I would very much welcome any advice before having to call a plumber out; my problem is I have 2 rads (bedroom & ensuite) that won't heat up and I have exhausted my knowledge. I have carried out the following fault finding in an attempt to identify the problem but to no avail: With both rads removed, I have opened up the TRVs and Return Valves and I get hot water coming out of the pipework so I know hot water is getting to the rads. I then replace the rads, fill them up, bleed them, they heat up but then go cold and will not reheat again. Thinking the rads might be blocked I have given them a good hose pipe clearance in the garden and no sludge came out. I have even swapped the affected rad with a good one in the hosue and it works elsewhere so I know the rad is OK. With an affected rad in place, I loosen the return valve and the rad begins to heat up and draws hot water, at the same time, the other affected rad also heats up. As soon as I connect the return valve and tighten up all connections and re-bleed, the rads go cold. I am baffed as to what it could be (or just stupid and am missing something obvious?) All suggestions gratefully received, thank you.