Electric Heat Pump - and turn off or turn down

districtgirrrlDecember 6, 2007

Hello - my husband and I just bought our first home - a 800 sq foot condo with an electric heat pump. We arent' very familiar with how exactly they work.

We had no problems running the AC, however now that it's cooler outside I have some questions for how to run it most effectively.

We have been turning it off for 10-12 hours during the day when we leave for work and during the night. The past few weeks its only taken 10-20 minutes to get the condo warmed up when we turn it back on. Now that the outdoor temperate is much cooler, it's running for 1-1.5 hours straight to bring the inside temperate up from 60-something degress to 70 degrees.

Instead of turning it off and on is it more cost effective to just keep the temperate at a steady 65 degrees or something during the day and at night?

The heat pump has a manual thermostat with AC/Heat/Auxillary (Emergency).

Thanks for your help.

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hiltonheadacguy

Great question and congrats on your first place. Yes, it is more efficient to set it-- leave it-- and let it do its thing. When you can, replace the thermostat with one that has "auto changeover function". That way, it will modulate between heating setting and cooling setting and automatically change between them. There is a 3 degree "dead band" on most models so they dont counter-act each other.

    Bookmark   December 6, 2007 at 1:03PM
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jeffnette

If you have 2 stages of heat like most are, and the second stage is electric strip heaters, you might get an unwelcomed surprise when you get you elctric bill in the mail........Heat pumps should be set at one temperature to be efficient. by shutting it off during the day and letting the temperature drop drastically in the house , when you turn it back on, it instantly goes to the second stage of heat ( electric strips) to catch back up with the set temp.$$$$$ By leaving it set at a normal temp all the time, the heat pump will kick on and off as needed to sustain that temp, which is way more efficient than the strips. If and when it really gets cold and the heat pump can't keep up, the 2nd stage (heat strips) will come on but will not run as long to achieve the set temp.
Hope this helps......

    Bookmark   December 6, 2007 at 1:18PM
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jeffnette

One thing I forgot to say, I would not recomend a setback thermostat for a heat pump setup, just for the reasons stated above.

    Bookmark   December 6, 2007 at 1:21PM
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davidandkasie

in my experience, and this is ONLY with my system, with the correct TSTAT you can set it back 8-10 degrees. my XL14i has yet to kick onthe emergency heat to recover. when we are gone for the day teh tstat shuts it back from 70 to 60. the built in recovery turns it back on around 4 and by the 5:15, the set time for it to be back at 70, we are at set temp. i ran it with no set back on a couple days, and it ran MORE during the day than just running the 1:15 it takes to recover.

again, this is with my NEW system that was designed 1/2 ton oversized for my house. your may vary.

if you do get a setback tstat, make sure it has a smart recovery option to handle this, or it will be turning onthe emergency heat every day.

    Bookmark   December 6, 2007 at 1:41PM
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dadoes

DavidAndKasie is correct.

Heat pumps CAN be set back (turned down) effectively with a setback thermostat that has smart recovery.

First, since you say that you don't understand much about how heat pumps work, let's cover that.

A heat pump basically is the same as an ordinary airconditioner, with the addition of a reversing valve that changes its functioning to HEAT the indoors instead of cooling. Air conditioners don't so much MAKE cold as they MOVE heat from one place to another. In cooling mode, heat is moved from inside to outside. That's the hot air you feel blowing from the unit outside (the inside blows cool air). In heating mode, the unit picks up heat from outside and moves it IN (the outside unit blows COLD air, the inside WARM). There technically is heat in the air (or an object) down to scientific absolute zero (-459.67F) when molecular motion stops.

It's cheaper from an electric energy expenditure point-of-view to MOVE heat than to CREATE heat. Thus heat pumps are cheaper to operate electrically than glowing-red heating elements.

However, when the outdoor temperature drops, particularly to the point of ice/frosting, it's harder for the heat pump to pick up the heat and move it inside. So all heat pump system have an auxiliary heat source, typically standard electric heating elements, that activate as needed when it can't maintain the desired indoor temperature without help. This is called the "balance point" -- the point at which the heat pump can't move heat in fast enough from outside to counteract the heat loss that's occurring through doors, windows, walls, etc. Every system and house is different in that respect. The indoor temp drops below the setpoint (thermostat set on 70°F, room temp is 68°F for example), and a 2nd pair of contacts trigger in the thermostat which turns on the heating elements. Electronic thermostats may also do this based on time. If the system can't reach the setpoint (70°F) within a specific time period, or the thermostat senses that the indoor temperature isn't increasing or isn't increasing fast enough, the elements turn on. Of course, the heating elements pull a lot more electricity and aren't as efficient as the heat pump.

So the trick after a setback (or having turned the system completely off) is to raise the temperature SLOWLY so the heating elements DON'T turn on. Electronic setback thermostats designed for heat pumps do this automatically. The computer figures out how soon to start the recovery process so it can raise the temp slowly and hit the target at the specified time without using the auxiliary elements (or minimizing use of them). It may take a few cycles for the computer to "learn" the characteristics of a particular system and get the timing right.

You can also do it manually, but you have to PAY ATTENTION and not raise the temp too fast or too high at a time. This may take a couple hours at lower outdoor temps. Beware also that if the outdoor temp is well below the aforementioned "balance point" the system may NOT be able to recover the desired indoor temp at all without use of the auxiliary heating elements.

Another aspect of heat pumps that you should know about is defrosting. As mentioned earlier, the outdoor unit in heating mode is basically cooling the outside. During some conditions, particularly wet/freezing weather, the coils can ice-up, like a food freezer that needs defrosting. Air can't blow through the coils when they're clogged with ice, so like a freezer they must be defrosted. The system does that by switching back to AIR CONDITIONING mode for a few mins so the outside coils get warm and melt the ice. Now remember that when running as an air conditioner, the system blows COLD air inside the house. During defrost, the auxiliary heating elements turn on, NOT for the purpose of melting the ice on the coils outside, but to warm up the cold air that would otherwise be blowing in the house. There's no way to prevent defrost from happening, the system does that automatically as needed. Some people say that if the outdoor temp is so low for an extended period that the system is running continuously, the outdoor unit is repeatedly icing over and defrosting, then it's best to set the thermostat to Emergency Heat, which shuts off the heat pump and runs ONLY the heating elements ... since they're being used a lot anyway during the frequent defrost cycles.

    Bookmark   December 6, 2007 at 2:34PM
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garyg

Solid post from dadoes.

The amount of setback for a heat pump depends on home construction, outdoor temperature, and heat pump capacity. Outdoor temperature (and thus the balance point) is crucial.

For example, if it's around 40 outside, I'll have a 4 degree setback (from 64 to 68) while the boss and I are at work. If it's 20 outside, I may not be able to have any setback because I'll be running on auxillary heat anyway.

I have a Honeywell 7500 t-stat with SmartRecovery.

    Bookmark   December 6, 2007 at 3:07PM
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dadoes

When I was working days and night both (until April of 2007), I did a 10°F setback (to 60°F). I left the house about 11 AM, got back 2 or 3 AM, which is about 15 to 16 hours. The central TX coast doesn't often have extremes of freezing weather, so it worked very well. I have my auxiliary locked-out until the ambient reaches 25°F, and under typical "cold snap" conditions recovery could take up to 2.5 or 3 hrs. In one case of ambient in the mid/upper 20°Fs, recovery took five to six hours ... but it DID recover, and felt perfectly comfortable to me as the system was blowing warm air the entire time. Even with a few defrost cycles thrown in I think I came out ahead against leaving the 'stat at the normal 70°F for 15 hours all day. My house is only 4 years old, insulated pretty well, so there's very little run-time during the setback period.

But that's my circumstance, which doesn't apply to everyone.

    Bookmark   December 7, 2007 at 12:57AM
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forzainter_att_net

I have an electric heat pump for my 900 sq ft condo. The HVAC tech came out yesterday and said I should leave it in the "emergency" heat mode all the time in the winter. Since I live in a town which has very cheap electricty, sicne we have our own electric company, cost isnt' an issue, but I wanted to make sure I wont be damaging the unit by leaving it in emergency mode all winter long? thanks.

    Bookmark   December 24, 2010 at 2:51PM
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zheng1975_comcast_net

HELP !!!
Can someone please describe HOW to allow my heat pump which gives 85 degrees - to run LONGER with 85 degrees and avoid the heat strip heating.
I do realize I will need heat strip heating on colder days.
I do not believe the heat strip should go on when it is 40 degrees outside - If I could get the system to blow 85 degrees longer - I believe I can avoid $600.00 electric bills.

Ummm Mike - very cheap electricity - WHERE DO YOU LIVE?

    Bookmark   March 26, 2011 at 1:39PM
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kandmsmith_xtra_co_nz

My lowest setting is 17. Does a heatpump use power if its on but not actually blowing out heat??? I hear yes and no. What is right???

    Bookmark   June 20, 2011 at 6:45PM
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