~~~AAA~~~ wrote:
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:

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.

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?

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.