Max Metral wrote:

Well, the uncertainty about why this happened after two working heating
seasons will be a great unanswered question in my life. (ok, well, not that
great) BUT, the problem is solved.

So, as suspected, the digital therm was simply losing access to power when
it routed R to W. The additional variable that I didn't realize is that
there's a summer/winter hard switch on the furnace. I assumed that normally
I had switched this to winter in the past. However, with meter in hand if
the switch is on summer, I measure 24VAC across R and Y. If the switch is
on summer, I measure 0VAC. I don't understand exactly what this switch does
that other than I suppose forcibly disabling the compressor. But given this
fact, there's no way for the therm to steal power when it requests heat, and
it shuts off.
snip

So which is it when it's on summer -- 24VAC or 0VAC? (I think you
mis-typed in the above quote.) If this is an attic-mount furnace,
then my guess would be zero, which keeps the heat from coming on
when, e.g., an attic exhaust fan might be running (which could
lead to dangerous levels of CO).

It sounds to me like whoever installed the digital thermostat didn't
cover all the bases.

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