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I want to swap out the old air handler for a new one and the old straight AC for a heat pump. I will not be putting a heat package in; I will use my boiler as the emergency heat.

The boiler is in the basement and only feeding baseboards. No hydronic coil up at the air handling unit. I only have the evaporating coil that comes in the unit to work with the heat pump. I’m not sure what kind of boiler it is.

Is this possible with any 1 cool 2 heat thermostats?

Separate wires are run already and I do have the right amount of conductors to switch to a heat pump instead of straight AC.

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  • Will the boiler feed a hydronic fan coil in the new air handler? If so, that shouldn't be a problem. You might want to consider the AUX HEAT connection on the tstat rather than EM heat for your boiler. That would make the process completely automagic (pun intended). One thermostat could easily accommodate your needs. Apr 11 at 22:36
  • Yes -- is your boiler feeding a hydronic fan coil air handler (with the air conditioning evaporator coil in it as well), or some other system, such as baseboard or radiant heat? What make and model is your boiler for that matter? Apr 12 at 0:19
  • The boiler is in the basement and only feeding baseboard. No hydronic coil up at the air handling unit. I only have the evaporating coil that comes in the unit to work with the heat pump. I’m not sure what kind of boiler it is. I won’t be back to that building today either to look.
    – jt2626
    Apr 12 at 10:01
  • Can you do it yes you use the output that controls the contactor to pull in the electric emergency heat to turn your boiler this is fairly common in some areas. Do you have enough wires? It depends on the system and controller you choose then how many wires you have.
    – Ed Beal
    Apr 13 at 21:48
  • "I won’t be back to that building today either to look" So this isn't your home AC/heating system?
    – JACK
    Apr 13 at 23:33
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I'm currently running such a setup with an Aprilaire 8620W thermostat. AC from the heat pump, Heat from the heat pump when outside temp is >=40F, Emergency Heat from the boiler kicks in when outside temp is <40F.

The thermostat initially installed was a Honeywell model, and that could be configured to support AC, or could be configured to support Heat, but could not be configured to support both. It took about 18 months for the contractor to figure that out (because it only became an issue at season cutovers, e.g. spring and fall; they'd fix it to whatever season was appropriate and leave without realizing the other season had been broken). I believe the model was VisionPRO 8000 Wifi.

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This is quite definitely possible, and actually a relatively good setup

What you describe (a heat pump with hydronic backup heat) is quite supportable by most thermostats that can support a two-transformer, single-stage heat pump system. Rc, C, Y, G, and O/B from the thermostat are brought to the air handler, with C, Y, and O/B continuing to the heat pump outdoor unit, while Rh and W head off to the thermostat terminals on a zone valve, zone controller, or circulator relay to control the boiler side of the system, depending on how that side of the system functions.

You'll want to make sure your air handler has an internal fan/blower relay, though, as you'll need to configure your thermostat for "fossil" or "oil/gas" blower operation unless you want the air handler fan to run constantly when the system is in emergency heat mode. Speaking of emergency heat, you are free to let the thermostat use the boiler as a 2nd heating stage to the heatpump. This is unlike normal "dual fuel" operation with a gas furnace, where the heatpump and the furnace can't run at the same time to avoid damage to the heatpump.

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