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
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
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.
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?
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
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
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
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
Electric back up heat is NOT inefficient. It is actually 100%
efficient. That doesnt mean it isnt expensive to operate.
You guys are also missing one simple point. A heat pump can be wired
to the stat or at the air handler several different ways. If the aux
heat light came on everytime the furnace came on, maybe some brilliant
dillweed wired it so that the heat pump and one stage of back up heat
(or 2 or all stages) came on at the same time?? Ive seen it all the
time. If there is more than 2 thermostat wires its just too complex
for most people. :-)
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.
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