One Cold Radiator In Extension

I was asked to a look at a radiator recently that had not been working since the heating had been turned back on. The radiator was upstairs in the master bedroom which had been extended a few years previously and the radiator relocated. All the other radiators were nice and hot, and there was 16 other radiators. The customer had tried bleeding the radiator and to remove the head of the thermostatic radiator valve, but neither of these attempts got the radiator working successfully.

I removed the remainders of the head from the thermostatic radiator valve and pushed on the pin which was moving up and down perfectly. I then checked that the lockshield valve was fully open at the opposite end which it was. If there was an issue with balancing or circulation I would have expected one of the bigger radiators below to have been affected rather than an upstairs bedroom radiator.

I ensured the room thermostat was turned up fully and went around the house turning off all the other radiators. I checked back at the problem radiator and it was getting hotter to the pipe but the water was still not circulating through it. I looked in the airing cupboard and the system had two zone valves fitted and a bypass. The hot water was satisfied and the zone valve was closed accordingly, and I closed the bypass off. As soon as I turned off the bypass the full force of the pump pushed some air out of the pipework to the radiator and I heard it gurgle. Almost instantly the radiator was hot all over, and so I went around and turned all the other radiators and bypass back on.

The radiator continued to heat with the rest of the system up and running. I expect that over the summer air settles in the pipework to the radiator and prevents the circulation around that radiator. I expect when the radiator was relocated the installer used plastic pipe which because of the way it is often coiled never lies completely flat and leaves high points in the pipework for air to collect in.