Heat pumps - do the heat pump and heat strips run simultaneously?

[joecalvin]

Hello all, I had an employee point out this code reference:
220.82 (C) 3 - 100 percent of the nameplate rating(s) of the heat pump compressor and 65 percent of the supplemental electric heating for central electric space-heating systems. If the heat pump compressor is prevented from operating at the same time as the supplementary heat, it does not need to be added to the supplementary heat for the total central space-heating load.

I have always calculated the load for a heat pump system based on the heat strips only. Should I be including both the heat pump motor and electric heat strips in the load calculation?

Thanks
Joe

 

[ptonsparky]

Both of mine run when it's cold enough. Water source.

 

[Ken_S]

I guess would depend on how the system is setup. Some will only run the electrical heat when in defrost or emergency heat. When a system is in defrost both would be on.

 

[suemarkp]

Emergency heat is for when the compressor fails, so that is heat strips only. Aux heat is for when the compressor can't keep up and more heat is needed, so they both run in that case. Defrost they also both run. The installer can usually disable any of those, especially in a southern climate, so there is no definitive answer. You need to talk to the installer or assume they all can be on at the same time.

 

[augie47]

I think Post #4 has the best advice:
You need to talk to the installer or assume they all can be on at the same time.

Over the years I have seen setups where the compressor and heat strips never ran simultaneously, compressor plus 1 heat strip, compressor plus 2 heat strips, etc.

 

[Dennis Alwon]

It is dependent on how the installer hooks up the thermostat. They can be wired to come on together or not. Around here they usually set it up to come on together but only when the temp is low- not sure- but around 30° or so. Basically, if the heat pump can't keep up then the heat strips will kick in to help.

 

[drcampbell]

I guess would depend on how the system is setup. ...

99% agreement: It will depend on how the system is set up, no guesswork involved.

You should be fine following the guideline in 220.82 (C) 3, especially in Texas.

In the South, the compressor will be sized for the peak summer cooling load. Not just the ΔT, but also the humidity and internal heat load. (warm bodies, lights and appliances) When reversed, it will be big enough to heat the house by itself until the outdoor temperature is well below freezing.

When it's moderately cold, the resistance heat will cycle on & off, not operate continuously. And when it's severely cold, the compressor efficiency will drop to the point where the compressor should be shut off.

But there's always the possibility that it has been configured by someone who doesn't really understand what they're doing.

All of which is largely academic. Unless Austin has its own independent not-for-profit publicly-owned power system, Texas isn't going to have electricity during severe cold snaps anyway.

 

[drcampbell]

Both of mine run when it's cold enough. Water source.

Yes, water-source is a completely different animal.

Since the heat source/sink remains at a fairly-constant temperature all year 'round, the outdoor air temperature becomes irrelevant and there isn't any low air-temperature threshold at which the compressor shuts down; it runs whenever the thermostat calls for heat.

When the outdoor air temperature drops to the point that the compressor can't provide as much heat as is leaking out of the house, then the resistance heaters will switch on. If they're properly sized, they'll be on 100% of the time on the coldest days of the year, and the circuit ought to be sized for that.

But they're almost always oversized. Resistance heaters are cheap, oversizing them doesn't reduce their efficiency, and HVACR contractors tend to oversize things because they'll surely get callbacks if the house can't stay warm. 220.82 (C) 3 ought to be fine. (remember that the NEC already includes fairly generous margins -- such as the assumption that everything will be in a 40°C ambient -- and this cable will only be energized on the coldest days of the year)