I need a better thermostat

Discussion in 'HVAC Heating & Cooling' started by got_nailed, May 30, 2007.

  1. got_nailed

    got_nailed DIY Senior Member

    Jan 11, 2006
    I have a large 2 story house.

    Both floors have train HVAC systems there about 9 months old.

    I have a 500 gallon wood boiler that I would like to use for heat. I did have the heat exchangers installed when they installed my systems.

    As far as the controls I’m running 2 thermostats on both units; one for the train and one for the wood boiler.

    I would like a thermostat that would control both systems and have a water temp sencer so if I don’t have the boiler running then it would use the heat pump.

    I do have it working the way I want it to right now but it’s using 4 relays on both systems and it dose not always work right.

    Anyone have any solutions for me? I talked to the people at the train place and no one knows of anything to help.
  2. CHH

    CHH New Member

    May 25, 2007
    Denver, CO
    Is there an electrician in the house?

    Hmmm. Seems to me that only 3 relays are required:
    1 double pole on the thermostats to potentially close the TT circuits for the boiler and the heat pump when the thermostats call for heat

    1 aquastat which opens on low temperature mounted on the wood boiler and in the TT circuit for the boiler. This aquastat prevents the wood boiler circulator from running when the boiler temperature is below the set point.

    1 aquastat which opens on high temperature mounted on the wood boiler and in the TT circuit for the heat pump. This aquastat prevents the heat pump from running whent the boiler temperature is above the set point.

    Of course the set points for each aquastat should be identical.
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  4. got_nailed

    got_nailed DIY Senior Member

    Jan 11, 2006
    This pic is how I currently have it set up.

    The pic is for one floor. The same set up is on both floors.

    2 thermostats
    1 for HVAC
    1 for boiler
    Both of the fan wires have diodes for one way flow of power

    The water temp sencer controls the relay to send the power to the compressor or the boiler manifold. It will only let one work at a time.
    Default for the relay is the compressor in case something fails so I still have heat.

    Both thermostats are set at the same temp.

    The reason I’m using 2 thermostats is because I didn’t know how to wire one and still have the AC work.

    I hope this helps with how I have everything currently set up…


    I don’t totally understand.

    I think what your tell me is I need a double poll thermostat. It would send for the boiler and the compressor at the same time.

    The relay would tell it where to send the power (boiler or compressor) when heat is needed, based off the temp controller for the boiler.

    The double poll thermostat would send for cooling in the summer with out having to change any thing.

    I’m off to look at a double pole thermostat. This would help me under stand what your talking about.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Most HVAC controls use a/c (24vac) for control. Adding a diode can give you big problems, as it only blocks about 1/2 of the sinewave. You need something other than that to isolate things.
  6. CHH

    CHH New Member

    May 25, 2007
    Denver, CO
    Hopefully the attached diagram will help you see what I tried to explain. There are probably several ways to handle the aquastat switching so only one aquastat would be needed. I think that all you really have to have is a low limit controller. One of the boiler supply houses should be able to recommend a suitable aquastat.

    I just went through the aquastat selection problem when I added an indirect water heater to my system. The controls appear to be simple but it was a whole new language for me to learn...

    Attached Files:

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