Heat strips coming on too soon?

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Old 12-05-10, 06:49 AM
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Heat strips coming on too soon?

I'm logging current on my inside and outside heat pump units. It appears that the inside heat strips are coming on when outside temp reaches 43.488degF falling, then back off at 45.082degF rising (obviously with much cycling between). Meanwhile, I suspect my second stage compression is never coming on. Note that the outside unit remains on all night, at the same time as the heat strips one. The system is not switching the heat pump compressor off while the heat strips run.

I built this house myself. Very tight and green. ICF (insulating concrete form) walls. Sealed attic. Spray foam roofline. Earthcraft certification (SE USA green org). I am suspicious that the system is factory tuned for most lousy built leaky houses. I am suspicious that the heat strips are being turned on under cold conditions by some dumb logic making assumptions based on outside temperature, while the second compressor stage is never used based on run time and inside temp swing, made better than typical by my energy efficient home with thermal storage concrete walls. I found a webpage somewhere suggesting an optimization that might be getting used, where such optimization assumes heat strips are more efficient than heat pump below some exterior temp. But this same assumption wouldn't be true for my house.

Meanwhile, this thermostat does NOT have an exterior temp sensor on it.

Any suggestions? I suspect my energy bills are double what they should be!

I've read about a "lock out" setting in the honeywell IAQ thermostat for the heat strips, but on neither this or the other system in my house, do the specific setting numbers appear. They get bypassed. Dunno if that's due to other setting causing lock out to get disabled, or if these tstats just don't include that feature.
 
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Old 12-07-10, 03:00 AM
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Arrow Opposite problem

I have a related but opposite problem. After new windows the house is so tight the strip heaters never activate and the heat pump runs excessively when the ouistide temp is below 30 or so. I would like to be able to set the thermostat so below a specified outdoor temperature the heat pump doesn't activate and the strip heaters take over. Can anyone recommend a thermostat model with this capability?
 
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Old 12-07-10, 06:22 AM
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I would suggest, Dave-VA, that you're actually "good". If you're able to somehow log the on-time and current consumption of your equipment, you should be able to figure it out. I purchased a data logger with two clamp-on AC current sensors plus a temp sensor on a long cable (details far below). Log over night. Calculate the amount of "on" time for your heat pump. Multiply by the typical amperage. Measure your 240V (mine is 250V) and multiply by that as well. The result is Whr. Divide by 1000 to get kWhr. Do the same for the heat strips.

Obviously, to compare apples-to-apples you'll have to tailor this recipe. For example, you'll have difficulty gathering the heat strip data at all, since they won't turn on. You may have to go through test mode on your thermostat and manually turn the test mode on and off, as if *you* are the thermostat, in order to get a measure of the duty cycle (on-time) required to maintain internal temp. Also, you need the outside temp to be about the same. Same solar heat gain, too (do it at night to avoid this issue).

I *believe* you'll find the heat pump number is still less than the heat strip number. In my case, the first stage heating draws 5A. Second stage by book should draw maybe 7A. Heat strips draw 23A. Guess-work looking at changing mixture of the two as temp falls suggests there is NOT NEARLY a 3 or 4 fold greater effectiveness for the heat strips, and therefore they're less efficient. (Duh! We all knew that already.) More importantly, in my tight house with thermal storage walls (concrete), the first stage heat is almost capable of maintaining temp. So if second stage could ever come on, I'd never need the less efficient heat strips.

Note I bought from Data Loggers: World Leader in Data Logger Solutions one U12-006, two CTV-C, one TMC50-HD, one BHW-LITE; $368+s&h. Clamped into breaker panel, carefully.
 
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Old 12-07-10, 06:53 AM
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Once again...I'm moving this thread. To heat pumps and electric home heating...under the same General Forum of Heating, Cooling and A/C
 
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Old 12-08-10, 04:30 PM
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Grumps - Thanks for moving us to the correct forum.

Dr. Lightning, I definitely agree with you on the point about efficiency. The old electric bill will double if I run straight E-heat for a month. But the heat pump technician told me to save wear on the equipment (and my nerves) I should run e-heat when the temp drops below 30-35 degrees. So in Virginia the temp drops below 30 at night for a couple of months a year but during the day it is usually above 35. I wish I had an automatic means of changing between Eheat and heat pump mode based on outside temp. I know it would raise my electric bill somewhat.

Does anyone know of a thermostat capable of this?
 
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