What size home? What size heat strip?

No, I wouldn't lock it out.

Now below 10 degrees probably OK to lock out.

IMO

I am new to a heat pump and am having issues myself but let me tell you my experience. Last year I had two different HVAC guys tell me that my heat pump was doing nothing but wasting power once temps got down around 20 degrees. I let mine run and one night we were forecasted at 20 below with wind chill temps between 40-50 below. I put my system in emergency heat, set the stat at 65 degrees and went to bed. When I got up the house was at around 58 degrees and the system ran all night. I called another guy who is an HVAC installer and he told me that 20-30 years ago heat pumps would not work in the cold weather but units designed today can generate BTU's when temps are below zero. I turned my outside unit back on and within a couple hours my house was back up to 65. Granted my emergency heat is under powered but that said my heat pump was still able to generate some heat. He told me I should never shut my outside unit off unless it is having mechanical problems or failure.

What flyin-lowe said.

The heat pump compressor is designed to run almost constantly, except for defrosting. Heat strips are designed to supplement the output of the heat pump. Let the system do the work and don't lock out the heat pump unless you want to waste your money on a higher electric bill.

In fact, many systems allow you to lock out the strips down to very low temperatures. Our strips are locked out to 5F because I'm green/efficiency minded/cheap and this works in our home to keep the desired temperatures. Our strips are also set to stay off during the defrost cycle, which happens occasionally and usually lasts 3-5 minutes. This is not really a big deal unless you stand on a register. I have a setting on our system to lower the regular compressor on air speed of the fan coil for comfort. I set the low speed fan to "on" for comfort and better air distribution when the compressor is off. Comfort versus efficiency, experiment with the settings and make your choices. It varies a lot by home.

It depends on weather you have backup propane or not.

There are physical laws governing the maximum heat that can be extracted from exterior air. This is why the real top of the line heat pump-only systems (in cold climates) are all geothermal. My installer - who was terrible - set the propane backup xover on my Carrier supposedly high end Infinity system to 44F (!) and said I'd be crazy to set it lower. The following winter I set it to 22F IIRC, because at the time I thought propane was a hassle to deal with. I only keep my house at 66 to 68F. (well, cold to some people - warm by New England standards) It seemed to keep up okay...but guess what? Running all the time at night and morning when the temp is down in the twenties cranks your electric bill way up, no matter how efficient the unit. My house has insulation problems but still, an eletric bill over $700 for a 2000 sqt ft house seemed crazy. The next winter I set it at 35F. The propane backup (which is of course cheaper per btu than electrical) takes over at night in the typical weather here in eastern MD. That seems like a good compromise to me. I suspect the higher you keep your inside temps the more it make sense just to burn the propane. The colder it gets too, the more propane it uses anyhow because it seems to need more defrost cycles.

This post was edited by davidrt28 on Mon, Jan 12, 15 at 11:28

It sounds to me like something is up with your system. My house is almost 3500 sq feet and my unit is undersized and will run all the time like you describe. Last year I had a couple bills over $400.00 and that is with temps below zero. We had spans in Indiana last year were it was below zero for over a week at a time with wind chill temps 30 below. My unit would run for days at a time without shutting off. I am all electric with electric strip heat for backup. $700 seem crazy high, especially since your unit was going to propane at 22f.

Well I did find the local utility tended to have higher than national average kw/h charges, so I switched last year to another one and that probably saved me 10% or so. Also I have some insulation issues I've been bad about dealing with.

Finally, I had put in a radon removal fan myself just before the HP install, because of the difficulty in locating a local contractor who had any interest in the issue, and it turns out it was way overpowered. Because of the unusual design of my house, it was probably sucking air mostly out of the conditioned space of the house. (You are supposed to close slab openings but a couple were impossible...directly under a bathtub for example.) What I did the year after the high bills was to reduce the wattage of it by 2/3 to slow the fan down. The radon meter showed almost the same level of reduction, from 9.8 pc/l to about 0.8 pc/l instead of .5 pc/l. So that was another problem.

I live in south west Pa temps have been hovering @ 20 during the day and 10 @ night wind chill has taken temps down to 0 degrees. any way I am straight electric. My new system installed in October 2014 has a 20 kw backup (aux heat) Some have said it is to much. That being said I quit locking out the Hp and it might run longer but it does shut off, so I need to learn how it works and most of all most efficient. Bottom line is when would I need to lock out HP and use just Aux heat...right now I am letting the thermostat take care of things, I was worried that running HP was not efficient at these temps.

I would think any modern unit does a reasonable job calculating that for you, and "knows" the best point to start using the coils. I think the reason propane backup systems give a crossover point adjustment is because unlike electric backup - well, mine at least works this way - the two do not run at the same time. While, when I had a electric backup in my former house, I seem to recall "aux heat" would come on when the heat pump was still running, while "emergency heat" meant you were only getting heat from the coils.
Thus with a propane backup the crossover adjustment allows you to take into account the relative costs of energy sources. If you had your own hydro-dam or something (LOL) and electricity was practically free, you could set the crossover very low. With electric backup that doesn't come into play...at worse, the heat pump is only as inefficent as resistance. (I'm actually not sure if that's the physical laws of the universe at play, or just how a more advanced unit would manage its own crossover. Mostly the former, I think, at least until very low temps around 0F) At best of course the heat pump is far more efficent than resistance...that's why people use them.

So...short answer, don't worry about it.

I think davidrt28 kind of confused some people as I do not have propane, I am electric all electric. He kind of jumped on my thread, shouldn't do that., but thanks to everyone putting their thoughts out there. It reassured me that I should let the therm stat do it's job

some HP condensers actually have a safety cutoff at low temps and will not come back on until outdoor temps increase. Of course if you ave outdoor temp sensor, you can lock out the condenser yourself at a preset temperature. You will get conflicting opinions about this but I would go all emergency heat around 10 degrees fah.

IMO