Question about radiators and TRV

9 Apr 2013
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United Kingdom
We recently had a new boiler and controls (both Vaillant) installed, we also got TRVs on all of the radiators except the one in the room where the controls are - we were told this is because it could interfere with the thermostat. We were also told that one of the radiators must be left without a TRV, something to do with it protecting the boiler (because of the pressure or something) but I can't remember exactly why.

Anyway, we are having a problem because the room in which the thermostat is placed gets warm more quickly than some of the other rooms in the house and stays warm. This is because it is well insulated (we have external wall insulation at the back of the house and recently had new windows at the back too), and because there is no TRV on that radiator it is always hot when the heating is on. So when that room gets warm, the heating goes off, but the other rooms in the house are too cold, especially at the front where it is not insulated and we haven't yet replaced the windows.

The only way I can get a comfortable temperature throughout the house is to turn off the radiator in the room where the thermostat is, so this room doesn't get as hot and the heating stays on for longer. There isn't really anywhere else I can put the controls, so they need to stay in that room.

My question is, it is okay to turn this radiator off without damaging the boiler? If so, do I need to keep a different radiator turned on fully? The others all have a TRV but I am assuming if I set it to the maximum position, that radiator will always be on.

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You just have to ensure that at least one other radiator is always turned on, then the pump will not be stressed by being unable to pump the water around the system when all the other TRVs are closed.

Maybe choose the room which seems hardest to heat at the moment and if you want to play safe you could remove the TRV top cover so it can't easily be turned off.

To do things properly the thermostat really should be in that same room and if moving the cables is a problem you could try installing an RF thermostat, there are even portable ones which can be carried around the house.
Thanks for the quick reply, it has confirmed what I thought. So if the other TRVs are all completely turned off, one radiator needs to be left on. What if the other TRVs are partly open, does the same apply?

Also, does that one radiator need to be on fully to protect the boiler, or can the TRV be set to 4 for example (5 is the maximum)?

We do have wireless controls but we have had some problems with the signal. It is better since we hardwired the radio unit into the boiler, but I can't move it further away (e.g. in the hall or the lounge) because the signal gets too weak. And if I move it closer to the boiler it will be in another of the back rooms, all of which are pretty warm.

So I think I'll leave it where it is but turn the radiator off, that seems to work pretty well. I just didn't want to risk damaging the boiler.
As long as you can guarantee there will be a path open through one or more radiators the pump will be happy. The only way to be really sure is to have one radiator where the TRV can't possibly be closed. Even one set to 4 (or 5 !) could close if the room was very warm.
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Thanks - so in theory I can have the TRV partly open, and the pump will be okay? As long as it isn't completely closed?
No I'm afraid you can't guarantee it will be OK because if the room warms up it may close. That's why there should always be one radiator without a TRV.
We recently had a new boiler and controls (both Vaillant) installed, we also got TRVs on all of the radiators except the one in the room where the controls are - we were told this is because it could interfere with the thermostat.
The installer was correct about a TRV interfering with a thermostat. If the TRV is set to a lower temperature than the stat, the room will never get up to the stat temperature, so the stat will never turn the boiler off.

Do you know if the system was balanced? The installer would have gone round adjusting the lockshield valves (opposite end from TRV).

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