This thermostat I have has a mechanical relay. But.... When closed is a
closed circuit or dead short, fine that works for me. BUT when it's open
it still has 20 ohms and will not shut off a circulator. Why in hell do
they make it have 20 ohms when it's open?????? It should be infinity.

I suspect that might be the resistance of the "anticipator"... a small
built-in heater, which is normally energized when the furnace is
running. The heat it generates, helps cancel out the effect of
thermal lag (i.e. the room air heats up faster than the bulky
thermostat does) and improves the ability of the thermostat to maintain
good temperature control.

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Dave Platt AE6EO
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