Just to clarify a point that's tangential to this thread...

"Rick C." <rixy@VVALLEY.COM> wrote:
> Most thermostats are passive. No electronic parts inside. There's usually
> a two circuit mercury bulb that will turn the heat/ac on or off. There's
> also an anticipator resistor circuit that works on the current flowing
> through the heat/ac relay circuit. This keeps the unit from cycling to
> quickly.

Actually, the anticipator is used to prevent overshoot of the room
temperature by removing the demand for heat before the zone fully reaches
the setpoint temperature. Most heating systems continue to dump a certain
amount of heat into a heating zone even after the demand goes away, and
this is a way of compensating for that. It also can compensate for any lag
between the zone temperature and the temperature of the thermostat itself.
If anything, the heating system will cycle slightly more often, but in
return you get tighter control of the zone temperature.

On non-electronic thermostats, the anticipator is simply an adjustable
power resistor that locally heats up the temperature sensor whenever
there's demand for heat. There's no corresponding mechanism for cooling.

To prevent rapid cycling, non-electronic thermostats often put a small
magnet on the end of the bimetallic element in order to create a certain
amount of hysteresis. The amount of hysteresis is usually not adjustable.
Sometimes the weight of the mercury shifting back and forth in the mercury
switch is enough to create the required amount of hysteresis. (This works
for heating or cooling.)

Obviously, electronic thermostats do both of these functions (and others,
such as timed setbacks, etc.) in firmware.

-- Dave Tweed

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