Heat pump vs Heat Strip and register temp
Please ignore my spelling mistakes
I'm wondering if you can draw a correllation between a heat pump effectiveness and the temp of the air coming out of the register.
I don't know the exact temp tonight, but it was +-30 degrees
The house was 64, so I changed the set point to 70. The heat pump kicked on and the air coming out of the register I tested was 81 degrees. I saw the temp change to 66 in an hour or two. I then kicked over to heat strips and upped the stat to 72. It went from 66 to 72 within an hour (or something like that...it was fast).
I'm guessing the heat strips use 2-4 times the electricity of a heat pump, but I'm also guessing that in colder temps there becomes a point where heat pump efficiency equals heat strip efficiency, then the heat pump becomes less efficient as it gets colder.
Is there a general rule of thumb as to when you should kick over to heat strips (I've heard 30 degrees)?
And can it be tied to the air temp coming out of the registers?
Thanks in advance
Heatpumps are almost always more energy efficient than heat strips.
Your t-stat should be turning on the strips automatically when required (if set up properly, the heat strips and pump will run simultaneously)
When it's too cold for the heatpump to operate, an outdoor thermostat should automatically make the switch.
I'm guessing the heat strips use 2-4 times the electricity of a heat pump
With electric strip heat as aux heat.
As long as the COP is not below 1.3 to 1.5(depending on area, and type of defrost control), the HP should be used.
EG: 1,000,000 BTU's of strip heat at 100% efficiency, at 10 cents a KW delivered = $29.30
1,000,000 BTU's of heat from the heat pump at a COP of 2.5, at 10 cents a KW delivered = $11.72 ( COP of 2.5 at 30° outdoor temp, is low, for most modern heat pumps).
1,000,000 BTU's of heat from the heat pump at a COP of 1.3, at 10 cents a KW delivered = $22.54
Depending on heat pump model, it may have a COP of 1.3 at outdoor temps at 0°F or lower, so it would still be cheaper to let the HP run when its at that temp.
Shutting a HP off at 30°, and using strip heat only, would cost you 2 to 3 times as much, as using the HP and strip heat.
The only accurate way to determine when it would be more efficient to run strip heat vs the heat pump would be to determine the balance point of your house. AKA: at what temp your heat loss outweighs your heating capacity.
Example: a 3 ton heat pump produces approx 36,000 btu's at 47 degrees F.
This same heat pump will only produce 24,000 btu's at 17 degrees F
The heating load for the house might only be 20,000 btu's at 47 degrees F, but at 17 degrees F it might be...... say 35,000 btu's.
So, If you split the difference to 30 degrees F the heat pump would maintain the home as designed with no aux assist and it would still be an efficient way to heat the house.
Now, when you get down to 17 degrees F, and you have say 15kw of strip heat in your air handler. Using the first stage of the 15kw (5kw=15,700)
in conjunction with your heat pump would give you a heating btu of approx 40,000 which matches your heating load but uses a more efficient heat because roughly 63% of you heat is being provided by your heatpump at a 2.5 COP. vs 1.0 COP for the strip heat.
Regiser temp is not an accurate way to determine switch point beacuse with a H/P it changes frequently due to defrost initiation.
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