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Combo hot water system and heat pump

I had a new heat pump system installed in my house this year.  Previously I had an old hot water system with cast iron radiators.  The old system is still working, and I wanted to keep it as a backup and to use during colder weather since the heat pump is not as efficient in colder temps.  The contractor said he could set it up so that the heat pump would run first and if the auxillary heat was needed it would switch to the hot water system instead of running the electric heat strips.  So far it hasn't been working like he said, and I'm not sure he knew what he was doing to begin with.  The only way to get the radiators to come on is to manually switch the thermostat over to auxillary heat, and then the fan continues to blow while the radiators are running.  Also, any time there is more than a 1 degree temperature difference between the set temperature and actual temperature, the thermostat switches to auxillary heat, and the heat pump continues to run.  I'm not sure if the heat strips are running at this point, but it never switches over to the boiler on its own.

Is it possible to run two seperate systems like this off the same thermostat?  Should it have an outdoor temperature sensor to regulate at what point one or the other comes on?

The thermostat is a Honeywell FocusPRO TH6220D

Heat pump is Trane XL15i model# 4TWX5042A1000A

Thanks for your help


  • Ironman
    Ironman Member Posts: 7,119
    Wrong Thermostat

    You'll need a TH8320 with an outdoor sensor, plus a double pole double throw relay. Connect the coil of the relay to "aux/W" and common. Connect the"T T" terminals from the boiler through the normally open contacts on one pole. Connect "G" wire from the stat to the Normally Closed contacts on the other pole, then on to "G" on the AHU. Make sure the stat is set up for "2heat/ 1cool, heat pump with aux. heat" in the installer set up.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
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