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Atheist, in answer to your questions : We have been on holiday and when we returned the house was cold so we put the heating on to warm it up. There are only 2 underperforming rads, both in the lounge.One gets reasonably hot the other just gets warm. I had an engineer from Corgi attend last September who diagnosed a stuck TRV. He took the top off of both valves (one on each rad) and constantly pressed the brass piece that was visible, up and down for about 5 minutes. He shut off every other rad in the house and then fired up. All was ok until recently when the one rad in particular didn't really get very warm. Pressure does not apply as we do not have a Combi boiler, ours is a condensing boiler.
FBG40