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Two thermostats running one boiler

ChrisLChrisL Member Posts: 121
I've got a unique situation where I am heating an apartment building with apartments having different wind exposure.  With a steady wind, one apartment can be 75, and another 68.  Averaging temps will not solve this problem.  Other than going to an internet based thermostat, and manually switching it as needed, I thought I could run two thermostats.  That way, I would be assured of keeping a minimum temperature in different parts of the building.

Is there any issues this might present?  If one thermostat is calling for heat, would the control voltage on the back side of the other thermostat present any problems?

Basically, I envision two Honeywell visions, each with one external temp sensor to be installed in an apartment, and then both hooked up to the boiler.




  • Paul48Paul48 Member Posts: 4,492

    that fix, the apartment that was 68 will now be 75, and the apartment that was 75, will now be 82.You need to insulate, upgrade windows, etc.
  • STEVEusaPASTEVEusaPA Member Posts: 4,419
    how exactly.....

    are you heating the different apts?  I see boiler, but is it one zone to multiple apts?  Baseboard, radiators?  One pipe, 2 pipe, etc.
  • ChrisLChrisL Member Posts: 121
    Other info

    This is a hydro coil system.  Basically, one boiler, with an air handler for each apartment.

    The building is very well insulated, and tight.  The issue is the apartments have different exposures to wind and sun.  Its not too big of a deal under normal winter temps, but becomes more problematic when we get 35 or higher, as the less cycles on the boiler magnify differences among apartments.  You guys that do primarily single family, would be surprised how much wind makes a difference on heat loss...even on a tight well insulated building.  I did not believe it myself at first, but temp readings have proven it to be the case.  With a building shaped like a cross, with apartments on each spoke, you can now see why the temps can be so different.

    My idea was to run a thermostat in two of the cooler running apartments, with different exposures.  These would operate the boiler.  Each apartment has its own thermostat that operates the blower.  I could then put a limit stat in each duct or apartment to limit the temp to max of 73 or so.  That way I would be covered on the low end, and high end as well.  So, back to the original question.....can I wire two thermostats onto one boiler?


  • lchmblchmb Member Posts: 2,992
    why not

    zone each appt with electric dampers to maintain a closer temp?
  • Paul48Paul48 Member Posts: 4,492

    each apartment has a t-stat wired back to the boiler room,they can all have control of the boiler.
  • ChrisLChrisL Member Posts: 121

    There really is no need to zone with dampers, as I can control the heat in each apartment, since they each have their own blower and ductwork.

    Its just a matter of making sure there is hot water available when the coldest apartment needs it.  The boiler has a circulator that runs continuously and pumps through each of the apartment coils.  (one large loop)  I could run the aquastat on a minimum water temp that would still provide some heat, but that would be wasteful.

    So I have a remote thermostat in the coldest apartment that operates the boiler.  When it calls for heat, it turns the whole building boiler on.  When the water temp reaches  130, a switch kicks in that then lets the blowers that are calling for heat turn on.  But if there is an apartment colder than the one I have the remote sensor in, it will never get the heat it needs, because the boiler will not turn on until the remote sensor apartment gets cold enough.  So that is my dilemma.  If I can have the boiler come on from two apartments, I should have my bases covered.  So, that is my question...can I operate a boiler from two thermostats? 


  • lchmblchmb Member Posts: 2,992
    you could

    use a taco 502. Wire the t-stat in, run the blower off the 120 volt leads and use xx to call the boiler on.
  • WesmanWesman Member Posts: 16
    Let me know if you need a drawing

    but I'll try to do it in words.  I don't think you would have a problem with 2 stats, however, your concern is having both stats call at the same time.  Assume the boiler is looking for 24vac, not a contact closure.  Assume using 1 transformer for both stats.

    You can isolate the 24v+ with a simple spdt relay.  Take the - side of the transformer to the - side of the relay coil and jump to the - boiler.  Take Tstat1 W and go to the NC side of the SPDT relay (like an RIBU1C).  Take the common of the relay and go to the + of the boiler. 

    Take Tstat2 W and go to the + side of the relay coil and jump to the NO side of the SPDT relay.  You're done.

    If tstat 1 calls, the relay doesnt come into play except to pass the signal through the NC side of the contact, to the common and then to the boiler.  If tstat 2 calls, the coil pulls in and opens the NC, closes the NO.  This allows the boiler to continue to run but isolates 1W and 2W.  the boiler is now run only by 2W.
  • ChrisLChrisL Member Posts: 121
    RIBU1C Solution

    Thanks Wesman,  That will do the trick, and is fairly simple and reliable.  An elegant solution.

    I have since hooked the two vision thermostats in parallel, and they are working fine.  Whether one calls for heat, or both, either one, it worked correctly. 

    So now the question is, will the added relay add reliability, or reduce it.  Seems like a tough call.
  • WesmanWesman Member Posts: 16
    Relay or not?

    I would say as long as you are using the same transformer for both tstats you should be fine.  Adding a relay would decrease reliability in that there is 1 more thing to go bad.  When it goes bad, tstat 2 could not make a call because the relay could not pull in.
  • Paul48Paul48 Member Posts: 4,492

    Could you please explain the need for any of that. I've been trying to figure out the need for a reversing relay of sorts, and I don't see it.Off the same transformer?
  • John Mills_5John Mills_5 Member Posts: 935
    stat for each?

    Seems to me, each air handler should have its own stat. When that apt needs heat, the blower turns on, zone valve opens or pump comes on and boiler fires. When satisfies, blower shuts off, zone valve closes or pump shuts off as does boiler. I can't see why it would be wired with 1 stat controlling both air handlers.

    If there aren't zone valves and both coils get hot water regardless of need, the stats for each could control the blower. At least that way each apt. would have their own comfort and you wouldn't be paying to overheat 1 of them. Then have to use relays as has been discussed to bring on the boiler if either apt needed heat.

    You'll be dealing with 3 transformers, 1 each AH and the boilers so the relays keep them all separate.
  • ChrisLChrisL Member Posts: 121

    Yes John,

    Each apartment does have its own stat, which controls its air handler.  There is a temp sensor on the supply pipe to be sure the water is hot in the loop before allowing any air handler to turn on.

    So far, its working real nice with the two stats.  The final piece is to put a temp limiting cut-off in the apartments to prevent over heating.  I found a product called Temp-Limiter by Jackson.  Basically is a box like a remote temp sensor that you mount next to the thermostat and wire in series,  It will open if temp gets above either 70 or 73 depending on the model. 


  • IronmanIronman Member Posts: 5,806
    2 Stats, 1 boiler

    The correct method is a zone panel such as the Taco SR502 if you have two circs. Taco also has a "hydro air relay". You'd need one for each AHU.

    You say that you have "Vision" Stats. If you mean Honeywell "Vision Pro", those have temp stops built in to the programming. All you have to do is go into the installer setup and set them for where you want (heating and cooling).
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Paul48Paul48 Member Posts: 4,492

    You're not heating all those air handlers, regardless if the apartment needs heat, are you?
  • ChrisLChrisL Member Posts: 121

    Well guys,

    There is only one circ for the whole building.  It circulates through every air handler (10 of them), and is on 24/7.  Its a B+G PD 37.  I would love to upgrade it, but it also does chilled water in the summer, so my choices are limited.  I leave it on 24/7 for reliability issues, but am re-thinking this part. 

    Now, I suppose there is some convective air flow in the ductwork even if the blower isn't on, but it is pretty minimal from my observations.

    Bottom line is, I could put zone valves or circs for each air handler, but it would make it unnecessarily complex with regard to maintenance issues.  Its a 45 min drive for me, so travel costs will quickly eat into small savings over time.


  • furnacefigher15furnacefigher15 Member Posts: 514

    I think you hit it on the head. Savings at what cost. In a building of that magnitude, continuous pump operation is the way to go.

    In order to truly save on pump operations, you'd need to upgrade to a variable speed pump, and add zone valves to all the air handlers to vary the flow of water, to give the variable pump something to respond to.

    When it comes to rental property, keeping it simple will save more in the long run.

    With an upgraded pump, you might save a few hundred a year pending on the horsepower of the pump, but it would cost more then ten times that to set it up. Then factor in system complexity for future repairs, and more parts to replace when they fail, just not worth it in my opinion.

    As far as boiler control, have you looked into outdoor reset?
  • ChrisLChrisL Member Posts: 121
    Outdoor reset

    Actually, the boiler has a very old outdoor reset type device that basically limits boiler on-time according to outdoor temps.  However, I've not been using it the last few years, as I only run about 160 degree boiler water temp most of the time.  Running any cooler keeps the air handlers on too long.  Don't forget that this is a 3rd story building, with all the air handlers in the basement, so there is some temp loss in the ductwork, even though its all insulated.

    Believe it or not, I am still running the original Weil Mclain cast iron boiler from 1965.  I was planning on replacing it any year now, but am going to hold off a little, as gas prices are so low, and I want to see where we go as far as technology.   Hard to believe that  I was looking at heating with electrical just two years ago, as it looked like gas prices were going to stay high, and it seemed like a cheap way to convert to individually heated units that would still be somewhat competitive to gas. So, once I finish converting the last few apartments to individual AC condensers, I'll be able to re-claim the space now occupied by the central AC system in the boiler room, and will be able to run any combination of boilers I like. 

  • zape74zape74 Member Posts: 1
    2 thermoststs one boiler

    I have a 2 1\2  story house with a one pipe steam boiler,however there is a hot water zone (run by therm. connected to a circulator) that heats the lowest level.I know this is not an optimal situation. It is what it is.The lower unit only get heat obviously if the therm upstairs calls for the boiler to fire and the therm. downstairs calls for heat.turning on the circulator pump. I was wondering if I could wire 2 therms.(somehow) to both control the boiler. I know ,if I can do it, that the boiler  would fire when the downstairs apt calls for heat but the upstairs O.K. ....However the lower apt is small 450 sq. ft. and right next to the boiler room.I figured,if it can be done ....the downstairs therm. calls for heat...boiler turns on...circulator turns temp rises and therm is satisfied......if the upstairs therm. calls for heat the system just works normally.
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