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The traditional North American split system heat pump (ignoring "dual fuel" systems for now) relies on a backup heat source to provide both makeup heat during low-temperature (below the system balance point) operation and, more importantly for this, outdoor unit defrost cycles. This backup heat is typically supplied by heat strips, and generously sized ones at that, considering that during defrost, the backup heat must carry the entire house heating load.

However, as documented in this Mitsubishi application note and this government research paper (as well as this research paper here), mini-split heat pumps stop or radically slow their indoor fan when defrosting to avoid filling the room with cold air, since they do not have a backup heat source to run during defrost cycles. This has the downside that they are not actively heating during defrost, but even without that, they are able to provide heating in most climactic conditions using this defrost strategy and sensor-based defrost cycle triggering without aid from a standby heat source.

Is there a reason that one could not use a couple extra wires in the outdoor unit cable and an inexpensive fan relay to retrofit a split system heat pump with the ability to perform a "fan stop" defrost cycle instead of turning the standby (backup, emergency) heat on? This seems like an obvious target for energy savings, given that emergency heat is far more power-hungry than heat-pump heat. Furthermore, even without any energy-efficiency benefits, this would likely allow the backup heat to be significantly downsized in climates where it's not heavily relied upon, as now it only has to be sized to make up the heat pump's capacity deficit at the heating design (99%) outdoor air temperature, not the entire house heating load. This can reduce electrical service sizes, and also position the system to be more easily handled by a backup generator.

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  • Wait, if we're defrosting the outside unit by reversing and pumping heat from inside to outside... What happens when 30 seconds have gone by, the inside coil has turned the coil box into a freezer, and there's no more heat to be had inside?
    – Harper - Reinstate Monica
    Commented Aug 21, 2019 at 3:35
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    Don’t forget the compressor is always hot, using that heat for modern splits is how they defrost not with e-heat.
    – Ed Beal
    Commented Aug 21, 2019 at 13:27

2 Answers 2

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During the defrost cycle, inside heat is not being pushed to the outside as you stated "the parasite heat load of heat that is being pushed from inside to outside to defrost the coils themselves". The inside fan has to run to insure that the liquid refrigerant entering the indoor coil is vaporized so that liquid refrigerant does not flow back to the compressor. The outdoor coil is defrosted by the "heat of compression". The temperature of the outdoor coil is elevated by the compressor pumping the refrigerant vapor to a very high pressure which produces a very hot gas to defrost the coil. Since the outdoor fan is turned off during the defrost cycle, the refrigerant does not condense and the coil pressure rises as does the coil temperature. The colder the outside temperature the need for defrost cycles is reduced due to less water vapor (humidity) being on the air. Less humidity results in less water vapor to coat the outdoor coil. The back-up heat is turned on to temper the now cold air that is being generated by the defrost cycle.-------- Add-on to answer; actually @ThreePhaseEel my explanation does answer your question; you asked if you could add a relay to shut off the indoor fan. You can , but do you want to gamble on slugging liquid into the compressor which may screw up the compressor. The indoor fan runs because it should. The heat strips turn on to keep the discharge air at a reasonable temperature. They do not have to run for the defrost to work.

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  • Thanks for the explanation of where the heat comes from during defrost, although this doesn't directly address my question...
    – ThreePhaseEel
    Commented Aug 21, 2019 at 11:46
  • In the last few split systems I have installed there is NO emergency heat at all. a small crank case heater on the compressor is all that is there this is to keep any refrigerant from changing to a liquid and cause slugging of the compressor, it may not provide the exact answer but is how modern units work ( some in the single digits) without emergency heat at all. I think it is a great answer. Since the article mentions mini splits and many do not have emergency heat available. +
    – Ed Beal
    Commented Aug 21, 2019 at 13:23
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The air handler fan is required during defrost to prevent the evaporator coils from freezing up. The heat pump at my first house didn't do this, and an HVAC tech installed a retrofit circuit board that turned the fan on during defrost.

Turning on the heat strips is purely a comfort option. The heat pump I have now only turns on one third of the heat strips during defrost. You could wire in an additional thermostat and a relay to prevent the heat strips from coming on until the temperature in the house drops too low.

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  • If what you say about the AHU fan needing to run in defrost to prevent the indoor coil from freezing is true, then how do mini-splits get away without it?
    – ThreePhaseEel
    Commented Aug 21, 2019 at 22:20
  • I never installed or serviced a mini-split so I do not have an answer for the operation of those devices.
    – d.george
    Commented Aug 21, 2019 at 22:53

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