Damp patch under radiator. Possible causes?

Oakeshott

I noticed a patch of damp 50cm in diameter around the pipe on the left of my living room radiator this morning. What could the cause of that be? (I’d rather try and fix it myself before calling out a plumber.)

tacpot12

It's likely to be a leaking valve, but it could be on any of the joints on the valve (including where the radiator tail is screwed into the radiator) or the valve itself. If you dry the valve carefully with some tissues, and then use a torch, you are likely to be able to see the water glistening as it emerges from wherever it is coming from. 

Another technique is to put some tissue down on a peice of plastic over the damp patch and watch for the first drip hitting the tissue. The water might be dripping, in which case it is likely to fall vertically onto the tissue and you just have to look up to see where it is coming from, or it might be running down the pipe, in which case the technique doesn't provide any insight.

This video will probably help: https://youtu.be/56QcMmp0VNQ

Oakeshott

Thanks. Turned out to be leaky thermostat. I managed to fix it by tightening the nut.