Thermostat dial is the wrong way around

[Matty F]

I have a Cotherm TSD thermostat for my hot water cylinder that has been staying on indefinitely, i.e. the water is getting far too hot. When I turned the dial from end to end, there was no clicking sound, so the contacts were closed all the time (which I tested with a resistance meter). Since I've always wondered how these things are supposed to work, I took it apart and cleaned the contacts. There's a wire that goes down the tube that expands to open the contacts. I suspect that wire has stretched over the years. When I put it back together (there's only one way of doing that), it works fine except that when I set the temperature to 30 the contacts are open circuit and for temperature 80 the contacts are closed. So it's working the wrong way around. However if I just set the temperature to the middle, the contacts open and close correctly when the temperature varies. So the dial is the wrong way around. When I first installed the thermostat I set it to 55 and that seemed about right, so I never noticed a problem. Surely it's not possible for the manufacturer to label the dial wrong, but I can't see anything I have done wrong,apart from taking it apart that is, but this is a DIY group!.

[PeterC]

If you bring it to the Northern Hemisphere it'll be OK.

[Max Quad]

ISTR some thermostats have a switch to tell the various controllers that the required temperature has been reached. There are labels CALL and SAT. I assume SAT is short for 'satisfied', and this would become closed in the way you describe. If there is only one switch, then by default, I think it will be CALL (i.e. turn the heating on).

[Roger Mills]

Are you sure that it isn't a changeover switch with 3 contacts? If so, the central (common) contact will be connected to one of the others below the set temperature and to the other one above the set temperature. This enables the stat to be used for both heating and cooling applications (or sometimes for more complex heating arrangements). Which way it operates depends on which of the contacts you use.

If you get that wrong, it still won't work properly even at the mid position because it will never switch on when the temperature is too low.

[Davey]

Surely:

If you set it to a lower setpoint (30) than the ambient temperature, the contacts are open, so it's not telling the heater to heat. If you set it to a higher temp. (80), the contacts close, and tells the heater to come on. Sounds correct.

[Mathew Newton]

Sounds fine to me! Why would you expect it to work the other way round?

[Ericp]

Are you using the aircon contacts?

[Matty F]

Yes of course you are correct. I just had a mental block. It's working fine now. I only looked inside because the contacts were always closed. Perhaps the metal rod slowly stretches over 5 years, although it seems to have gone wrong over a few days. The water from the tap was much hotter than usual. I have now fitted another thermostat.

[Davey]

I had a similar problem, and it was caused by the zone valve on the side of the hot water tank staying stuck open, so it never told the boiler to shut off. The only control was the boiler's internal thermostat, the result was much hotter water at the tap than normal.