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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:

  1. Move the circuit for the heat pump from the main electrical panel to the generator electrical panel.
  2. Have a 24V doorbell transformer running on a circuit powered by the generator (I already have one as part of the HVAC system.)
  3. 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.
  4. 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?

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  • I wonder if you're over-thinking this. What you're proposing is akin to an "off light" which is lit when power is off. You don't need that complexity. Think of it differently: you need to load-shed when generator power is on. Can't the 24V transformer tap the transfer/interlock panel before any interlocks so the transformer is energized only when generator is running?
    – Harper - Reinstate Monica
    Commented Feb 13, 2022 at 22:18
  • I don't think I have a convenient place to hook into the power from the generator before it reaches the circuits. I'd have to hack the generator transfer switch to get that. In any case relays come in both normally open and normally closed varieties, so it should be possible to do it either way. And I need to relay in any case because the generator doesn't produce 24 volt power.
    – Stephen Ostermiller
    Commented Feb 13, 2022 at 22:33
  • I’m just saying you don’t need a relay at all. Relays don’t change 120/240V to 24V, transformers do. If you are stuck with one of those Reliance style 6/8/10 circuit transfer switches with an inlet on the switch, they may not give you access. Beware,it’s a code violation to mount one indoors, that invites cords through doorways, and that kills whole families from CO entry.
    – Harper - Reinstate Monica
    Commented Feb 14, 2022 at 0:55
  • Have you looked at programmability of the air handler? Maybe it can ignore the communications error if programmed by pins or settings? I
    – jay613
    Commented Feb 14, 2022 at 5:13
  • @jay613 That would be nice, but I don't think it is possible. The service manual doesn't say anything useful that I can find. My contractor had asked Trane about this issue and Trane said that "they needed to run 24V AC to the heat pumps" to solve this issue. They took that to mean that the 24V AC would power the circuit board of the air handler, but the manual for it says load shed.
    – Stephen Ostermiller
    Commented Feb 14, 2022 at 9:42

1 Answer 1

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This is fairly practical, although you'll want a different relay

According to the service manual for the current Trane variable speed heat pumps (XV20i), you should be able to open the connection between terminals T3 and T4 on the outdoor unit's control board to put it in load shed, once you turn the load shedding function on in the options menu, of course.

You'll want to use a standard 24VAC HVAC fan relay for this (made by Honeywell, White-Rodgers, or the like, and available at a hardware store) instead of some random piece of junk from Amazon (which is absolutely the wrong place to order such things from). You can then run its coil power back to a doorbell/HVAC transformer run off of utility (not backed up) power, which also lets you use thermostat wire for this run.

If you don't have a convenient transformer, or would rather run 120VAC wiring out to the outdoor unit, you can use an Air Products & Controls PAM-1 relay, with standard 120V utility mains running from a convenient utility-only supply (it can be on a general branch circuit) out to the air conditioner. Note that either way, you'll need some crimp-on female tab terminals to fit the male tabs on the control board.

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  • I was trying to avoid introducing a power gremlin (extra 24v transformer) by using a 120V relay rather than a 24V relay. I appreciate that the relay I chose my be crap, but there should be high quality 120V relays available that would be suitable. Maybe something like this one? I wouldn't use the double pole functionality. It says it is double throw which I think means it supports both "normally open" and "normally closed" configurations.
    – Stephen Ostermiller
    Commented Feb 14, 2022 at 9:52
  • Also, I had been looking for a solid-state relay because magnetic relays with moving parts are prone to wear out. I was thinking that in this application, the first indication that the relay failed would be that your generator got overloaded during a power outage. A longer lasting solid state relay might prevent that from happening.
    – Stephen Ostermiller
    Commented Feb 14, 2022 at 10:15
  • @StephenOstermiller -- the Siemens/Furnas relay is suitable if you want something with a 24VAC coil. SSRs aren't really suited for this sort of low-power switching, though
    – ThreePhaseEel
    Commented Feb 14, 2022 at 12:42

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