New thermostat

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I have a Trane BWD724A100A1 heat pump system servicing my 600 s.f. apartment. The original Weathertron stat appeared to be inop in that whenever heat is called for the aux heat blue light indicates. I purchased a Honeywell TH8320U and outside temp indicator that I appeared to wire correctly to this stat that I just installed. Now, a few ?s:

In the installer setup, should I set the compressor and/or aux heat lockouts? If so, to what temps? I am in NYC area if that matters.

Is there much of an energy-use difference between using the compressor or aux heat?

On occasion, after the new stat reaches the set temp during heat cycle, the "heat on" indicator will turn off but the vents will continue to blow out air, at decreasing temp. I put a thermometer in the ceiling duct and at times this air continues to blow out for 20 - 30 mins going down to 63 F which of course lowers the room temp, causing the heat to have to turn on again. Even if I remove the stat from the wall plate the vents continue to push out this cold air for at least several more minutes. As I said, this only happens sometimes. Sometimes it appears to work correctly and the ducts shut down shortly after the 'heat on' indicator turns off. How can I stop this from happening?

Is there somewhere that I can see the correct Installer setup settings for this configuration?

[Travis Jordan]

Why do you say that the original thermostat wasn't operational?

Aux heat (AKA emergency heat) comes from resistance-heating "heat strips" which at outside temperatures above about 20 degrees are terribly inefficient compared to a heat pump.

Sounds like a stuck fan relay on the air handler.

I'll speculate that your heat pump component isn't working at all, that the new thermostat did nothing to change this, and that you've got an intermittently stuck fan relay. Time to call a service tech.

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The old stat was 22 years old and each and every time that the heat would automatically go on, the blue auxiliary light would light up. Should the auxiliary heat always be on during calls for heat? Also, the room temperature and set temperature were not in sink, being off about 5 degrees.

AKA is the same as emergency heat? I thought they were separate issues. My new TH8320U has Heat on, Aux Heat on, and then a separate Emergency heat indicator. Am I not understanding this correctly?

If aux heat is inefficient above 20 degrees should I simply set the aux heat lockout at 20 degrees and above so that only the compressor would operate?

Maybe an intermittently stuck fan relay? If so I guess I should get a professional to look at this and fix if necessary.

Re the heat pump, I do get the Heat On indicator and warm air comes into the room, and sometimes it will switch to saying Aux Heat On, also with warm air. Since Heat On brings heated air, does that mean that the compressor is in fact working?

If the room temp is indicated at 70 and the set temp is 72 I will see the Heat On indication and warm air. If I move the set temp higher even a few degrees, the Aux Heat On indicates. Normal?

Last Q: Is it better or not to utilize the temp lockouts for compressor and aux since I added the outside temp accessory to this new stat?

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[Travis Jordan]

Aux heat should only be on when the heat pump can't keep the temperature at the set point. If it was really cold outside (say below 20 degrees F) it might be normal for aux heat to be on virtually all the time. OTOH the temperature error is certainly a good reason to change the 'stat. Your new 'stat is a very good piece of equipment.

On an all-electric (no gas, no oil....) system there is only one source of supplemental (aux or emerg) heat - resistive heat strips. "Auxiliary" heat means the strips are on in addition to the heat pump, "Emergency" heat means the strips are on but the heat pump is off. Uusally the selection of aux vs. emerg mode is made at the thermostat. Emergency heat will also be selected by the heat pump when it is in defrost mode.

Not necessarily. Since your systems is "all electric" in most US climates you are better off letting the heat pump run regardless of the outdoor temperature. You will almost alwasy get cheaper BTU's from the heat pump than from resistive heating. At lower temperatures the Aux heat will be on most, if not all of the time.

That would be my recommendation.

That would be the logical conclusion. If you have access to the outdoor condensor you can check to see if the compressor is running; you should also get colder-than-ambient air out of the exhaust side of the condenser.

Perfectly normal. The thermostat thinks that the heat pump can't keep up with the set point demand, so it turns on the heat strips (aux mode). Most thermostats turn on aux heat when there is a 2 degree (F) difference between the current temperature and the set point temperature.

I wouldn't use the lockouts unless you are sure you know the temperature at which your heat pump costs more to run than it produces in heat energy - which is going to be well below 20 degrees, perhaps as low as zero or below. You might be able to divine this information from the manufacturer of the system - assuming that the system was 'matched' (outdoor condensor, inside coil and expansion valve) when it was installed, or from a very experienced service tech in your area. Here in my part of Florida it never gets cold enough to lock out the heat pump.

[Roy Starrin]

[Bubba]

[Travis Jordan]

Hi Bubba - I didn't say anything about the efficiency of resistive heat.

I agree that a hack might have miswired the aux heat to come on whenever the heat pump was operational - and they could have done that with a jumper at the old thermostat. From the OP's description it sounds like the new thermostat is wired correctly in this regard.