# Boiler return temperature control



## maineDIY (Apr 12, 2012)

Sorry if this topic is old and boring but I'm planning on some system modifications during the off heating season and have been unable to get clear answers from searching the archives. I have (nearly) one heating season under my belt with a Vigas 60kw and 1000 gallons pressurized storage in a small building 75 ft from house. For me, having the mess and smoke and and work flow for wood handling outside away from the house outweighed the advantages of inside placement. The house was built way before the boiler idea came along. 
Anyways, here is the question: is there a three way thermostatic valve that will close the bypass arm as it opens the return allowing full flow? The valve I have now requires that the bypass arm be manually closed once the return is up to temp (140) or manually set the bypass valve partly open and leave it there which seems sub optimal. Mixing valves with motorized  controls seem excessively complicated and expensive as do other previously posted systems with multiple pumps.


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## huffdawg (Apr 12, 2012)

One way around that is a delta T boiler pump.  I'm in the same predicament as you .  Its a pain in the butt because sometimes you forget to open before you light a new fire .

I think delta t pumps you can set the temp of boiler in water. also I think they are variable speed

Huff


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## Hunderliggur (Apr 12, 2012)

The Danfoss valve does that.  The bypass setting is to adjust the pressure/flow when boiler output is being mixed with the return temp <140.  I have mine set the same way for a few seasons and I never touch it.  The thermostat in the Danfoss handles adjusting the return flow to full open at 140F.  Maybe I only get 99% flow, or maybe I get 100%.  The system is working fine for me.  You can use a mixing pump, delta t circulator, and other options, all of which cost more money.  I like the K.I.S.S. system myself.


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## ewdudley (Apr 13, 2012)

Hunderliggur said:


> ... You can use a mixing pump, delta t circulator, and other options, *all of which cost more money*. I like the K.I.S.S. system myself.


 
PID controller: $35, RIB relay: $15, boiler loop injection pump: 0$ salvage ($100 retail).

A Danfoss type valve should cost $45, but typically they go for three of four times that amount, plus balancing valve and fittings.

I went with the boiler loop injection pump because it controls return temperature according to a programmable setpoint, the fact that in only cost me $50 matters little.

My boiler happens to require minimum return temperature of 150degF, but seems to be even happier with a return temperature of 165 degF. Adjustable return temperature does the trick.

Adjustable return temperature control is also nice because it means you can in effect set supply temperature anywhere you want it, 190 degF in the dead of winter, 170 degF during DHW seasons, whatever you please.

And adjustable supply temperature solves the two-laps-through-storage problem, where storage heats to about 160 degF on the first lap and then goes to 180 degF on the second lap, with possible idling problems on the second lap. Of course this is not a problem for systems that do not require hotter water for some of the loads, but in my case I have a couple baseboard zones that do better not having to wait for storage to get up to full temperature on the second lap.

Then again, if I had known before I started that I wanted 160 degF or higher return temperature I could have gone with a 160 degF Danfoss type valve, which would have been simple and adequate for not much more money.

--ewd


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## maineDIY (Apr 13, 2012)

EWD--have you posted the details of your design elsewhere? Sounds interesting. What's a PID controller?
GG


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## Fred61 (Apr 13, 2012)

You shouldn't have to touch the Danfoss bypass throttling valve once you find the "sweet spot". Do you know the function of the bypass loop? I suspect you do but might have been misinformed about how much flow needs to go through it. Please don't take this as an insult to your intelligence but I would like to go over it as to the way I see it.
* Boiler gets up to circ launch temp. (probably 165-175 degrees)
* Water circulates but automotive thermostat is closed because sensing bulb is inboard (on boiler side) of valve.
* Water is pumped through by-pass, contacts sensing bulb and auto type thermostat starts to open.
* Small amount of colder return water starts flowing and thermostat starts regulating temp by opening and/or closing depending on temp on the bulb.
* When return water is up to the temperature you have chosen for your Danfoss, the thermostat stays open allowing flow from system.

If you set your throttling valve so that there is just enough flow to bath that sensing bulb it won't be enough to have much effect on the efficiency, taking into account the whole scheme of things. I haven't touched my by-pass in four years.


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## maple1 (Apr 13, 2012)

That sounds simple enough, but there is something about the operation of a Danfoss that is not quite as straightforward as it seems, at least in my head.

Is the bypass side of the Danfoss always open?

If so, it would seem to me that throttling bypass flow on start up would not do anything, because the flow you're trying to divert to return by throttling won't go anywhere because it will be blocked on the other end by the thermostat. At least at first - or maybe that is only a very brief period of time.

If not, there would be no need for a throttling valve as while return flow increases as the water warms up, the bypass flow would correspondingly decrease, on its own.

Hence, I think, the questions of the OP.

Maybe.


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## fuelfarmer (Apr 13, 2012)

Is this what you need

http://www.fpevalves.com/files/specsThermostatic\1010.pdf


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## Fred61 (Apr 13, 2012)

maple1 said:


> That sounds simple enough, but there is something about the operation of a Danfoss that is not quite as straightforward as it seems, at least in my head.
> 
> Is the bypass side of the Danfoss always open?
> 
> ...


 If that's happening, then your Danfoss is in backwards. There is free flow through the bypass and no particular temperature needs to be achieved in order for it to circulate. Throttling allows less circulation through the bypass thus allowing more flow through the return port once it is open. You don't want the same heated water to keep circulating within the boiler. You want to take away that heat as fast as you can by circulating it through the system or storage to keep your boiler from going into idle mode.


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## maple1 (Apr 13, 2012)

Fred61 said:


> If that's happening, then your Danfoss is in backwards. There is free flow through the bypass and no particular temperature needs to be achieved in order for it to circulate. Throttling allows less circulation through the bypass thus allowing more flow through the return port once it is open. You don't want the same heated water to keep circulating within the boiler. You want to take away that heat as fast as you can by circulating it through the system or storage to keep your boiler from going into idle mode.


 
Yes, that's my point, and I think the OPs: is there a Danfoss type of valve that closes the bypass the same rate it opens the return.

'allowing more flow through the return port once it is open.'

The question I have is, how long would it take the return port to open - because until it opens, throttling the bypass flow would have no effect since the return port would be closed.

It would seem to me to be a better working solution to have the thermostat within the valve be a diverting mechanism, rather than a simple opening & closing mechanism. In my mind, using valves to throttle flows is just compensating for some sort of poor original flow design. But, if it's a solution, well, it's a solution.


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## Fred61 (Apr 13, 2012)

maple1 said:


> *1*The question I have is, how long would it take the return port to open - because until it opens, throttling the bypass flow would have no effect since the return port would be closed.
> 
> *2* It would seem to me to be a better working solution to have the thermostat within the valve be a diverting mechanism, rather than a simple opening & closing mechanism. In my mind, using valves to throttle flows is just compensating for some sort of poor original flow design. But, if it's a solution, well, it's a solution.


 
I haven't got my head wrapped around what you are saying in sentence # 1.  

I do agree with sentence #2. A diverting mechanism would be ideal but I question if it's needed. The throttling valve is primitive but it works very well for me. There might be some sharp engineer out there that could develop a diverter type valve for not many more $ than the cost of a danfoss.


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## woodsmaster (Apr 14, 2012)

I havn't adjusted my bypass valve for a couple months and will probably leave it where it is from now on. Seems to work good and is a simple device with no motors or fancy stuff to fail. The thermostat could fail, but would be easy to change. I havn't heard of anyone having to replace theres yet.


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## maple1 (Apr 14, 2012)

Fred61 said:


> I haven't got my head wrapped around what you are saying in sentence # 1.


 
OK, try this - at cold startup, with a danfoss & no throttling valve at all, all the flow will go around the bypass since the return thermostat in the danfoss has the return flow blocked.

Adding a throttling valve will not change that, as the flow that you're trying to divert with the throttling valve will still be blocked by the danfoss return thermostat. All you'd be doing is cutting flow around the bypass and adding flow resistance to your circ pump, which might stress it, maybe?

Until the return thermostat starts to open, that is.


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## DaveBP (Apr 14, 2012)

Try thinking about it using a different term.
It isn't a throttling valve, it's a balancing valve.Saying throttle makes you think that you need to stand there with your foot on it and working it  up and down as you go through traffic.

The purpose of the thing is to balance the flows from the different parts of the system. Every system is piped differently with more or less resistance to flow on the various routes the water takes as the burn cycle progresses. The size of the pipes and their lengths along each route, The circulator size and the resistance to flow internally through the boiler all effect the amount of flow through the different legs of the danfoss (or whichever brand) thermostatic valve your have in there. The balancing valve just helps to optimize the flow for best performance as part of your particular system. The thermostatic valve is not a snap-action device. It slowly changes the flow over time as the water temps change.
It's never going to be perfect, it's just pretty good and simple.

If you want to upgrade from there you can go to the loading units which have 2 valves and circulator combined in a single housing. Very elegant but rather steeply priced. Or you can go electronic if you have the skills required. That can be expensive, too, if you don't happen know exactly what you need and have all the stuff sitting on your workshop shelf.


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## ALASKAPF185 (Apr 14, 2012)

Just put in a boiler bypass , can't get any more simple. Once set you never have to touch it again.  You can't put it in backwards and when have you seen a ball valve fail. It is required on all indoor boilers and some brands and models actually have them built in already for boiler protection. Its part of the warranty


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## DaveBP (Apr 14, 2012)

I'm not sure that a boiler bypass is the most appropriate scheme for a wood-fired boiler used with a hot water storage tank, 185.

Using a boiler bypass, as I understand the term, would cool the supply water going to storage at the beginning of the burn cycle. That would put cooler water into the top of the tank and mess up the ideal stratification in there.
That may be why the European systems we see on the net invariably use a system bypass arrangement so water doesn't start going to storage until it is quite hot. The boiler is more or less short circuited until the water it offers to storage is likely hotter than the water at the top of the tank.

In systems without a large storage tank the boiler bypass water comes back around to the boiler pretty quickly so it catches up before anyone notices anything.


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## maineDIY (Apr 15, 2012)

maple1 said:


> OK, try this - at cold startup, with a danfoss & no throttling valve at all, all the flow will go around the bypass since the return thermostat in the danfoss has the return flow blocked.
> 
> Adding a throttling valve will not change that, as the flow that you're trying to divert with the throttling valve will still be blocked by the danfoss return thermostat. All you'd be doing is cutting flow around the bypass and adding flow resistance to your circ pump, which might stress it, maybe?
> Until the return thermostat starts to open, that is.



I agree with maple1 comments. If no valve is placed in the bypass arm, then it will most likely continue to route heated water round and round, rather than sending heated water to storage or to the house distribution system and taking return water from storage or from house. By placing a partially closed valve in the bypass loop, one seems to be just stressing the pump until the retun arm thermostatically opens. Also once up to temp, it would seem best to stop all bypass flow and to allow full unrestricted flow from the storage tank bottom or house return as long as its up to set temps.


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## Fred61 (Apr 15, 2012)

mainedyi said:


> I agree with maple1 comments. If no valve is placed in the bypass arm, then it will most likely continue to route heated water round and round, rather than sending heated water to storage or to the house distribution system and taking return water from storage or from house. By placing a partially closed valve in the bypass loop, one seems to be just stressing the pump until the retun arm thermostatically opens. Also once up to temp, it would seem best to stop all bypass flow and to allow full unrestricted flow from the storage tank bottom or house return as long as its up to set temps.


What makes you think that a reduced supply to the circulator puts "stress" on it? Even if it is a receipe for the possibility of cavitation, if your pump launch temperature is high enough, ie, 170 to 180, the low flow situation time is very short.


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## Hunderliggur (Apr 15, 2012)

ewdudley said:


> ...Then again, if I had known before I started that I wanted 160 degF or higher return temperature I could have gone with a 160 degF Danfoss type valve, which would have been simple and adequate for not much more money.
> 
> --ewd


 
I like a lot of the electronic stuff - I even have a couple of PIDs on the shelf waiting for me to hook them up to control the circulator when the fire is out and the tanks are warm - something my Paxo controller does not do well.

There is no perfect way or just one way to solve the return temp protection, all have positives and negatives.

Trying to clear the OP question, the term balancing valve is much better than a throttling valve.  With a Danfoss arrangement, the water being pushed by the circulator into the boiler has two places to go on the output - to the boiler again (via the bypass loop) or to the loads/storage.  The head (resistance) is the two loops is dramatically different (without a balancing valve).  Plumbed with the same pipe, the boiler bypass will always be a lower head than the load/storage.  Water will always take the path of LEAST resistance.  The balancing valve adds resistance to the bypass loop, making it approximately equal to the load/storage loop.  Then the only difference in the loop resistance is the open/close state of the Danfoss valve.  Since the Danfoss only opens at its setpoint (140F usually) the loop will circulate in the bypass.  As the Danfoss opens, some water will go to the load/storage.  Full temperature (whatever your launch temperature is - mine is 170) will always be sent to storage. As the return temp increases, more flow comes from storage until storage reaches the Danfoss set point.

As far a  load on the circulator, the balancing valve makes the circulator see a constant load.  Circulators don't really care anyway, they are designed for these kind of applications.

Assuming a 170F launch point and a 140F Danfoss, you get the following circulation:

Return Temp 110F - 50% return flow 
Return Temp 120F - 60%
Return Temp 130F - 75%
Return Temp 140F - 100%

If your boiler can produce a  30F rise, the circulator will run 100F.  If not, the circulator will cycle around the launch point.

Always a lot of information and experiences here on Hearth.  Also a lot of different opinions!  But we share one common thread, we all choose (for one reason or another) to heat with wood.

Mark


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## ewdudley (Apr 15, 2012)

DaveBP said:


> ...
> If you want to upgrade from there you can go to the loading units which have 2 valves and circulator combined in a single housing. Very elegant but rather steeply priced.


A 160 degF Laddomat type device was pretty much exactly what I needed, but being cheap and stubborn I went the roll-your-own route.

The must-have feature I needed was a low forward-resistance check-valve for thermo-siphon flow in the event of power failure, but had very little luck finding one. I found a floating ball check valve that was perfect for $150, plus nipples and elbows. (In the end I found a bronze swing-check laying around in the back of the shop and replaced the swing-plate with a lighter hand-made polyethylene piece, which works great -- if you're cheap and stubborn.)

Then all you need is the pump, isolation valves, temperature wells, the thermostatic valve, one or more gauges, and enough space to fit in all the pieces, which is possible to do inexpensively with some 'free' time, scrounging, and good planning.

But if space is at a premium, and you need the low resistance back-flow preventer check-valve, and you need to get the show on the road, the Laddomat type device could definitely be seen as a good cost effective choice.

--ewd


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## fuelfarmer (Apr 15, 2012)

I hope photos are not frowned on in a mostly text thread.
The LEI boiler came with a mixing or boiler protection valve. Because I have an open storage tank the and the boiler is open also, I had to change from "pumping away" to a "pumping to" the boiler configuration. But that is another story. Now the mixing valve is used as a diverter valve. It does the same thing. The water circulates only through the boiler until the water gets hot enough to activate the valve. The valve opens or closes to allow more or less hot water out and cold water into the boiler loop to maintain a preset temperature. This valve if fully open at 140 F.  The pump sees the same flow rate, it just gets the water from different sources and at  different percentages depending on what the valve is doing.


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## Hunderliggur (Apr 15, 2012)

fuelfarmer said:


> I hope photos are not frowned on in a mostly text thread.


 
We always like pictures!  Some of us are just too lazy (or embarrassed) to post our own ;-)


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## Elmer70 (Sep 21, 2016)

I know it's an old thread but...  I have a question on return temps and storage.  Is it better to have a 140 or 160 degree return temp for protection when using a 1000 gallon storage system?  My worry is that the boiler would always see return temps marginally high enough to prevent condensation.  I'm looking at a Caleffi ThermoBloc for return protection and you can get thermostats for 115, 130, 140, and 160 degrees.


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## maple1 (Sep 22, 2016)

I think 140 is the accepted number -and size of storage doesn't really matter. You would have to keep your boiler pretty hot all the time to get 160 back - efficiency would take a bit of a hit.

There have been writings on here before about what value thermostat that corresponds to though. Might not apply to the Caleffi, and might be brand dependant - but some have had to either go up or down on their stat element to get the return temps they wanted. Had something to do with exactly what temp the element was full open, vs. just starting to open, how quick it went from one to the other, and how that corresponded to the actual rating on it.

The vendor should be able to help. You could likely also turn some of that prior talk up by searching the forum for those older talks.


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## boilermanjr (Sep 22, 2016)

Scott from Tarm Biomass here.  140 F is ideal for wood boilers.  Make sure that whatever loading unit you consider has ample flow characteristics for a remote installation with a 60kW boiler.  The LK loading unit we sell won't handle the job (12 gallons/min) with integral Grundfos 15-58.  I don't think the Caleffi unit will give you the flow you need either.  Check with them.  Because Europeans deal with low distribution temperatures, they have large temperature deltas to work with.  This enables them to rate loading units more aggressively than we can.  As your boiler begins to equalize in temperature with the HST, even 15 gpm isn't going to keep the boiler from reaching operating limit.  We do have a return water temperature lifting valve that has a built in balancing valve that closes completely when the the tank temperature exceeds the minimum return water temperature.  The manual balancing valve is no longer necessary.  When we provide return water protection for boilers 40 kW- 60kW we generally recommend a valve and separate circulator, often a Grundfos 26-99 to ensure adequate flow.


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## Bob Rohr (Sep 22, 2016)

Good to draw attention to the valve flow capacity Scott.
The Caleffi 280 valve only has a 14Cv in the 1-1/4" version.  Size the pump according to required flow, I'd agree a 15-58 may be on the small side.

Also pay attention to the differential of the valve.  The Caleffi valve will close off the bypass 100% at temperature + 18°, So a 140° valve closes bypass completely at 158°.

I actually dropped mine down to a 115° + 18 for a 133° boiler return temperature protection.

Then issue I had with the 140° was my EKO would start ramping down before my bypass was closed 100%.

I have an early "metric" control and the temperature knob setting would not allow the boiler to go above 170F, and the blower started ramping down before that.


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## velvetfoot (Sep 22, 2016)

For kicks, I bypassed around my measly 110 gal buffer tank with a motor operated valve and controller because I figured higher temperature water could get to the baseboard that way, instead of the big cool off with the buffer water.  Not sure it was worth it though.  With that 1000 gal tank, maybe it would be.  What's nice about the motor valve is that it can be tuned.  What's not nice about is that it has to be tuned.  These are only my observations.


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## maple1 (Sep 23, 2016)

I don't think the 'thread reviver' ('New OP'?) mentioned how far away his boiler was from storage - therefore my assumption was they were next to each other. So the typical 15-58 boiler circ may cover his needs quite adequately. My unit (LK) has a 15-58 3 speed circ. It handles my 40kw boiler just fine on low speed. I tend to run things on the lower side of system temps though, so my fire is pretty well out by the time the second lap through storage is coming to an end. If I was to get into a third lap, or try to run higher supply temps - I might be looking to shift up to a higher speed. But it seems to have some flow in reserve.


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## Elmer70 (Sep 23, 2016)

Thanks for the insight guys.  Yes maple1 my boiler is very close to my storage, about 8-10' fitting to fitting.  They are both located in a boiler room inside my pool house.  It is connected to my house with insulated 1-1/2" Uponor he-pex.  Runs are less than 40' from pool house to house.  Near boiler to storage piping will be plumbed with 1-1/2" and some 2" black steel.  My boiler install is going to set a record for longest project.  I purchased my boiler biomass 60 in 2008 along with some parts.  Life and other priorities got in the way.   I have a 3 speed TACO 0010 circ. and an 1-1/2" Danfoss mixing valve.  But was under the impression these are no longer favored and that better methods have become available.

I'm am not new to this forum, I've been here at hearth.com for a long time.  Perhaps before there was even a separate group known as the boiler room.  I rarely post as you can see.  I used to have a different username but lost it during a major web redesign "whitepine".  

I think for this heating season I just want to get my boiler plumbed and my garage heated.


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