Flaky Furnace Thermostat

BFL13 wrote:
Doug's link to the Tstat shows that on "Gas"- the WF (white) is energized.
Heat pump has white/black energized.

I take it that the Gas switch position connects the two parts of the white wire so that acts like connecting the two ends of the blue wires.

I assume the red wire is just to power the Tstat. When you go to the Heat pump position, that energizes both the gas and the HP so they do that "gap" routine.

Your mystery is why when on Gas only and you have set 66F, it sometimes sets itself to 72F, the default after a power interruption to the Tstat.

The Red wire power to the Tstat is not from the furnace as such (which should have its own 12v power supply that the Tstat switches), but it might come from the same connection the furnace gets its power from. Say the Tstat's connection is loose but the furnace's connection is tight.

Or not making contact inside one of those wire nuts mentioned?

Assumes the furnace keeps running after the Tstat jumps to 72 and now the problem is just that it gets too warm in the RV. Assumes the furnace will cycle off at 72.

You said the problem does not happen when in heat pump mode where both the HP and the furnace are energized and you get to where you are on furnace assist. Mystery why a loose red wire would not lose its contact sometimes then too.