Why You Shouldn’t Use Heat Pumps in the Northeast US

Frosted heat pump
Frosted heat pump

Heat pumps behave like bidirectional refrigerators: they cool the building by heating the outside air or heat the building by cooling the outside air. In relatively mild, dry weather, this works perfectly.

Here in the Northeast US, it’s not such a bright idea. For about half the year, the ambient temperature is low enough and the humidity high enough that pumping heat out of the exchanger drops its temperature below the dew point, whereupon ambient moisture condenses on the fins and, given the temperature differential between ambient and coil, freezes solid.

In that situation, the efficiency of the heat exchanger drops well below zero: it turns on electric resistance heating bars to warm the inside air and runs a defrost cycle on the exterior heat exchanger.

Lather, rinse, repeat.

Perhaps the defrost cycle hadn’t started yet?

8 thoughts on “Why You Shouldn’t Use Heat Pumps in the Northeast US

  1. A friend built a heat pump out of a hacked air conditioner, and added a temp and humidity sensor (LM34, and HIH4000, if I recall correctly) that try to predict when it’s getting in trouble and turn it off. That is probably more effective when you live somewhere with 25% humidity most of the time, though.
    So do you think the heat pump is overall a good idea? I know they’re supposed to be markedly more efficient when they’re working than forced air.

    1. heat pump is overall a good idea

      The only heat pumps that seem to make sense north of the frost line use geothermal transfer: pump heat into or out of groundwater or earth. Anything involving air simply frosts over; you wind up using electric resistance heating far too often.

      Although geothermal can be done poorly. I knew a guy who had a once-through system that pumped water from a well, ran it through the exchanger, and dumped it down the drain. That just can’t be a good idea, no matter how it’s dressed up.

  2. Just defrost more. There should be a jumper on the defrost control board to set the amount of time it can run between defrosts– 30, 60, or 90 mins.

    A heat pump is still great in the north, you have months of weather between 30-70 deg F, and can save a lot of money then.

    You don’t have to use it with electric resistance heat. I have a ‘dual fuel’ system with a heat pump on a 93% efficient gas furnace. The honeywell tstat cuts out the heatpump at 30deg and runs the furnace. The furnace also kicks on during defrost cycle.

    Also it is a good idea to calculate the cost of electric resistance vs gas heat. The price of gas has gone up so much that in many areas it is no longer cheaper.

    1. a jumper on the defrost control board

      This is outside the local DMV / County Clerk office in a rent-a-building that’s obviously maintained to the lowest level consistent with not having the doors fall off; nuances like choosing the appropriate defrost time are well above anybody’s pay code. For example, you can just barely make out the turnbuckle attached to two coathangers (?) holding the top plate in place…

      The building is surrounded by heat exchangers, most in similar states of decrepitude, with insulated tubing coming directly through the wall panels in blobs of yellowing urethane foam. A single, much larger, and far more efficient unit for the entire building would have made more environmental sense, but that seems not to have been part of the design. Or, perhaps, there’s a dead unit on the roof and these clunkers represent the fallback plan.

      From what I’ve read, the current crop of heat pumps can figure out when they’re frosted over and act accordingly: jumpers are a thing of the past. Maybe that makes enough sense to justify them, but I’m not at all convinced.

      And, at $0.15/kWh and climbing, anything’s got to be cheaper than electric heating!

  3. A standard efficiency heat pump operating at 10 degrees F has the same cost per BTU as natural gas. At 32 degrees a heat pump is 30% more efficient than a 96% efficient natural gas furnace. Pull out your manual J and figure the heating hours at 32 compared to heating hours at 10 degrees. I know some of you are more creative than your posts appear. You make it look as though it’s an either or proposition with heat pumps

    1. Remember those cold, humid winter days that exacerbate ice buildup on the condenser coils; the DOE points out that standard heat pumps use resistive electric heaters below about 40 F, which isn’t particularly efficient.

      Around here, the average low temperature is under 40 F for more than half the year.

      Newer heat pumps should be significantly better than that old clunker, but a heater that spends time defrosting itself won’t be a net win compared to, say, plain old natural gas. If you have a gas line passing your house, that is, which isn’t true for most housing around here.

  4. I love that we live in a world where 100% “isn’t particularly efficient.” :)

    1. 100% “isn’t particularly efficient.” :)

      As Eks put it: “If you want heat, burn something!”

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