How do thermostats work?

[Sunrise6875]

I am aware this is a very stupid question!

We have quite an old central heating system - fairly new boiler but the old water tank situation, so not a combi. There’s a thermostat in the hallway which seems to glow when the radiators are on.

The heating is set to a timer - however sometimes I find that even though the central heating is allegedly on (the screen tells me it is and it’s the right time), the radiators sometimes aren’t on?? Is this because the temperature isn’t low enough?

I’ve got the thermostat set to about 20/21. Today I cannot get warm - the heating came on as normal first thing and was toasty when I got up. I pressed what I thought was the button to keep it on for longer but I don’t think it happened - maybe because it is too warm??

About an hour ago I tried to boost the central heating by putting it on again but the radiators have only just turned themselves on and now the house is warming up. Is this because the house was already at 21 (didn’t feel like it!!) and has only just dropped below, triggering the radiators?

Please be kind!! We haven’t lived in this house long and I’ve previously had combi boilers that seemed a bit more straightforward.

Oh and boiler was serviced last week and all working fine apparently!

I think your right. I think it's because it's already at that temperature.

yes. You set the thermostat at the temperature you want to be. If its warmer than that, then the heat wont come on.

We are getting used to having a thermostat that actually works and it had me confused for a while!!

Incidentally our house had 2 thermostats and neither worked, but the boiler service didnt check those, so it is possible yours don't work.

Maybe buy or borrow a stand alone room thermometer (theyre cheap) so you can double check what the tempersture actually is.

That's exactly how a thermostat should work. Think of it like an override to whatever other controls you have. If you set the thermostat to 21deg it will switch the heating off once it reaches that temperature.

We have a nest smart thermostat, but the theory is the same, just without the timer. To check how yours is working - if you set the timer to be on all the time and set the temperature on your thermostat it should be controlled.

there are a load of things to watch for - if the thermostat is in direct sunlight, or very close to a radiator (or directly above one) the thermostat can heat up while the rest of the house is cold, this can stop the heating from warming all the other rooms up - to combat this you may need to turn the heating near the thermostat down (TRV valve on the radiator) ton 'balance' the heating across the house.

I think previous posters are right. Our thermostat frequently says 21 but it doesn't feel like it, so we up it

It doesn't help the thermostat is in the hall where the sun shines through and we think it overides the temp

We have a thermostat in the hall.

However we also have thermostats on every radiator in the house. This means we can control each room separately (with hall thermostat set v.high).

It depends where the thermostat is.

If it's near a radiator or in direct sunlight, it will be "tricked" into thinking the house is warmer than it is, so the heating won't kick in when it should.

We have ours set at 18 degrees and it's on at the moment.

We were always cold in our house even though the thermostat in our hall was set to 21 or so.

Someone then bought us a digital thermometer (the kind to keep on your sideboard). It turned out that our hall thermostat was about 4 degrees wrong so 20 was really 16. We put them next to each other.

No wonder we were cold! It had seemed extravagant to turn the thermostat up to, say, 25 but as soon as we did (bearing in mind that was really about 20/21) that, it was all fine.

I asked the guy servicing the boiler to adjust the thermostat in the hall to show the real temperature but apparently I'd have needed a new thermostat.

So OP, buy or borrow a digital room thermometer and check if the temperature on your main thermostat and the digital room thermometer match. Adjust accordingly.

Thanks all - i am going to get a digital room thermometer, great idea.

The hallway one isn’t near a radiator really or direct sunlight but I suppose it is quite old so might not be working too well!

A thermostat does a really simple job. You set a desired temperature (e.g. 21 C) then if the air temperature is below 21 C it will turn on the heating supply to the radiators. When the air temperature reachers 21 C it will turn off the heating supply to the radiators. That's all it does*

The most common reason why people become disatisfied with what it's doing - is because the thermostat is measuring the temperature somewhere other than where you are spending time!

So - it might be an area that tends to warm up more easily than the rest of the house. Or an area that always tends to be cooler than the rest of the house.

In your situation - the air temperature near the thermostat might reach 21 C (switching the heating off) - but in the room where you're spending time it might still only be 18/19 C.

The solutions are - either move the thermostat to be closer to where you want most control over the temperature. Or just acknowledge that - perhaps to get the temperature up to 21 C in that room, you might need to set the thermostat (postioned somewhere else) to something higher (e.g. 22 C).

*Some people think if the house is really cold (e.g 12 C) and they want it to be 21 C that if they set the thermostat to a high value (e.g. 27/28 C) that somehow the boiler will "work harder" and put out heat "full blast" to get there. Doesn't work like that - it's just an automatic on/off switch. It'll just turn the heating on until the air heats up to the temperature you want. If you want 21 C set it to 21 C.

Is your hot water on? My hot water and heating have separate controls but I can't have heating without the hot water being on too.

You should consider the Google Nest one. See here for documentation: support.google.com/googlenest/answer/9248184?hl=en

If you are getting maintenance/service done, ask if it is compatible. It's very easy to use.

Don't forget. the thermostat is only measuring in the room it's in, so if that room is warmer than the rest of the house it won't click on.

Oooops cross post.