I electrified my heating system by replacing my air conditioning condenser with a heat pump. I hired a contractor to install a Trane variable speed heat pump and matching air handler. The air handler is also connected to the old hydronic heat from the oil boiler for when its too cold for the heat-pump or for when there is so much heat demand that the heat-pump needs a boost. In general, this new system is working fine.
I was expecting this system to function during power outages without the heat pump. My generator is not large enough to power the heat pump so it on a circuit that is not powered by the generator. However, the air handler and boiler are on a circuits that are powered by the generator. This setup is how the old air conditioning condenser and old air handler with hydronics were configured and it worked fine for that system.
Unfortunately, the new air handler freaks out when the heat-pump loses power. It shows an error code ("communication error") and refuses to operate at all!
Reading the installation manual for the heat-pump, there is a solution. The heat pump supports "load shedding." There are two terminals on its circuit board through which I can pass 24V AC power which would tell it not to operate. To make this work, I think I would need to:
- Move the circuit for the heat pump from the main electrical panel to the generator electrical panel.
- Have a 24V doorbell transformer running on a circuit powered by the generator (I already have one as part of the HVAC system.)
- Create a circuit off the main panel (not powered by the generator) connected to the input of a relay (like this one). The output of the relay would allow the 24 volt power from the doorbell transformer to pass through only when the electric mains are off (ie. 24V connected to the "normally open" circuit of the relay). I'd plan to put this in a new 2-gang electrical box next to the main panel.
- Run thermostat wire to between the doorbell transformer, relay, and heat pump in a circuit that uses the doorbell transformer as the power source, the relay as the switch, and the heat-pump load shedding terminals as the load.
This would make it so that the heat pump is always powered (even in a power outage when the generator is on) and able to communicate with the air handler. However, the heat pump would know that it is not allowed to operate its motors and pumps so that it wouldn't put too much stress on the generator.
There is also the "slight" wrinkle that my generator circuit panel is full, and I would need to install a 100 amp sub-panel following the instructions from this video.
I'm planning to do all this work myself. I'm comfortable adding circuits to panels and I live in New Hampshire where it is legal for homeowners to do their own electrical work. Is my plan reasonable?