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We had a new Trane XR15 heat pump installed three months ago. I’ve noticed that aux heat is kicking in more than I think it should, given the temperatures. For instance the other day…..
The outside temperature was 48 degrees.
The inside of the house was already 64 degrees.
I bumped the thermostat up to 68 and the Aux heat kicked in. That’s only four degrees difference with the outside already almost 50 degrees.
This morning it was 39 degrees outside. The inside of the house was already 68. I bumped it up to 70 to see what would happen. Aux heat kicked on immediately and stayed on until it hit 70 degrees, then it went back to normal heat mode to maintain. I can't understand why Aux heat was needd to go up two degrees when it’s almost 40 degrees outside.
Is this normal or do I need to call the HVAC guy.
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I don't know for sure, but I think that's normal, atleast it was for an older Trane unit (XL1200) we had. The air source HP can't generate enough heat for a sudden increase, so the furnace comes on to get up to temp, then the HP takes over. I have no idea what the parameters are in the system logics for that to happen, but if it's only 3 months old you're under warranty....it wouldn't hurt to ask.
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The auxiliary heat comes on when it is 1.5 degrees cooler in the house than the thermostat set point.
With heat pumps, it's best to set the temp you want and leave it alone.
If there is an outdoor thermostat, the wall thermostat may be calling for aux heat but the aux won't actually be on unless the outdoor temp is below the outdoor thermostats set point.
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Contact your HVAC installer and have a discussion with him.
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thooks said:
You didn't say where you live, but another issue you are probably going to notice is that the system will go into defrost more often when it is 38-48 degrees and very humid or misting or slighting raining. They seem to make ice better in those conditions than heat a structure. 
Roger that, although ours was hooked to a gas furnace and the defrost didn't take all that long.
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daddo said:
The auxiliary heat comes on when it is 1.5 degrees cooler in the house than the thermostat set point.
With heat pumps, it's best to set the temp you want and leave it alone.
If there is an outdoor thermostat, the wall thermostat may be calling for aux heat but the aux won't actually be on unless the outdoor temp is below the outdoor thermostats set point.
Not yet a heat pump owner, but working on it. Existing gas furnace has a dead band of something like 2 to 4 degrees. Do you set up a heat pump thermostat to have a dead band of 1 degree? Else, at a 1.5 degree differential, you would always have the strip heat cycle with the heat pump, at least for a while.
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Thanks for the feedback. To answer a couple of questions, I'm in Zone 7. Upper West South Carolina foothills. There is no outside thermostat, just the one indoors.
I called my HVAC guy and he said pretty much the same thing you all did. The unit has no idea if it's 20 or 50 degees outside, it just responds to the indoor thermostat. If there is a 2 degree or greater heat demand, it kicks on the Aux heat to quickly get it up to the demand temp.
I'm surprised. I would have thought that it would be at least a 5 degree (or more) difference before it would kick on the expensive to run heat strips.
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Mr_Mike said:
[blockquote]daddo said:
The auxiliary heat comes on when it is 1.5 degrees cooler in the house than the thermostat set point.
With heat pumps, it's best to set the temp you want and leave it alone.
If there is an outdoor thermostat, the wall thermostat may be calling for aux heat but the aux won't actually be on unless the outdoor temp is below the outdoor thermostats set point.
Not yet a heat pump owner, but working on it. Existing gas furnace has a dead band of something like 2 to 4 degrees. Do you set up a heat pump thermostat to have a dead band of 1 degree? Else, at a 1.5 degree differential, you would always have the strip heat cycle with the heat pump, at least for a while.
[/blockquote] Dead band is the temp difference setting between heating and cooling settings on a thermostat set up in "auto" changeover mode. This prevents the system from cycling from heat to cooling at one set point. The 1.5 degree for aux heat is different and can also be different in some thermostats. There are "economy" settings where the thermostat will wait a default set determined amount of time before using the aux heat- be it another 1 or 2 degrees or in minutes. Then there is the "comfort" setting where comfort over rides energy savings and starts the aux heat at the 1.5 degrees regardless. Some thermostats "Intelligent recovery" where it records the running times and temperatures allowing it to already know how long it takes the home to warm up or cool down and will cycle the system earlier or later to maintain the set temp. In doing this, it may bring the heating cycle on sooner as to prevent the aux heat from being called for. Manually changing the temp set point on the thermostat prevents this option from working at that time. That, among other reasons is why a heat pump in heating mode is best set at one temp and not changed manually, unless of course you have a more effective way to recover like Thooks' gas furnace. But basically, the 1.5 degrees is the norm and isn't commonly a changeable feature.
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That part (the outside temp) is different than the one I had (XL1200) and the geo I have now. The Trane had a thermostat in side the condenser housing that was adjustable to the cutover temp. Mine would cut ver to furnace only when the temp dropped below 30º. My current unit (Carrier) cuts over to the furnace when it hits 5º (also a gas furnace). It's interesting to see what your guy told you. The place I just bought has geo, but it's an older unit with no out side thermometer (that I'm aware of), I'll bet it works just as you described
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Terry W said:
Thanks for the feedback. To answer a couple of questions, I'm in Zone 7. Upper West South Carolina foothills. There is no outside thermostat, just the one indoors.
I called my HVAC guy and he said pretty much the same thing you all did. The unit has no idea if it's 20 or 50 degees outside, it just responds to the indoor thermostat. If there is a 2 degree or greater heat demand, it kicks on the Aux heat to quickly get it up to the demand temp.
I'm surprised. I would have thought that it would be at least a 5 degree (or more) difference before it would kick on the expensive to run heat strips.
If your home dropped 5 degrees all the time (Which actually means 6-7 degrees), you would be calling for service and demand that your home be comfortable all the time. Keep in mind that most thermostats show the degree in single digits. When it reads 70*, it might actually be 70.8*- the internal of the thermostat reads the degrees in 10ths, so it will react differently, as it may bring the heat on when it is reading 70*, but the temp is actually 69.4 and falling rapidly. "Recovery intelligence".
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