1 of my upstairs radiators has suddenly stopped working, and now it's permenantly cold. It has a thermostatic radiator valve (Honeywell) and was working fine until Friday. Now it won't heat up regardless of the setting of the TRV. The pipes into the TRV are also cold, as well as the outlet pipe. I have tried bleeding the air from the radiator, but there was no air in it to bleed - all that came out was clear water. I also tried turning off all the other upstairs radiators, but it made no difference. All of the other radiators seem to be working fine. Is it more likely to be the TRV that's failed or a blockage in the pipe (it is a hard water area). Is there any way to force a stuck TRV to open manually? The house and entire heating system are 5 years old.