Max Metral wrote:

Yes, I mistyped, it should read:

...with meter in hand if the switch is on summer, I measure 24VAC across R
and Y. If the switch is on winter, I measure 0VAC across R and Y.

Well, I think now I'm the one who mis-spoke. :-)

Having 24 VAC across R and Y in the summer is what you want, because
then you can effectively call for cooling. In winter, you'd want it
across R and W instead, to allow calling for heat. This could still
be part of an interlock with an attic fan.

"CJT" wrote in message
...

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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