Heat Pumps and Electric Heating - heat pump question

does the size of the heat pump determine when the aux. will kick on in cold weather? I have heard differant answers as to what temp the heat pump is effective. A co-worker had new ac installed this summer, so he went with a heat pump for heat in milder weather, he said his gas furnace is set to back up when outside temp gets below 40.
My heat pump is just for upstairs, 3 bedrooms 2 bath, geothermal for downstairs.
When it was very cold, in the teens and low 20's, I was told I should switch over to emergancy heat because it was too cold for the outside unit to do any good?
Shouldn't the system reconize when it is too cold and switch on its own?
And will the outside unit shut off and switch to all elec in very cold weather? Or will the compressor always run?
Thanks for any info.

no way to know w/out knowing everything in your system.

You are asking about what is known as the balance point. This is where the heat pump costs the same as running the aux heat and as stated without specific unit data that will remain a mystery and even with all the info it will depend upon the install that you have. If you have a good install with properly sized duct and a clean well running system the manufacturers recommendations should be followed. On a lot of newer systems the balance point can be down to near zero degrees they have done much better with their defrost controls and efficiency so that makes a big difference. A lot of the older cheaper heat pump systems simply threw the unit into deforst for a set period of time at set periods wasting gobs of energy at the lower temps and even at the higher temps. Post your info and someone may be able to find the manufacturers recommendations for you. A nice honeywell t-stat like the 8000 series can lock out the compressor at selected set temps so you can easily control your balance point cutout the technology has far surpassed what was available only a few years ago. That Geothermal system sounds interesting how is that working out for you?

That Geothermal system sounds interesting how is that working out for you?


The geothermal works great both summer and winter. I have a wood burner I burn, especially when its real cold and the elec. aux kicks on., I wish the previous owners would have installed a bigger one and ran the whole house with it.I am all elec and my monthly budget is $153
The upstairs heat pump in question is a Arcoaire. It was installed in Sept. 2005 just before I bought the house. The model of the outside unit CHP218AKC3, the fan unit in the attic is model EBX2400. Honeywell tstat model T8411R.

I have heard differant answers as to what temp the heat pump is effective. A co-worker had new ac installed this summer, so he went with a heat pump for heat in milder weather, he said his gas furnace is set to back up when outside temp gets below 40.
That is correct but maybe for a different reason. Mine is set to also lock-out the furnace above 40 because the HP may enter defrost cycle at 40 or below and you definitely want to temper the air when that happens.


My heat pump is just for upstairs, 3 bedrooms 2 bath, geothermal for downstairs.
When it was very cold, in the teens and low 20's, I was told I should switch over to emergancy heat because it was too cold for the outside unit to do any good?
Shouldn't the system reconize when it is too cold and switch on its own?
And will the outside unit shut off and switch to all elec in very cold weather? Or will the compressor always run?
Thanks for any info.

If comparing the efficiency of resistive electric heat strips to an HP, the HP's Coefficient of Performance (COP) is greater than 1 down to some low temps, even your model. If you have the COP curve you'll know where that is, but I bet at 0 degrees it may still be above a COP of 1, which is the best those heat strips will be.

As the outdoor to indoor delta gets bigger, your HPs BTU output will go down (COP goes down) and at some point not be able to meet the needed BTUs, at which point your heat strips will kick on. The tstat will know when to do that.

I would not lock out the HP above 0 until you get the COP numbers, and let the tstat decide. Call the manufacturer and ask for the COP numbers for your unit and repost. Then we'll know for sure.

You are asking about what is known as the balance point. This is where the heat pump costs the same as running the aux heat and as stated without specific unit data that will remain a mystery and even with all the info it will depend upon the install that you have.

I like to call this the economic balance point, as opposed to a comfort balance point. Some folks don't like the cooler air coming from their registers and the tstat allowing a temp drop off before engaging the backup/emergency heat.

Cheapskates like me only think of the economic balance point while the wifey only thinks of the comfort balance point. :)

Call the manufacturer and ask for the COP numbers for your unit and repost.

I emailed them and this is all they said in their reply:

"Normally, -10* is where you the efficiency of the outside unit drops
below the efficiency of the electric heaters."

I emailed them and this is all they said in their reply:

"Normally, -10* is where you the efficiency of the outside unit drops
below the efficiency of the electric heaters."

That is true for most models. And since you are comparing electric resistive to electric HP efficiency it is easy to figure. So don't lock out your HP and let it run to the lowest temp possible. In fact there is probably a lockout inside the HPs control module that will keep it from running below that, so just make sure your tstat won't lockup above -10.

If you have a fossil fuel backup furnace (propane/NG or oil) like my setup you need the COP numbers to compare the BTU cost, but in your case you don't need them.