Central Heating

[Bazile]

How does a room thermostat work in conjunction with the radiator thermostat ?

[nicebloke1]

Independantly.

[nicebloke1]

Once the radiator reaches lets say number 3 it will shut off regardless of what the room thermo is set at.

[Peter Pedant]

radiator therm dicatate how hot the radiator gets - I set mine low

Room therm senses the room temp and tells the boiler to go on or  not

and yes it is poss to set your radiators so low ( which I do intentionally) so that the  boiler is hardly ever heating

I  also have a gas fire - heating is a way of  life ( open flame etc, very victorian) and not just " is it warm enough"

[Canary42]

The first one to detect temperature reached will cut out. In the case of the radiator it will only cut that radiator, in the case of the room thermostat it will (usually) cut the total system (i.e. the boiler and pump).

[netherfield]

Room stat should shut the whole house off, or in more modern  systems shut a section off, e.g. upstairs or downstairs, radiator stat just shuts that radiator off.

[nicebloke1]

The idea of radiator/ temp valves is so you can have high temp in some rooms and low in others.

[New Judge]

Quite simply, it doesn't.

The room thermostat will turn the boiler on or off in accordance with the temperature it is set to do so. If you have a TRV (Thermostatic Radiator Valve) in the same room the only time it will have any effect is if it is set to a lower temperature than the room thermostat. It will then turn the rad off before the room thermostat shuts the boiler down. If it is set higher, the boiler will be shut off before it reaches that temperature.

The two work independently.

[Bazile]

So if you want one room to be at 10° and another to be at 15° and the room thermostat is in the room for the 10° then the other room will not reach 15° ?

[nicebloke1]

The boiler will not switch off until the room set at 15 reaches 15.

[New Judge]

"So if you want one room to be at 10° and another to be at 15° and the room thermostat is in the room for the 10° then the other room will not reach 15° ?"

Correct.

I installed my central heating system (apart from connecting the boiler to the gas supply). I fitted TRVs on every rad and never had a room thermostat. When I had a new boiler installed  few years ago I was told a room 'stat was mandatory under the regs. (Climate Change, don'tcha know). So I simply set it to its maximum on day one and haven't touched it since.

[New Judge]

"The boiler will not switch off until the room set at 15 reaches 15."

Not if the room stat set at 10 controls the boiler. 

[Canary42]

Sorry nicebloke, but I don't agree, my understanding is that the room thermostat set at 10 will switch the heating off when the room reaches that temperature, so the other room will not reach 15.

[Bazile]

Thanks 

What temperatures corresponds to the numbers on the trv ?

[bhg481]

Canary - not necessarily. If the room set to 10 has a very small radiator and the window left open and the room set to 15 has a very large radiator and all windows closed the 15 room will heat up more quickly and can reach 15 before the other room reaches 10. When the 15 room reaches 15 the TRV will shut off the radiator but the 10 room will continue to heat until the room stat turns the whole system off at 10 deg in that room.

[nicebloke1]

In practice there should be one radiator without a TRV, the room stat should be near or in the same room. Mine is in the hallway the coldest part of the house in winter.

[bhg481]

nicebloke - it's also common practice for the non-TRV radiator to be in the bathroom. If the room stat is set very high the pump will continue to run pumping water to all the radiators. If all rads have a TRV and they all close the pump will be running and pushing against a closed system - not good. By leaving one radiator always open without a TRV there is always a release for the pumped water. This also a reason the bathroom is often chosen for that radiator.

[Bazile]

Sorry , why is the rad without the trv sometimes chosen to be in the bathroom any different to the hallway ?

[New Judge]

"In practice there should be one radiator without a TRV, the room stat should be near or in the same room. Mine is in the hallway the coldest part of the house in winter."

Nothing wrong with that. It's unnecessary, though. and simply introduces a complication to the control over your heating . There's no reason why you should not have a TRV on every rad. 

"If all rads have a TRV and they all close the pump will be running and pushing against a closed system - not good."

That's why closed systems should either have a bypass fitted or (more common these days) have one incorporated in the boiler itself.

[nicebloke1]

21.50. Correct explanation of why one rad should never be closed off. Spot on! 

[nicebloke1]

Also spot on NJ.