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I have recently replaced 9 rads on my Combi system. Most have been replaced in the same positions apart from 3. Downstairs a large curved bay rad has been replaced with two smaller rads on the opposite side of the room. In another downstairs room and in an upstairs bedroom one rad under a window has been replaced with two smaller ones either side of the window. I have put TRV’s on every rad. When the system is turned on everything works fine until the rads reach their individual temperatures. The rads then cool down but then don’t re-heat when the room goes cold. Thinking the TVR’s may be faulty, I have replaced all of them with a better quality TVR – no change. I have drained the system endless times which has made no difference. I have done this type of work many times before and had no problems. Should the rads heat up again when the room temperature cools down? The rads are on no. 3 and there are no room thermostats. Can anyone come up with any suggestions what the problem may be.

TVR is a make of a rather nice but expensive car.

TRV stands for Thermostatic Radiator Valve, I know it doesn't matter but now you know.

yes when the radiators cut off because they have reached temperature, they should then re-open when it cools down.

It could be that the TRV's are faulty, but I would first check the TRV heads are on correctly and are not screwed down too tight. The pins may become stuck. You don't need any grips etc screw on loosely just hand tight will do.

Also I have had problems with valves where when set below 3 they don't work correctly.

Try turning the valves up to 4 or maybe 5 and see what happens - they may work better. Obviously they are there for comfort and energy effieciency so you shouldn't have to set to 4 or 5 and leave there. If you find it works on 4 or 5 but not on 3 take them back to where you bought them.

this sounds very strange dosnt sound like your trvs as you have changed them i would look towards the boiler maybe introducing extra rads the boiler is struggling is it turned up full if so try just balancing them turn them all off and open one at a time slowly see if one stays hot then try the second etc sorry i cant be of any more help here but never heard this one before

good luck mate

COASTER if that was the case and the boiler wasn't coping, how would they all heat up, then cool down ?

If the boiler was struggling as you say - the radiators wouldn't get hot in the first place.

sorry engineer i think i worded it wrong there i meant boiler getting up to temp then cutting out

This surely is a strange one.

When the rads are cold and you think they should be warming up, is the pump running, and are the flow and return near the boiler hot?

When you say you have fitted TRV's to ALL the rads, is it possible that when all the rads reach temp the water flow in the system is stopped entirely, and the boiler is overheating?

Open two or three valves fully, and see if they continuously remain hot and if so see what happens to the remainder.

It is normal practice to have one rad. with simple irv's, set wide open, in the hallway, where you also fit a wall thermostat, to cause cycling of the boiler wrt rads.
Also, I have found that a setting of just 3 on a trv is barely requiring heat ;-- I set my bedroom trv to 3 1/2 in chilly weather, but leave a window cracked open, as well as 2 vents.
The thermostat in the hall gets left at 21 c.

YOU SHOULD NOT FIT TRVS TO ALL RADS ONE MUST HAVE LOCKSHIELDS TO BOTH SIDES TO REMAIN OPEN TO PROTECT THE PUMP

Many thanks to all who replied. Happily, problem now solved - massive air lock.

Thanks again, envapyms :D