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Steam boilers and heat pumps playing together?

Love my steam heat, but got tired of the window AC units and had a heat pump installed this Spring. A second thermostat was added for the heat pump. I am wanting advice of how to best utilize both sources of heat this winter, especially if there is a thermostat that can properly work with both the boiler and the heat pump. Any ideas?


  • Jamie Hall
    Jamie Hall Member Posts: 20,401
    I presume the heat pump is forced air, and is the usual air to air variety? In which case, you need to look at the specifications of the heat pump to see how its efficiency changes with source air temperature -- in particular where (or if) it switches to resistance heat backup. Which you don't really want to use.

    So the change from one to the other isn't dependent on the interior air temperature or temperature setting, but on the exterior or source air temperature.

    I imagine with a little ingenious wiring involving indoor and outdoor thermostats and perhaps a relay or two could do it...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • njtommy
    njtommy Member Posts: 1,105
    Yes there are many T-stats out there that will work to control your heat pump and steam boiler. I have a honeywell prestige for my own home that controls my heat pump and boiler.

    What fuel are you running for your steam boiler?
  • JStar
    JStar Member Posts: 2,752
    You should make a balance-point chart for the heat pump performance at various outdoor temperatures. Run the heat pump down to its practical limit, then let the steam take over at the coldest temperatures. Although, I would probably just stick with the steam full time.
  • njtommy
    njtommy Member Posts: 1,105
    You really can't beat steam or hydronic heating, but if your heating with oil or propane the heat pump will generally save money.
    JUGHNE Member Posts: 10,278
    If it were mine, I would try a heating season by manually running the T-stats as needed. If you haven't experienced heat pump heating you may find it not comfortable after living with steam.

    It is a great heat source for the shoulder seasons of fall & spring with moderate outdoor temps. Heat pump 70 degrees is not like steam heat 70 degrees, with HP there is no one place to go in the house to "feel" heat. But a CI radiator will iron your jeans with you in them and then you know heat. (Wood stove without the ashes).
    Anything is possible with controls but there is the complexity sometimes not needed.
    When it freezes out turn the steam on. IMHO
  • Champ6
    Champ6 Member Posts: 19
    edited September 2015
    Thanks for the replies. I lost my hard drive a few months back, and only now thought of checking back in. I do love my steam heat. It is a 10 year old Weil McLain gas boiler. I think the last comment sums it up well, it's mostly the shoulder seasons that I am thinking heat pump. But we have a moderate climate, so often in the winter, we have days that need little heat, and the temperature over-shoots as the radiators continue to radiate heat long after the boiler has shut down. This is what I hope to minimize this winter, without having to decide which heat source I want on any given day.
  • vaporvac
    vaporvac Member Posts: 1,520
    so maybe folks here can help you minimize the overshoot with the steam. Then the choice won't be so tricky. Do you have two or one-pipe steam?
    Two-pipe Trane vaporvacuum system; 1466 edr
    Twinned, staged Slantfin TR50s piped into 4" header with Riello G400 burners; 240K lead, 200K lag Btus. Controlled by Taco Relay and Honeywell RTH6580WF
  • njtommy
    njtommy Member Posts: 1,105
    You can just have the heat pump turn off once it's gets into the colder days by using an outdoor air sensor. It will not allow you boiler to come on say above 40 and it would turn your heat pump off below 40f ODA.
    Any honeywell t-stat That has a out side air sensor option and is rated for a heat pump can control your system no problems.
  • Champ6
    Champ6 Member Posts: 19
    Vaporvac: It's a one pipe system. NJtommy, I have a Honeywell t'stat (left the model number at home, but it's only months old) I have 5 wires to the heat pump, and as expected only 2 to the boiler. How do I wire this, and how do I add the outdoor sensor? Will the sensor help with overshoot on interior temps?
    Thanks all!
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,572
    Have a look at the documents for the thermostat and see if it has provision for warm weather shutdown. This temperature could be set to prevent the boiler from firing during moderate cold snaps.
    In colder weather, the separate heat pump thermostat will be satisfied by the steam heating temperature, and will not go on.
    Some thermostats will have an auto changeover feature from heat to cooling, but it is the WWSD which will cut off the steam boiler.--NBC
  • njtommy
    njtommy Member Posts: 1,105
    Once you see what T-stat you have you can figure out which option you can go with.

    Your over shooting most likely will not stop with this setup. I would start another post for your over heating problem. The steam guys on this page will really be able to help you more so then me.

    I've it before using a Honeywell bulb type temperature control that has 3 terminals on it and a relay, but It would require a bit of leg work and wiring. Cost and labor wise you would be better off replacing the t-stat.

    I wil call a local supply house tomorrow to see if they have any other options i'm missing

    JUGHNE Member Posts: 10,278
    edited October 2015
    If you want one T-stat to do everything, I believe you need a T-stat that will handle a heat pump with fossil fuel as back-up heat. It would need an outdoor sensor. You can set it to use HP only for heating down to some desired ODT, say 35 degrees. At that point HP is off and boiler only would do heating. It gives you the options of cycles per hour adjustment for each stage of heat, needed for steam heating, different than HP heating.

    The HW IAQ also has a "Heat temperature control" which gives an adjustment if room is warmer than set temp. (Don't know how that would affect steam overshooting temp).

    In the fossil fuel mode, if the heat pump goes into a defrost cycle it would turn the steam on. You might not want to run the HP at low ambient temps that would require a defrost cycle, (which is thought by some to put a strain on the compressor anyway.)

    I would probably have the heat pump 24 volt power supply run the t-stat and use an isolation relay to start the boiler with it's own 24 volt transformer.
    If you have resistance heaters on the HP air handler you would not want that to be used as stage 2 of heat.

    The IAQ t-stat I have info on only needs 3 wires from t-stat to an interface module which was located on the furnace. Newer versions may have changed, but the wiring design would keep in mind that you may have limited conductors at the T-stat location and unable to add more.