Honeywell T10 - Avoiding Aux Heat for Defrost

[nailbird]

Greetings all,

I have a dual-heat (2H-1C HP + gas furnace) setup controlled by a T-10 which includes an outdoor temp sensor. It's an all-Trane system (XL14i HP and XR90 furnace) - the system is about 12 years old. The T10 and a new furnace blower were installed exactly one year ago.

As winter approaches I noticed the T10 calling for Aux Heat at 40F when the lockout is set for 30F. I haven't yet turned the gas on but I heard the furnace blower spinning up repeatedly indicating it was trying to fire up the furnace. I'm guessing this is triggered by a defrost call (I sure hope it is!) but I don't want to burn expensive gas for defrost. My home is passive solar heated and has plenty of energy for defrost without needing gas.

My old 8000series t-stat didn't use droop and wouldn't permit running the furnace above 30F. In the T10 settings, can I nix this by setting the droop temp to 5F in ISU 3090 (Backup Heat Differential - currently set to 2F) and ISU 3120 (Backup Heat Lockout), which is set to OFF to 5F?

ISU 3110 (Backup Heat Upstage Timer) is currently set to OFF.

The T10 doesn't have any indicator signaling a defrost cycle that I can see. Smart device and in some way not very informative.

Thanks for the assist.

[Admin]

I think it's the heat pump that initiates the defrost mode, not the thermostat.  When it activates defrost mode it usually sends power to W2 and activates the gas furnace.  If you wanted to take advantage of the solar energy you need an electric air handler so an electric element is activated instead of gas.

Is 2000 and 2010 set to heat pump?  And 2180 and 2185 set to gas?

If you set 3120 to 5F then the gas furnace should only run when the outdoor temp is 5F or below, or when the heat pump is in defrost mode.

Didn't the old gas furnace run during a defrost before it was replaced?

[nailbird]

Yep - the t-stat doesn't control defrost, but the T10 has this "backup heat lockout" in addition to the "compressor lockout" under 3120. It notes that some installers set the aux heat lockout as a "...warm weather shut down setting." To me, this would make the furnace unavailable during defrost even if the HP called for it.

I used to have an 8000 t-stat and whatever the installer did the furnace would not assist in defrost and we got cool air for a short while when a defrost occurred. But defrost was only something we had to contend with only on really cold, wet days which were rare and not really a big deal. The house is a passive solar design with a lot of thermal mass, so a bit of cool air was no big deal, and still isn't.

When the 8000 got fried in a surge and he installed the T10 the behavior changed. The T10 being new is not something many local techs have familiarity with. It works fine but I was quite happy with the old way and wondered if some droop settings and the backup heat lockout could make the T10 act like the old install without much fuss.