Heat Pump back up heating

[Mike Tao]

Dear All:

I am a little confused now.
I remembered that a heat pump is not working efficiently when exterior temperature is below 38-40F since the evaporator can not withdraw heat from exterior. Then when we inspect the system in a cold day below 38 F, the system is running using the back up heating. So whether 2 degree or 3-4 degree increment, we are testing the backup heating. Is that correct?


Thanks

[Dom D'Agostino]

It really depends on the system and t-stat. Some will kick the supplemental/emergency heat (gas, electric, oil, hydronic) on once you raise the desired temp up more than 2 degrees. Most will indicate this is happening, but there are other ways to tell.

And there are newer heat pumps that can work in temps down to 0 degrees, I believe.

Dom.

[Mike Tao]

Hey, Dom:

Thanks for your info. I guess my question is if the exterior temperature is below 38F and the house is using old generation heat pump for heating, then even raise 1 degree, the system will run the back up heating?

Just curious, what refrigerant in the newer heat pumps if you can remember?

Thanks

Mike


It really depends on the system and t-stat. Some will kick the supplemental/emergency heat (gas, electric, oil, hydronic) on once you raise the desired temp up more than 2 degrees. Most will indicate this is happening, but there are other ways to tell.

And there are newer heat pumps that can work in temps down to 0 degrees, I believe.

Dom.

[Jack Feldmann]

I have a new multi stage, multi speed heat pump, and it runs in heat pump mode in very low temps (in the 20's). We just finished our first year and when the tech came out for service, he could tell me just how many times the heat strips came on, and which stage of heat strips.
I don't remember all of the stats, but the 3rd stage heat strips only came on 4 times in the past 12 months, and we had a very cold winter of 2014, as well as the end of 2014 and start of 2015. They projected I would have an $800 savings over old system. It ended up at $1200 for the year.

Most heat pumps ought to be able to run at 38 degrees. You can always tell if the heat strips are on with a simple electrical tester.

[Mark Reinmiller]

Heat pumps loose efficiency at low temperatures, but still produce some heat. If the backup is electric resistance than the heat pump usually continues to run down to single digit temperatures or lower. If you raise the T-stat a couple degrees you should be bringing on the backup heat. I generally do that, but to just test the electric resistance heat I put the T-stat in the emergency heat mode. Then the HP turns off and you can tell for sure where the heat is coming from.

[John Kogel]

Yes if you aren't sure, go out and see if the outdoor unit is working. In emergency mode, the fan will be off outside, no? I would do that before screwing around with 240 volts and a meter.

If the unit is older, they should have it checked out anyway. Refrigerant leaks out of the system over time and that leads to bigger trouble like a knocking compressor.