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We have just moved into a house with a Lennox XP17 heat pump. It has two stages of electric heat strips in addition to the heat pump.

In observing the power bill, I have been astonished at the rate we are going through kWh. I did some more research last night to see if everything is working right, and in the process I discovered that the outdoor unit wasn't running, at all. Yet the air coming out of the vents was very warm, which explained the power consumption. It was about 20 degrees outside last night.

The thermostat (a plain Jane Honeywell non-programmable) only shows "Heat on" and gives no indication that the aux stages are running. Emergency Heat is not on.

I suspected a wiring issue but when I switch to emergency heat, same result.

I checked the thermostat setups too, it is set correctly as a heat pump. Surely HVAC manufacturers and installers don't still think that heat pumps are not efficient below 35 F. Is this a setting on the heat pump or on the air handler, or is this a thermostat problem?

  • It sounds like they have a low temperature cut out on the unit. They do lose efficiency as the temperature drops but you should still be able to get something out of it even close to zero degrees F. Look for the technical manuals on this unit and see if there is a setting or jumper you can change to lower the cut off temperature. Just a thought. – ArchonOSX Dec 11 '16 at 15:04
  • Low temperature operation is variable by specific heat pump. There are still many made that don't have much of a low temperature operating range - among other things, once it gets below freezing they have to manage frost on the coils correctly (or incorrectly and inefficiently, as some do.) Most of the very low outside temperature units I have found when looking are mini-splits, not whole-house units. Do you have gas service available? If you do it's often a better deal to add gas heat than to replace the system wholesale. – Ecnerwal Dec 11 '16 at 15:13
  • A quick look indicates that Lennox are not very proud of their COP performance data, as I can't find any on the web for that model. I'd infer negative things about its likely performance from that lack of data, but I'm a cynic that way. Companies that make heat pumps with good low temperature COP are generally quite happy to publish their data. – Ecnerwal Dec 11 '16 at 15:24
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    Is the unit throwing any fault codes on the control board LEDs? – ThreePhaseEel Dec 11 '16 at 15:28
  • @threephaseeel it's giving me the "idle" blink. This is like a 4 year old unit. We moved from a townhouse with an 8 year old cheapie unit that performed great down to 20 and below. – trpt4him Dec 11 '16 at 15:58
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The Lennox XP17 series is certainly capable of operating at lower outdoor temperatures than 35F with reasonable efficiency. The extended performance tables on these units are published by Lennox at the following url:

http://tech.lennoxintl.com/C03e7o14l/UfSSOXTazc/ehb_xp17_1512.pdf

Beginning on page 16, they list performance information for different indoor/outdoor combinations. For most combinations, the designed performance of the unit at an outdoor air temperature of 17F is about 60% of the full rated capacity.

One likely cause of the problem you describe is that your unit is equipped with an optional outdoor thermostat kit which is set much too conservatively (Lennox Part 56A87 or similar). This part is often used to automatically disable the outdoor unit during extremely cold temperatures and it is usually located somewhere in the control section of the outdoor condenser.

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