I have trane xr80 HVAC unit and is functional smooth (ignitor, flame sensor replacement etc couple of times but no big issues other than that) for quite a few yrs now. Also there is Honeywell rtf8580 thermostat (programmable with wifi ) , which is also in use for more than 4 yrs.

Since yesterday, this thermostat is restarting while making a heat call. It come back right away, connects to my wifi and when it sense it has to make heat call, a click sound is heard followed by shutdown of device.

I changed thermostat battery ( though it is not needed since thermostat takes power from HVAC unit ).

At HVAC unit, I changed high limit switch, changed circuit board , changed transformer (I keep these parts at home for such situations) but no luck. Cleaned air filter and reinstalled. I removed and installed all wires to thermostat. No luck.

There is no indication on circuit board red light flashing. It is showing normal steady blinking indicating a normal operation.

I tried to short R and W wire but that blow off circuit board fuse.

Any idea what could be reason for this malfunctioning ? Why thermostat could be restarting ? Thanks in advance.

 

A good thing about those Honeywell thermostats is that they have the ability to reset faster than the fuse can blow. You have a low voltage short.

I would remove the panels and look for a screw that might have fell into the furnace and landed on a limit switch, or look for a melted wire, or a wire that has rubbed into a short.

Does this still occur if you disconnect W at the furnace?