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Old 01-27-2014, 11:45 AM   #1
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Any reason to lock out heat pump


As long as the COP is above 1 and the heat pump is able to keep within a couple of degrees of the setpoint .... is there any reason to lock it out at extreme low temperatures? Emergency heat is heat strips in each air handler.

I have a roughly 3800sqft house with 2 Goodman heat pumps.

Unit 1 is a 4ton 13seer that heats/cools about 2500sqft of the house.

Unit 2 is a 2ton, 2 stage 16seer unit that has a zone controller and 4 zones.

I have each unit set to lock out when the temperature falls below -0- .... but the switches are a simple mechanical switch that seems to lock out closer to -10- than -0-. They cannot be set any lower.

I ran an experiment and just bypassed the lock out on the 4ton unit and it was putting out enough heat down to -0- with a windchill of -20.

So my question .... as long as the COP is greater than 1 the heat pump is more efficient than the heat strips and as long as they are able to keep up (within reason) ... .any reason to lock them out?

I have all my thermostats set with no auxiliary or emergency heat. I do not want to run the electric heat during periods of recovery for the zones.

Icing does not seem to be a problem either .... defrost cycles working properly on both units.

Edit: I live in southern Indiana .... this has been a colder than normal year with real temperatures seeing -10 -12 so far. for about 40 hours the temperature remained below zero and my air handler never stopped. My electric company has a daily usage tab on their website and my usage over those 2 days was about 700kw. My house never fell below the setpoint of 70*. My 4ton unit has a COP of 1.30 at -10 and my 2ton has a COP of 1.47.


Last edited by swilk; 01-27-2014 at 11:55 AM.
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Old 01-27-2014, 03:15 PM   #2
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Any reason to lock out heat pump


What??? Are you talking about locking out the heat strips until its 0 degrees out? Hey whatever. As long as your happy then let her rip. I wouldn't. Does it not get cold when the unit goes into defrost? If the heat pumps are keeping up with it 0 degrees out and no auxiliary heater kit, then I would say your equipment is oversized.

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Old 01-27-2014, 03:27 PM   #3
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Any reason to lock out heat pump


The heat strips will still run during defrost.

The equipment was spec'd for summer time load. It will run constantly to keep set point on the hottest days of the year (right around 100*) which I would say is sized just about perfectly.
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Old 01-29-2014, 09:34 AM   #4
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Any reason to lock out heat pump


Quote:
Originally Posted by Patrick Eubanks View Post
What??? Are you talking about locking out the heat strips until its 0 degrees out? Hey whatever. As long as your happy then let her rip. I wouldn't. Does it not get cold when the unit goes into defrost? If the heat pumps are keeping up with it 0 degrees out and no auxiliary heater kit, then I would say your equipment is oversized.
on day 3 of the experiment ... and the heat pumps will not completly keep up with the temps we have had since Monday. During the nighttime hours the inside temp has dropped to 65* and risen to 68* during the day.

That is with a real outside temp of no higher than 9* for the last 40 hours.

Now I just have to see what my elec usage was during that time and see if that 5* of comfort is worth whatever amount of money I may or may not have saved by running just the heat pumps.
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Old 02-03-2014, 12:41 PM   #5
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Any reason to lock out heat pump


I am sure it is not 100% apples to apples ..... but comparing usage and temperature it is as close as I can get to a side by side comparison of heat strip use and heat pump use in temperatures below 10*.

Looks like nightime, in house temperature will fall by 5* when the outside temperature is 0* and the home is running on heat pump only.

This 5* drop in inside temperature saves me about 140kwh.

That is nearly $14 per day.

On this colder than normal January that would have been about $100 savings if I had made the chance sooner.
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Old 02-03-2014, 12:51 PM   #6
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Any reason to lock out heat pump


Quote:
Originally Posted by swilk View Post
I am sure it is not 100% apples to apples ..... but comparing usage and temperature it is as close as I can get to a side by side comparison of heat strip use and heat pump use in temperatures below 10*.

Looks like nightime, in house temperature will fall by 5* when the outside temperature is 0* and the home is running on heat pump only.

This 5* drop in inside temperature saves me about 140kwh.

That is nearly $14 per day.

On this colder than normal January that would have been about $100 savings if I had made the chance sooner.

Is that 140 KWH per month, or per day?
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Old 02-03-2014, 02:08 PM   #7
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Any reason to lock out heat pump


Per day.

On a similar 24 hour period with my heat pumps locked out (outside temp below 10*) and my house being heated only by the heat strips my daily usage was 330kWh.

Locking out the heat strips and running only my heat pumps the usage was 190kWh during a 24 hour period.
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Old 02-03-2014, 02:15 PM   #8
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Any reason to lock out heat pump


On January 23 my electric company claims my average outdoor temperature was 8* with a low of 3*. During that 24 hour period my consumption was 329kWh.

On January 28 my electric company claims my average outdoor temperature was 7* with a low of 1*. During that 24 hour period my consumption was 191kWh.
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Old 02-03-2014, 02:21 PM   #9
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Any reason to lock out heat pump


There are only a handful of days that are cold enough to matter .... but this January I had daily usage of 242, 339, 321, 241, 329 and 269 before I changed the lock out setting.

If all of those had been knocked down to the 191 I experienced running just the heat pumps that would have been a saving of 595kWh in just those 6 days.

Nearly $60 and a house that was 65* instead of 70*.
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Old 02-03-2014, 06:34 PM   #10
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Any reason to lock out heat pump


If you comfortable at 65, okay.

I would set up the aux heat for staging. That only 5KW can come on.

Your 191 KWH day would have been around 225, 230. And the house would have been at 70.

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