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Having moved house, we find one particular radiator doesn't heat up, despite the pipes that lead to it getting hot. One of the valves must be stuck, and I've worked out which one by feeling the pipes either side.

But how to unstick it? I've whacked the end with a hammer, and repeatedly twisted it round and round as far as it will go each way (it does rotate a little) but not had any luck.

EDIT

Finally, I replaced all the valves. Looking at the old valve, it appeared to work fine, which puzzled me. Switching the boiler back on puzzled me even more - the radiator was as lukewarm as before!

Finally we realised all the hot water was flowing through the radiator immediately upstairs of the problem one - there must be some aspect of the pipework which means the water just can't be bothered to flow vertically downwards and back up for 7ft as well. If you turn the upstairs radiator valve right off, then the downstairs radiator will get almost as hot as the others. Almost.

Other downstairs radiators in the house have the same pipework coming down from the ceiling, and they get very hot. What could be different about the problem radiator?

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Is the inlet valve thermostatic? –  Jamie Oct 17 '12 at 12:15
    
Oops, forgot to say it's the old-fashioned type, not thermostatic. –  MerlinMags Oct 18 '12 at 13:04
    
There is sometimes a "lockshield" valve on the opposite side which must be open as well. You often need some sort of key to operate it. It's normally used to balance the system. –  bcworkz Jan 23 '13 at 22:23
    
Turns out it wasn't the valves at all. Arg! –  MerlinMags Jan 24 '13 at 17:43
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3 Answers

I had a radiator in one of my old houses that didn't heat up. I thought it was a valve, but it turned out that it just needed bleeding. There was a lot of air in the radiator, and the water couldn't get into where it needed. If you haven't already, bleed it.

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Yup, I've bled all the rads. –  MerlinMags Nov 1 '12 at 18:52
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If there is no resistance felt while rotating the valve handle and just goes from stop to stop, the valve is not functioning and is likely frozen closed - if it were open you'd have heat. That inlet valve probably should be removed and replaced.

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Answer to edit question:

The characteristics you describe could just be a variation of how the pipes are connected. Water tends to flow straight through a tee rather than to it's side outlet. Typically the overall flow and pressure of the system ensures flow is more or less equal straight through or through the side.

So first off, ensure your system is properly filled and at proper pressure. If that checks out, partly restricting the flow of the upstairs radiator can cause more heat to go down to the overly cool radiator. The idea would be to balance the flow so there is adequate heat from both radiators. This sort of tuning is where the lockshield valves are utilized to good effect.

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Yes, it is some oddity of the pipework. But no matter how I set the upstairs lockshield, the radiator below never achieves a decent temperature like all the others! –  MerlinMags Jan 26 '13 at 10:33
    
The problem may be more complex than the two radiators. The rest of the system may be taking so much of the supply, there's only enough left for one radiator. You may need to choke down the rest of the house so enough water goes to this one branch in enough volume to be useful. It's sounding like the circulating pump may be under sized, so the whole system will need to be finely tuned to get the best performance. –  bcworkz Jan 26 '13 at 20:35
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