Thermostat vs Radiator Valve

[itsthomas]

I have never lived in a house which had a thermostat to control the heating.
Soon we are moving to a house which has a thermostat to regulate the temperature in the house.
But I don't understand that if you can control the temperature via a thermostat, what's then the function of the radiator valves?
Let's say I set the thermostat on 21 Celsius, the radiator valve in the bedroom on 1 and the radiator valve in the living room on 5.
Would both rooms still have the same temperature of 21 Celsius? or the living room temperature would be more than 21 Celsius?

[MrVacca]

Hi @itsthomas - I'm not 100% sure, but I think the valve just limits the force of the 21 Celsius air that is being pushed into your home. The heat that will be consistently pushed out with be at that temperature that you set, it's just how fast it gets pumped into your room. 

Hope this makes sense!

Bill

[MartinJEvans]

You presumably won't have a thermostat in every room and just have one in a single place. When you set the temperature at the thermostat and it is too cold your heating will come on and heat will be pumped around your whole house and the heating won't go off until the temperature reaches the target you set in the room with the thermostat. Thermostatic Radiator Valves work locally on each radiator so if you set them to say 1 they close the valve to the radiator at a lower temperature than if you set them to 5.
For instance, we have a thermostat in the kitchen but TRVs on most radiators. The thermostat in the kitchen switches the boiler on when it is too cold and starts pumping hot water around the radiator system but we like the bedrooms cooler so we turn the TRVs down in bedrooms. We even have an intelligent TRV in the bedroom that closes the radiator valve in the evening until after 10pm when it opens so IF the heating is on, a little heat is added to the bedroom before we go to bed.

[itsthomas]

Thanks for your responses. I'm starting to understand a bit more. But now I have another question.

Let say, I set the main Thermostat on 18 Celsius but I want to have it warmer in the bedroom. Now if I set the heater's TRV in the bedroom on maximum level, will my bedroom temperature go behind 18 Celsius? Or because the main Thermostat is set on 18, the maximum temperatur that each heater can achieve is only 18 Celsius?

[MrVacca]

Hello again @itsthomas - From what I understand from the amazing @MartinJEvans' explanation, I think the maximum temp each heater would be able to achieve is 18 C.  

[MartinJEvans · Accepted solution]

Let me try and explain again.

Assumptions: you have one thermostat say in room1 and TRVs on most radiators

Let's say your heating is turned on, that is in a position to fire up should there be demand for heat. Now let's say you have your thermostat set to 18 but it is 19 in room1 where the thermostat is, then your boiler does not fire up so you have no heat. Now lets say the temperature in room1 drops to 17, the thermostat turns the boiler on and a pump pushes hot water around your heating system. Now what happens as to which radiators get hot or not is down to the individual TRVs. Let's say the TRV is room 2 is set low or off or frost protect, then the valve is closed and no heat goes into the radiator so you have no heat in room 2. However, room 3 has the TRV set high or the maximum, then the valve to that radiator is open and the radiator gets hot. Now lets say the radiator in room 1 where the thermostat is located is small and doesn't push out much heat so the temperature only goes up a little and never reaches 18 so your heating stays on. The hot water flows through the radiator in room 3 all the time and it gets hot. It could get ALOT hotter than 18 in that room because heat is circulating through the radiator all the time because room1 where the thermostat is located hasn't reached 18 yet so the heating stays on and the TRV in room 3 never closes because you have it on maximum (which is likely to be a lot hotter than 18). Any rooms where the TRV are set to say 20, will close the valve in the radiator when it reaches 20 so they won't get any hotter than that whilst the heating is on (not quite true as the radiator continues to radiate heat for a while after the valve closes).

Just think of the thermostat as a switch which turns your boiler on when it is too cold in that room and off when it reaches your desired temperature. IN THAT ROOM. The individual TRVs control the valves in those radiators to open or close the valve at the temperature you set but if you set them very high and the heating stays on the room could get a lot hotter than the room where your thermostat is located.

It is a bit more complex than my slight simplification because I'm assuming your radiators are balanced i.e. the valves at the non TRV end are adjusted so there is a specific drop in temperature between the inlet and outlet. If you don't do this you can get cold spots in your house because all the hot water is used up in radiators nearer the pump. If you balance them (turn the valves in a bit to reduce the water going through them, then more hot water can get to other radiators.

So in my house I have a nest thermostat in the kitchen (presently - we spend a lot of time there). In all the bedrooms I am not using I have turned the TRV to the frost position and close the doors. In my main bedroom I set the TRV low as I don't like a hot bedroom but that still irritates me as when I'm sat in the living room I'm also potentially heating the main bedroom if it is cold. For that reason I have a programmable TRV which works like a normal one re temperature but also has a clock in it which I can set to keep the valve closed until 10pm to put a little heat in the bedroom nearer the time I go to bed but not heat it whilst I'm in the living room in the evening.

Hopefully this is clearer.