines whether the heat turns on or off. A thermostat is nothing more than a temperature controlled switch that either makes or breaks depending on the temperature it's set at. By putting it in series you're basically switching the varistor, when turned off, into an open situation and that triggers the error condition that shuts down the stove. When the thermostat calls for heat it makes the connection and the error goes away and the stove starts up. Very simple and it works very nicely. I've been doing it for 2 years. It works equally well in either room temp automatic or manual modes.
> Ron





railfanron said:


> Yes there is. The temperature sensor is a varistor and it varies with the temperature in the room. This give the control board something to compare against its internal electronics and that determines whether the heat turns on or off. A thermostat is nothing more than a temperature controlled switch that either makes or breaks depending on the temperature it's set at. By putting it in series you're basically switching the varistor, when turned off, into an open situation and that triggers the error condition that shuts down the stove. When the thermostat calls for heat it makes the connection and the error goes away and the stove starts up. Very simple and it works very nicely. I've been doing it for 2 years. It works equally well in either room temp automatic or manual modes.
> Ron


  So if I us battery power and contacts R &W the stove sensor will control the stove and tstat can start & stop, tstat is just a timer.  Do mil-volts come in to play?


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