# Fan limit control or thermodisc or ??



## TCNC (Dec 7, 2008)

I fired up my US Stove 1537 for the first time today.  It's installed parallel to my newish Thermopride oil furnace.  

The oil furnace fan will kick on for a few seconds here and there, but nowhere near enough to actually push the warm air upstairs.  Meanwhile, the wood furnace's fans run constantly when the fire is going good.  The oil furnace's fan & limit control isn't in the plenum, but inside the cabinet, closer to the burner itself, I think and therefore not picking up on the plenum temp.  I've read where people have installed another f/l control in the plenum.  Is that the best solution?  Would a thermodisc (similar to what came with the wood furnace--only adjustable) work as well?


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## jdew1920 (Dec 7, 2008)

Nicole - I've been looking into these too.  A thermodisc & fan control esenntially do the same thing.  The main advantage with a traditional fan control is the flexibility you have to make adjustments.  The thermodiscs usually are built with 1 temp on & off setting, but I have seen adjustable ones but not with the range of a fan control.  I've been looking around and I think you could pick up a fan control off of ebay for probably $30-$40.  Thermodiscs new run <$20.   I'm installing a Daka furnace right now and am going to wire it so when the wood furnace fan comes on (its only 500 CFM) that the furnance fan will come on at low speed (it's a variable speed blower which I normally run all the time on low speed just to help even the temps out).  I'll see how that works then if needed I can add a separate fan control.


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## TCNC (Dec 7, 2008)

Thanks.  What are you doing to wire it to have the wood furnace and main furnace fans come on together?


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## jdew1920 (Dec 7, 2008)

Well - Now I am rethinking my plan.  But, I was intending on activating a 24 volt transformer by connecting it to the power supply after it ran through the thermodisc on my wood furnace - that is all fine.  My problem now is I'm wondering if using an outside 24v source to activate the fan on control on the furnace board is a bad idea.  My BIL suggested using a relay instead of a transformer and getting the 24 volt power from the furnace board (similar to how the thermostat get's it's power) and using the relay to turn it on or off.  But even that I'm not sure about because I'm not sure if have a constant signal on "fan on" will mess with how the furnace normally operates (It's a newer Rheem with variable speed fan), I guess I need to figure out what the regular furnace thermostat does with the "fan on" signal when it calls for heat.  If the 24v is still there, then it should work, if not, then I need to get creative.

If anybody has any input I'd appreciate it.

Thanks.


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## mike1234 (Dec 7, 2008)

In a similar setup my HVAC person suggested that I put a thermostat with a probe into the plenum of the wood burner, and have that thermostat activate the fan of the other furnace.   Set the thermostat to cool, not heat, and when the temp in the plenum goes above 100 (or whatever you set it at) the fan comes on.  
Mike


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## TCNC (Dec 8, 2008)

jdew1920--You lost me there!  Hopefully someone else will help you out.    I'm going to go with another fan/limit in the plenum.  Might be the more expensive route, but it seems like the easiest.  Have you burned in your Daka yet?  I'm toying with the idea of unhooking the power to one of my wood furnace's blowers.  I was wondering if the single blower did a good job moving the heat from the jacket.  They're each 550 cfm and today they were on non stop.  I was scared to go look at the electric meter.  I think that both of them also cool the stove down too much.  I'm know my chimney could be a bit higher, but it was burning nice and clean at 600 on the stove front thermometer.  It was just hard to keep it up that high.  I tried just using the oil furnace fan, but it wouldn't pull the heat from the wood furnace enough on it's own.

Mike--Interesting.  There being a million and one ways is making my head numb.  The stove manual has left a lot to be desired, I'm finding.


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## Snowdooer (Dec 8, 2008)

Those fans on the 1537G are junk.  They don't push enough air.  However if you run both I wouldn't worry about the electric bill too much.  They are only 1/25 HP motors.  I believe they draw about 1.5 amps.  That equates to about 350 watts per hour.  So for every three hours or fan run time you'll use just over a kilowatt of power.  Does that make sense? (1 KW of power runs about $0.10-0.12)

For your installation there are two ways to solve the problem.  You can either: 

1) Install a limit control in place of the thermo disc on the wood furnace and have it start both the wood furnace blowers and the gas furnace blower at the same time or 
2) Install a second limit control into the plenum of your gas furnace and have it turn the gas furnace blower on when it's plenum sees the heat from the wood furnace.

Personally I would go with option 2 in your situation.


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## smokinj (Dec 8, 2008)

Snowdooer said:
			
		

> Those fans on the 1537G are junk.  They don't push enough air.  However if you run both I wouldn't worry about the electric bill too much.  They are only 1/25 HP motors.  I believe they draw about 1.5 amps.  That equates to about 350 watts per hour.  So for every three hours or fan run time you'll use just over a kilowatt of power.  Does that make sense? (1 KW of power runs about $0.10-0.12)
> 
> For your installation there are two ways to solve the problem.  You can either:
> 
> ...


Same here!( in this case the 2 550 blower will be enough)


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## laynes69 (Dec 9, 2008)

One thing about running your furnaces blower with the wood furnace is you will temper the heat going through the ductwork. Those 2 little blowers do nothing to push the heat through the home. Whats the square footage you are heating? A fan limit in the main furnaces plenum would be your best bet. You may even be able to use the low voltage side and run 1 wire to r and 1 to g on the circuit board. That way you hit a set temp and your blower on your furnace defaults to on and runs. I opted to reinstall my furnace 3 years ago in series. It was the best thing I could have done. You need a good damper to block off the heat from the woodfurnace from backfeeding in to the main furnace. Parallel will work, but most people have installed a larger blower to assist in heating. Our main furnaces blower is the only blower in the system. It pushes the air around and out the jacket. Our blower runs almost constant, and we don't see an increase in our electric bill. I would place a limit/control where the thermodisc is on the furnace. That way you adjust your heat to where you need it. I wouldn't change the location into the plenum. Keep it in the woodfurnace.
As far as the question goes on the board of how the furnace would act, here is my scenario with a 15 year old propane furnace. 
Lets say that I'm burning wood, and it gets too cold for the woodfurnace to keep up. Its using my propane furnace to remove the heat from the woodfurnace. When the thermostat upstairs triggers the lp to kick in, the blower shuts off. Then the furnace cycles, and the heat comes on. Meanwhile its just pushing the heat and the woodfurnaces heat through the jacket. When the upstairs thermostat is satified, the lp burners shut off. Now as long as there is heat in the woodfurnace, then the blowers just keep running. Never had a problem with the system this way. From what I was told, most modern furnaces have the relays built in to allow this. The only part where it would get tricky, is if you have central air also. I set my LP furnace blower to low for the woodfurnace, and when the heat kicks in, its med high.


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## jdew1920 (Dec 9, 2008)

Can you use the fan controls as a low voltage relay?  IE can I use low voltage to activate my furnace fan?  For some reason I was thinking they were used to kick on the fan directly and were not for low voltage application.


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## laynes69 (Dec 9, 2008)

I thought I did it differently. I ran the r and g off the relay for the forced draft. Its been so long since I wired it I forgot how I did it. The limit control energizes the relay which in turn energizes the r and g on the board of the LP furnace. Hope this makes sense. Back then it did, now I really forgot.


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## TCNC (Dec 9, 2008)

I don't see a circuit board inside my furnace, but maybe I'm looking for the wrong thing.  My frame of reference for circuit boards is regular personal computers.  I've attached some pictures and the wiring schematic for my furnace and a picture of the blower in the rear.  I'm assuming those letters are the different fan speeds.  I think I'm at medium high now because the furnace guy that installed the oil furnace said he could bump up the blower if it wasn't giving me enough heat on my 2nd floor.  My two main floors are about 1800 square feet.  I don't have any central air to complicate things further.

What's the difference in using the low voltage side versus hooking another fan limit control in the plenum?  Honestly, I don't even know what all that means, but I really want to learn so I know what's what in my own home.  My father's going to hook up the new fan limit control but I'd love to understand all this myself.

There's another picture of the convoluted ducting into the oil warm air plenum.  With the wood furnace being on the other side of the oil furnace, I can duct it just up and over instead of this up, over and around crap.  That might allow me to only use one of the wood blower fans to move the heat from the jacket yet not cool it off too much.

Now this leaves me with another question/problem.  There's a picture here of the stove pipe hookup and I'm not happy with it.  I just had the class A chimney put in last week and I think I'm going to need another length on it to improve the draft because I'm having trouble keeping a clean burn.  However, it took me from the beginning of September until last week for all the chimney pipe  and wall kit to get in to the HVAC guy so I want to see what I can do to improve the draft right now because adding to the chimney won't happen probably until winter is almost over.  IMO, there are too many horizontal runs and too many sharp turns in my stove pipe.  Can I get suggestions from you guys as to how you'd run this pipe?  The part you can't see just runs straight out from the back of the stove.  Being able to get into that door behind isn't a concern.  It's just storage and a bomb shelter, which if I get bombed, that homemade louvered wooden door isn't going to save me anyway.  

Thank you all.  I really appreciate the advice.  I've been reading these boards since last winter gathering all my info to get back into wood burning after the high oil prices last year and what they did this summer.  Of course, with my luck, they've dropped so far down now,.   But I do prefer the wood heat and I'm sure oil prices will go right back up--if not this year, then next.


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## laynes69 (Dec 9, 2008)

I'm not sure what to tell you about your wiring, maybe an electrician could chime in. With my forced draft came a relay which energizes when the limit kicks on. When this happens it sends low voltage to the circuit board on the LP furnace, just like a thermostat would. Could you get a few pictures around your woodfurnace. You may end up having to install a larger blower on the unit. As far as the flue is concerned, you want as few bends as possible. You need to eliminate those 90's, thats your draft issue right there. I would run a gradual angle if you can. Maybe a 45?


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## TCNC (Dec 9, 2008)

I don't have the forced draft on my stove.  Even with the horrible ducting, the two blowers to a great job now moving the air into the oil plenum.  Upstairs I can feel a bit of heat blowing out of the ducts with the shortest runs.  No way I could heat with just those, but when I manually turn on the oil furnace fan, it does a really good job with the heat.  Pretty similar to how my old wood/oil unit did.  The 2nd floor with the bedrooms (3rd floor from the basement) is cooler, which is how it was before and how I prefer it anyway, but I had the main floor between 70-75  today and it was a high of 18 degrees today.  And that is with me not being able to keep my stove temp up enough, so I'd think that would give me even more heat.

I'll have to have my ex-FIL take a look at the stove pipe tomorrow.  He's a woodburner.  That'd be sweet if fixing that cleared up my draft issues.


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