Anyone know how a boiler thermostat works?

[Konan555]

Google failure...

I think the stat in my old boiler is a bit out of whack. It's heating the water over 100 even on the lowest setting. I'm wondering how the thermostat actually works.

As far as I can see, it's going to work one of two ways.... I think



a) The copper wire conducts the heat back to a temperature switch within control unit

b) The copper wire is in fact a tube and there's a pressure sensor in the control unit with a gas bulb located in the boiler at the other end.

Either way, it looks like a new one is worth a go. But it if works as described in a) I'd know it was broken as the copper wire/pipe/tube isn't hot to touch.

[peterb]

The copper wire is a tube, and the expansion of the working fluid (where fluid includes gas) with the increase of temperature operates a pressure switch in the main unit.

You might be able to test it by immersing the sensor in hot water and either listening fir the click as it operates or using a continuity meter on the connections.

[Konan555]

Cheers,

I've found one with a matching spec for £15, so I think I'm going to test it by replacement. It's been in the boiler for at least 36 years after all

[Domestic_Ginger]

Is it the heating that is getting too hot or your hot water from a tank?

Water temperature is usually controlled by a thermostat on the outside of the hot water tank; these usually come loose and that is the reason your hot water is too hot.

[Steve]

Yes, what Domestic Ginger said, unless you have a combi boiler, of course.

I don't know what the common failure mode for thermostats is, but one would think it would be open circuit, then again, that would be sensible, so who knows...

[peterb]

I think the failure mode of that particular thermostat would be closed cct - the stat opens to switch off the boiler when the required temperature is reached. If the working fluid leaks out, then the contacts will never open.

The tank stat usually controls a motorised valve or a pump which pumps the hot water round the system. In the case of a valve, it may have auxiliary contacts that control a pump or the boiler, but it depends in the system. If the boiler stat has failed, the water in the top of the tank will get very hot, simply because the water in the primary loop is much hotter than normal. (I am assuming a normal indirectly heated water system which seems reasonable, given the age of the system)

[Whiternoise]

Anyone know how inefficient this is compared to a PID+thermocouple or similar based system?

[peterb]

I'm not sure if efficiency is the right term!

It is certainly effective for a simple boiler controller, but a more modern boiler might benefit from the the more precise temperature measurement that a thermocouple/thermister detector would give, but only as part of a more sophisticated control system. It would make no difference to the OP's boiler, apart from adding unnecessary complication, and potentially less reliability.

[Whiternoise]

I'm not sure if efficiency is the right term!

It is certainly effective for a simple boiler controller, but a more modern boiler might benefit from the the more precise temperature measurement that a thermocouple/thermister detector would give, but only as part of a more sophisticated control system. It would make no difference to the OP's boiler, apart from adding unnecessary complication, and potentially less reliability.

Oh, not suggesting a refit!

Thinking more that you're probably going to get less temperature over/undershoot with a simple on/off device - though obviously it's going to be more than good enough for heating a house. In terms of efficiency, I was just thinking about whether it would save money to have a system that can accurately nail a set temperature.

More importantly the geek in me wonders when I have a boiler whether building my own gizmo would be more fun (not controlling the boiler, mind, more what's being injected into the house).

[kalniel]

More importantly the geek in me wonders when I have a boiler whether building my own gizmo would be more fun (not controlling the boiler, mind, more what's being injected into the house).

There's probably an app for that

[Konan555]

Is it the heating that is getting too hot or your hot water from a tank?

Water temperature is usually controlled by a thermostat on the outside of the hot water tank; these usually come loose and that is the reason your hot water is too hot.

It's the boiler itself getting too hot. It's starting to kettle a lot (which it's a little prone to doing at the moment anyway... needs a proper flush out this summer and a possible descale on the heat exchanger. No sign of corrosion on the thing so it should take it!).

There are no other electric thermostats or valves in the house. Hot water is gravity and the radiator pump is just switched with the boiler.