I'd like to think I understand how heat pumps work decently well. My home has one plus electric heat, which I try not to resort to because heat pump is more efficient, if slower.

When a traditional thermostat calls for the heat pump however, it turns on the big fan in the basement at the same time as the heat pump (or perhaps it's the heat pump calling for the fan right away when it's called on to pump heat). This uses electricity and makes nose to just to move cold air while the heat pump builds pressure and starts to heat the coils in the basement.

Couldn't a delay be added, or a thermocouple added to the indoor coils, so that the fan indoors doesn't run until the indoor coils are nice and warm?

For that matter. I would think the outdoor fan could be delayed a bit too to save electricity and reduce noise at the beginning of a cycle while a temperature delta builds up.

I realize that if you ran the heat pump or AC for 'too long'/indefinitely without the indoor fan, it could damage the system. But why don't heat pumps run for 'a bit' BEFORE starting the air circulation fan?

I could understand parts cost of a thermocouple (though that's what a marketing department is for), but just using an delay wouldn't cost any money, and would be pretty easy to add to the firmware, I would expect.

  • All heat pumps are also air conditioners and they use the same exact coils for heat and cool. What happens in summer? The thermocouple will prevent the fan from ever starting, since the coils will always be too cold. That said, I think it's ridiculous that furnaces need proprietary circuit boards that result in a good furnace being scrapped because a faulty board is difficult to source... but since the boards are there, they ought to work out stuff like that. Dec 25, 2017 at 5:39
  • Note that 'how long' a fixed delay should be will depend on both the indoor and outdoor temperatures -- the ideal time in one situation may damage the system in a different one. And conversely a time delay that is always safe may not be long enough to make much of a difference in typical operating circumstances. The thermocouple idea could work, but consider also that a broken wire on that thermocouple would mean the fan would never start, and the system would self destruct. I'm not surprised that the engineers who design these didn't think it worth the trouble.
    – Nate S.
    Dec 18, 2020 at 23:25

3 Answers 3


I don't think the problem you're having would be solved by a fan delay. When the fan turns on, it has to purge all the "cold" air from the ducts, before the warm air reaches the registers. There's not much that can be done to solve that problem, other than having really short duct runs.

If you did add a delay, the system would likely overheat. This could cause a bunch of problems, as well as reducing efficiency.

If you did want to add a delay. It would require a thermocouple and relay, or a time delay relay. I doubt your furnace has any firmware, unless it's a super fancy custom system.

  • Omg. I can't believe i didn't think of the volume of the ducts! How could i go about calculating the delay I should expect that to introduce? My nest thermostat certainly has firmware. Nest has a feature that will run the fan longer after the compressor has stopped, to get the last bit of goodness out of it. I just don't know if the fan runs during heating/ac because net 'calls' for fan or if the fan runs because a relay on the head pump's wire calls for fan. I know during a defrost cycle, the outdoor coil's fan is inhibited for a prescribed amount of time without causing damage. Dec 24, 2017 at 16:32
  • @BillyC. Your NEST is just a really fancy switch, that closes a contact when it gets cold, and a different contact when it gets hot. If you wanted the nest to control this delay, it would require additional circuitry between the NEST and the heat pump.
    – Tester101
    Dec 25, 2017 at 19:15

Many fan coils do have a delay for the fan. A solid state delay and relay could also be added to a fan coil unit. However the air is not actually cold but room temperature except for any air trapped in the ducts which needs to be purged anyway so a delay wouldn’t help. Any little heat being produced by the heat pump will go into the house so the only thing you might be gaining is not hearing the unit for an extra 30 seconds or a cold draft blowing on you if the registers are in a poor location. Any delay would be slightly less efficient and a long delay would cause undue wear to the heat pump.


Another consideration is head pressure. We don't want to under any circumstances to build unnecessary head pressure. Running the compressor at any time without dissipating the resultant heat generated could shorten the life of the compressor.

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