Just a dumb question, but, I've asked a lot in my lifetime. At what temperature does it become more cost efficient to run just the heat strips rather than both the heat pump and strips? I realize a lot rides on the efficiency of the heat pump. Most of the reading I've done indicated 20 deg. Thanks for your answers. Hope you're having a great new year.
Hi beachbum64! I'm not a professional, I'm a homeowner, like you. Nevertheless, the answer I thnk your looking for is the heat strips should not come into play until the heat pump can no longer re-place the heat loss that's occuring in your home. I beleive it's called design temperature. I also belive that the heat pump should run even when the heat strips do kick in because you are still getting some btu's from the heat pump and this will still help the heat strips so their job and shut off sooner. I hope this helps some and I'm sure there will be more added to this.
Originally Posted by beachbum64
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Per Goodman's Expanded Heating Data for my new 3-ton 14 SEER heat pump:
15F outside temp, COP (Coefficient of Performance) = 2.46
10F outside, COP = 2.26
5F outside, COP = 2.06
0F outside, COP = 1.84.
The COP is the ratio of the energy output of the heat pump divided by the electrical energy to run the heat pump.
So, even at 0F outside, the heat pump still puts out better efficiency than electric resistance strips which have a COP of 1. At 0F, the heat pump is 184% efficient at transferring heat from outdoors to indoors.
The heat pump is also cheaper to run as the outdoor temp drops. My unit draws 2.64 kw at 40F and 2.41 kw at 10F (9% less power draw). Of course, the heat output from the heat pump drops as well but it doesn't cost the same to run it.
Many people turn off the heat pump due to concerns about wear and tear - they don't like hearing it run non-stop.
The combination of heat pump plus electric strips will also provide 15 to 30 degree warmer air from the registers as compared to just running the strips by themselves (based on outside temps).
Best to you.
Thank you very much for your replies.
By definition of COP, single phase heat strips will be a value of 1.0. A heat pump is compared to the COP and will always be above 1.0 and most of the time quite a bit higher than 1.0.
As the outside temp drops, the COP of a heat pump will also drop. When this occurs, the amp draw of the heat pump drops also. The COP of heat strips will not change with the outside temp.
There are only 3 times when you want to use heat strips if you have a heat pump:
1) the heat pump is in defrost cycle
2) the heat pump has failed
3) the outside air temp has dropped below the thermal balance point for your home
The balance point is determined by your climate, home construction, insulation, windows and doors. This is the point at which the heat pump will run non stop and the temp in the house will no longer be maintained. It will still have a COP of over 1, meaning it is more efficient than your heat strips, however, it's performance has tanked big time.
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Usually some where under -10F
am I understanding this wrong?
Originally Posted by gary_g
as I read it, the heat pump produces 15 to 30 degrees of heat. I was told anything above 18 degree temp rise was no possible.
His temp rise was including the aux heat being on also. Not just the heat pump.
Presuming he was refering to colder OD temps.
Your HP may not be set up correctly if your only getting 18* even in mild temps.
I hope that where u live isnt cold most of the time.
Yes, you are mis-understanding my statement.
Originally Posted by pacnw
Using my 3 ton Goodman as a reference, and 70F inside temp:
30 degrees outside air temp, the Delta T across the coil is 23 degrees.
20 outside = 20 degrees Delta T.
10 outside = 16 degrees Delta T.
0 outside = 12 degrees.
So if you turn off the heat pump in bitter cold temps and run the electric strips only, you lose from 12 to 23 degrees from the registers depending on the outside temps.
Do a load calc to determine the balance point for your home. This is where the strips should come on. They should work with the heatpump, not by themselves unless you are in emergancy heat.
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Comfort, not efficiency, will determine when you should run your heat strips. If the heat pump can't keep up with the heat loss, it's time for the heat strips to help out.
Coincidentally (or not), that is how the thermostat operates the system.