heat pump temperature setback

[rkdjones]

I just got a heat pump and am trying to figure out the reason for the advice about temperature setbacks that I am getting from the techs. They say that, unlike an oil or gas furnace, one should not set the temperature back at night or during the day when no one is home. That the system is more efficient if I keep the thermostat at one temperature all the time. I cannot get an explanation of why this should be.

I firmly believe that heat loss is proportional to temperature difference between inside and outside (so when the thermostat is set to 63, my house loses less heat). I believe that all furnaces, including heat pumps, are more efficient when they are not cycling off and on (am I wrong). I can accept that it might take 2 hours for my heat pump to raise the house temperature 2 degrees. Why would a heat pump use less energy to keep a house at 70 all the time?

Robert

[beenthere]

Its because to recover from set back. The electric resistance aux heaters will come on. And they will cost far more to use for recovery then what you saved during the set back.

[rkdjones]

I failed to mention that my aux. heat is locked out until the temperature drops below 35 F (which doesn't happen than often here in Seattle). Is there any other reason why it is more efficient to operate a heat pump at a constant temperature?
Robert

[beenthere]

Thats the main reason why.
Next is slow recovery at temps below 50°F outdoor temp.

Next. is at temps of 45 and below. The long recovery can cause it to go into defrost and still use the aux heaters. Since they are not controlled by the thermostat when the heat pump goes into defrost.

You can try it and see how your works. does it use more electric or not. Does it recover quick enough, or do you have to wait 5 hours for it to recover 6°F.

[rkdjones]

Thanks.

Triggering a defrost would certainly make it less efficient. Unfortunately, it would be difficult to compare the energy usage with a setback versus keeping a constant temperature since the weather changes. And without standing out by the compressor to see if it goes into defrost, I would be hard pressed to know whether I am triggering a defrost. (Defrost control is already a contentious issue with me.)

My only hope to get a handle on this would be to keep a record of aux. heat usage. I would love to get some device that would tell me how often I am using the auxiliary electric heat. I have seen meters for 110 outlets that record energy use but don't know what I could use for the 240V line that feeds the 15KW aux heater (I have a 30 amp and 60 amp circuit feeding my inside unit). Any suggestions?

Robert

[beenthere]

A quick easy way to just see if they are coming on. Is a thermometer that has the ability to record and store the max high and low temp.

If you have a high temp that exceeds 110, aux was on. Won;t tell you how long though.
Next would be a data logger. Again just set at the closest supply. And sample the air temp every minute. or however often you wanted, and it can store.

[farbeondriven]

Choose a tstat with Adaptive Intelligent Recovery. They reconize the amount of time it takes to recover and adjust themselves by building a memory of previous cycles by reconizing trends in your systems recovery.. Is your aux heat fossil fuel or straight electric? Either way if its above the lockout point who cares. A 2-3 hour run cycle is better than 5-6 start ups through the night. The refrideration cycle doesn't stabilize for about 10-15 minutes so limiting the short run times and is beneficial..

[spotts]

Agreed. a vision pro IAQ with Outdoor sensor

Or better yet... a prestige!
with wireless outdoor temp sensor.

[kflack]

OK

I was like all of you and wondered what is the best way to determine how to maximize the savings using a heat pump...

Well you are right there is no device that measures everything (Data Logger yes but really expensive)......So I designed a device my self, and wrote a load of software to take into account electricity costs, secondary heating oil costs, and automatically determine the Balance Point....

I monitor everything including defrost cycles, length, secondary use, outside temperature, as well as input and output temperature Delta T....

Bottom Line is this...

Do Not use the set back at night if the temperature outside is forecast to be above the balance point of your Heat Pump.

Reasoning is as follows

If you use a set back, most thermostats will during the morning rise, attempt to raise the temperature but will do so slowly. Most thermostats will activate a secondary heat source if the rise in temperature is less than 1 degree per 30 minutes...AS this is normally the coldest time of the day the efficiency of the heat pump will be at its lowest, and the likely hood of having to use the secondary heat source is increased.

In our case Oil is used as a secondary heat source and it costs 8 times what the Heat pump cost is.... A No Brainer.....

On the other hand, if the forecast is for colder temperatures than your balance point, you will be heating with the secondary anyways, so it is cost effective to turn down the heat on nights when the temperature is below the balance point efficiency....If possible, try to turn the temperature up in the a.m. in 1 degree increments (After the temperature rises above your balance point) if at all possible....

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[beenthere]

kflack:

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