Thermostat Design

by George Lyle

The thermostats work with a liquid (probably an alcohol) that is a liquid below the thermostat's set temperature and a gas above that temperature. When the temperature drops below the set temperature, the alcohol attempts to liquify. This lowers the pressure inside the thermostat, which causes the atmospheric pressure outside to push the can to the collapsed position. Thus, the thermostat uses atmospheric pressure to push it to the closed position.

The partially collapsed thermostat wasn't caused by having too much alcohol. The amount of alcohol is not terribly important to the operation of the thermostat, as long as some is present and (important part here!) no other substances are inside the thermostat.

My theory is that this thermostat has a tiny leak and that air has found its way into the thermostat through that leak. It's unlikely that the alcohol would come out, since there is a partial vacuum inside the canister when it is cold (the canister is new, and never warmed up).

The air in the canister would never liquify, so the canister would never get any smaller than the air volume inside. The alcohol in the canister would expand the canister to its correct length if it were heated.

I'd hate to try to fix one. You'd need a vacuum pump and the correct alcohol, along with some way of opening and sealing the canister.

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