Have heat pump - No outside thermometer

[NoSpam]

I have a heat pump and no outside HVAC. We are having below freezing temps all day long. Should I turn on my emergency heat during this very cold (for us) weather?

Thanks!

[jamesgangnc]

Not unless the unit is unable to keep up. Otherwise let it do it's thing. Current heat pumps are still effective below 32f. They heat the air prior to it's entering the resistive heat stage so even if the aux strips kick in your are still getting something from the heat pump.

[NoSpam]

Thanks for your reply.

According to what I have been reading, heat pumps do not work well below 40 degrees, but my own observation of my 3 year old system seems to support what you said.

At some point the heat pump needs to spend so much time/energy de-icing the coils that it its efficiency drops below the aux heat. At what temp do you think the heat pump should no longer be used?

I don't know what that point is and it probably depends on the outside humidity. If the outside humidity was zero (admittedly not realistic) then ice would never form. I think I'll carefully watch how the heat pump performs vs outside temp. to see if it can maintain the house temp without too much aux heat.

The reason I mentioned turning on the emergency heat is because that turns off the heat pump which is what the outdoor thermometers are designed to do.

[Pete C.]

The newer heat pumps have adequate sensing and protection in the HP controls (as opposed to the thermostat) and the HP will call for AUX heat itself when it goes into a de-ice cycle or if it decides it can't work effectively.

[Edward Reid]

What you read is incorrect. Yes, there is a point at which the efficiency of the heat pump is less than that of the resistance heat, but that point is roughly 15F, not 40F. Newer, more efficient heat pumps might lower this point a little, but the problem is basic physics, so it won't get much lower.

And as already pointed out, the heat pump might switch to the resistance heat automatically. This likely depends on age and manufacturer. Also note that most heat pumps will add the resistance heat when the inside temperature drops about 5F below the set temperature, but this is separate from the efficiency cutover. This auto-add at 5F below is the reason you are always advised not to set the thermostat down (very much anyway) at night when you have a heat pump. If you disable the resistance heat (which is a viable course of action in the southern US), then you can ignore that advice and save some money, albeit at the tradeoff that it may take quite a while to bring the temperature back up.

Edward

[dpb]

Edward Reid wrote: ...

One alternate ploy rather than disabling them entirely is what the installer of our system did routinely -- he used a thermister inline w/ the emergency heat that kept them out of the control circuit w/ outside temperatures were above about 20-25F.

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[krw]

Ours throws in the resistance (aux) heat with only a 2F hill to climb; dumb. With our house in VT (hydronic system), we'd set-back the thermostat about 6F (65F down to 59F) at night and weekdays. Here we have to keep it set at

66-67F (no set-back capability and the forced-cold-air system) to be anywhere close to comfortable.

[krw]

Neat idea. The problem I see with this is that there is something else to go wrong and there is no way to know it's gone wrong until the worst possible time.

[dpb]

Trivial to bypass if want/need to but it's a passive solution and a device that has extremely long MTBF ratings...

(Which leads to a story of when sold the house, however; the buyer's inspection wrote up a defect that the emergency heat was non-functional; the buyer ended up paying to have the good idea removed 'cuz couldn't fathom the benefit even when explained to him and I refused to pay to "fix" something that wasn't broke but was a beneficial feature.)

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[dpb]

[e-mail] wrote: ...

That was (mayhaps still is?) pretty standard ordinary thermostat setting/logic.

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[krw]

There is *nothing* in a house that has extremely long MTBF, ratings notwithstanding. In some areas auxiliary heat may only come on once or twice a year. The owner may not even be home; frozen pipes.

Buyers are fickle. Buying a house is a very emotional time and anything out of the ordinary can cause strange reactions and some not so strange (e.g. not to believing the seller or agents).

[dpb]

[e-mail] wrote: ...

Nonsense. Thermistors are extremely reliable devices as are virtually all passive devices.

The likelihood of the thermistor being the failure point is multiple orders of magnitude lower than that of any of the other components it's associated with, including the power grid itself.

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[krw]

Nonsense yourself. IT's not about the device. More such stuff is damaged than fails on its own. MTBF is meaningless.

Other items are discovered at times other than the worst possible. This is a bad design.