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semi-OT: Weird thermostat?

Awl --

A perhaps inneresting electrical teaser:

I've got a chronothermIII thermostat on a gas furnace.

I found the two wires for the furnace (as opposed to the two wires for the A/C), and observed that the III actually energizes these two wires with 24 V -- iow, there does not seem to be an external or furnace-supplied 24 V for the heating circuit. ie, the III is not simply acting like a mercury switch, it is actually supplying 24 V power to the furnace controller.

Ergo, if I disonnect these two wires, and jump them, nothing should happen, and indeed nothing does. In an old mercury-type stat, jumping the wires

*would* kick in the furnace.

OK....

BUT,

If the wires are disconnected from the III, and I energize them with a separate 24 V, the furnace does not go on either!!

The furnace will only go on if the two wires are receiving the 24 V from the III.

Could the two wires for the A/C somehow be mixed up in this? Those wires DO have an external 24 V transformer, which the III "knows" about, as it will register a " -AC " on the lcd if those two are disconnected..

But still, with the furnace wires isolated (off the III), either jumping them OR providing them with 24 V should kick in the furnace, right? But no go.

Iny idears?

Reply to
Existential Angst
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The Chronotherm III that I installed years ago replaced a mercury thermostat. Perhaps these will help:

Model overview:

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Owner's manual
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Installation manual
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-Brian

Reply to
rtandems

My guess is that the transformer for your ac is also the transformer for the furnace. You probably have jumper or a common connection screw supplying the power.

Simply attaching a 24vac transformer at the thermostat side probably would not work. The wires to your thermo are just switch legs. Attaching a transformer here would run half the power backwards through the original transformer.

Paul K. Dickman

Reply to
Paul K. Dickman

That has indeed proved to be the case! I believe one of the "furnace wires" goes nowhere, and that the 'stat is really a three-wire system, with one side of the xformer being the common. So I was jumping the furnace wire and the nowhere wire, instead of the furnace wire and common wire, which I thought was just for the A/C.

But, as Stan alluded, soon they'll make these things impossible to navigate, with digital encoding etc, much like how repairs/adjustments on our cars are near-impossible.

If they duct-taped us and hung us from a hook, we proly wouldn't notice the loss of freedom.

Appreciate the replies.

Reply to
Existential Angst

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