Heat Pump Usage Questions

anon1111December 10, 2009

Hello, I am pretty new to heat pumps, and I have one with electric heat strips as auxiliary heat. Yesterday, when away for work, I set the temp down to 62, when I got home I turned it up to 68. During that time my aux. heat & heat pump came on and brought the temperature back up to about 65-66 at which point the auxiliary heat turned off. For the next 4-5 hours the heat pump was running solely & nonstop, but only able to raise the temperature a degree or two, never reaching the 68 before I went to bed, which I then turned the thermostat back down. During all this time, it was about 23 degrees outside. I thought it was inefficient to run the heat pump at this cold of a temperature, even more worse than the heat strips. So I am wondering this: Is this normal? Should a heat pump still be running at this outside temperature and lower? I mean it gets below 0 degrees here some days, should the heat pump be running at all at that temperature?

I called a local hvac company and they said it's ok, but I want a 2nd opinion.

Thank you.

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I just got a heat pump 2 months ago to work along with my gas furnace. I've been doing alot of experimentation with it regarding how the heat feels whether its warm out (in the 40's) compared to when its cold out (in the 20's). I've also experimented with setting the thermostat back and trying to go back up with just the heat pump. Bottom line is the heat pump can do a good job maintaining a termperature (even in the 20's), but it will run alot. That is ok. Its relatively inexpensive to run the heat pump compared to heat strips and fossil fuels. It took me 4 hours or so to go from 60 to 66 degrees with outdoor temps of 35 degrees. Just keep the temperature inside at a set number. Do you know the seer and hspf of your heat pump? I just got a 15 seer model, and I can run it alot and it doesn't add a ton of money to the electric bill.

    Bookmark   December 10, 2009 at 3:21PM
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heat pumps are designed to hold a constant temp. lowering the thermostat at night is not recommended.
unless they are set up shut off at a set temp most heat pump outside units will run all the time when the temp is cold and the heat strips will kick in ocassionally to help the unit keep up. at 0 the heat strips may be on most of the time helping the outside unit and the outside unit will be contributing very little heat. it does not hurt the unit to run when the temp gets below the balance point. some people will set up the unit so the outside unit will not kick in at a set cold temp in an effort to save unit runtime but i doubt it saves much energy. you can also do this yourseld by switching the unit over to emergency heat when the temp gets to zero. that will lock out the compressor and your heat will be only from the aux heat strips.

    Bookmark   December 10, 2009 at 3:29PM
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you will get different opinions on the condition you described. these are mine.

yes, I would say your HP is operating normally. However, I do not like a setback more than 3-4 degrees particularly when outdoor temp is in the low to mid 20s. A six degree setback is a poor cost saving strategy. The idea that the HP can not bring the temp back up to usual thermostat setting after several hours and is running continuously suggests not enough BTUs are being produced. Do you have a lockout on the aux(heat strip) at a certain temp?

Keep in mind a heat pump condenser by itself does produce heating BTUs at low temps and the aux heat(heat strips) are necessary to supplement the heat pump with additional BTUs. Each 5KW heat strip produces about 17 KBTUs in additional heat. Heat pumps can operate at 0 degrees and provide a comfortable home with the help of the heat strips. Any idea what size your HP condenser is and size of your heat strip?

hope this helps.


    Bookmark   December 10, 2009 at 3:36PM
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Thanks for the answers, I just wanted to make sure mine was working like it should, which it appears to. It seems it's not such a good idea to turn the thermostat down (with a heat pump & aux. heat) at night which is contrary to how ENERGY STAR programmable thermostats are setup. Interesting.

    Bookmark   December 10, 2009 at 4:44PM
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I've had a heat pump for 8 years. I don't do setbacks so much any more for the last couple years as I primarily work from home now, but previous to that, I did a 9°F or 10°F setback for 12 to 14 hrs (11:30 AM to usually 1:30 AM) on Mon - Fri. I have the auxiliary locked-out until the ambient temp is below 25°F (it still runs during defrost cycles). Being near Houston, there's not so much freezing weather, but in the few cases when there was, the system ran continuously for up to 6+ hrs to recover. I never considered that a problem. It produced airflow that was warmer than the room temperature (EVEN with the auxiliary locked-out), so the indoor temp kept rising, albeit slowly.

We had a record-making/breaking snowfall on 12/4. Snowed pretty much the whole day, below freezing through the night, but bottomed-out at 26°F so was still above my lock-out point. What with the precipitation, the outdoor unit was icy for pretty much the whole day and likely triggered defrost every time the control board checked for it. But ... it STILL kept the house warm, DID actually cycle-off a few times, and 20 mins or so of auxiliary during defrost every 90 mins (or whatever is the defrost check-period) is better IMO than running solely on auxiliary.

Anon1111, there are thermostats specific for heat pumps (which yours may be) that have a smart-recovery feature. When programmed for an automatic setback/recovery, they "learn" the operating characteristics of the system over time and start the recovery a little early, automatically raising the temp slowly to avoid triggering the auxiliary. Note that manually raising the temp does not invoke smart-recovery, so the auxiliary will come on in that case ... unless you have the lock-out option installed.

    Bookmark   December 11, 2009 at 4:03AM
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You have to realize that heat pumps lose capacity as ambient temps decrease.

At 55F, a 3-ton heat pump may put out 39,000 btu.

At 25F, that 3 ton heat pump is down to 25,000 btu.

That's a 36% drop in heating capacity.

So a 5 degree set back may work at 55F ambient because the heat pump has the heating capacity to recover. But at 25F, there may not even be enough capacity to keep up let alone recover.

I agree with Dadoes: you should have a t-stat that has Adaptive Intelleigence like the Honeywell VisionPro 8000 series. With AI enabled, the t-stat will try to recover from setback without using aux heat (or as minimal as possible). The heat pump may start to run hours before the setpoint time to reach setpoint temp.

Take care.

    Bookmark   December 11, 2009 at 10:05PM
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I vote for 3 degrees of setback and locking out the auxillary heat at 35. If something happened and your not home and the heat pump has a problem nothing will freeze up. I know my HP can supply the house with no problem below 35 but I also know how my luck goes.
I haven't set my heat pump lockout yet but think I will eventually lock it out at 10-15 degrees after checking register temps and amperage next time it gets close to that temp again.

    Bookmark   December 11, 2009 at 10:52PM
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