Radiator hot at top, cold bottom - why?

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Tony_Park | 22:44 Thu 02nd Dec 2010 | DIY
38 Answers
Hi, we have 1 rad (double)which is affected as above.

All other rads are fine, system has only been filled since Aug, when a couple of new rads were installed.

Cold rad has been drained today, no sludge present - water is nice and clean.
Rad has a new feed valve, but looks like plumber re-used an old one on return. Thermostats show water entering at 50+ oC, and return shows room temp. Temp seems hot across top, and warm centrally hightwise, and warm down side of rad on return.

Anyone any thoughts on why this is happening?


What could be causing this problem? I'm wondering if the return valve is faulty.


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some sort of air lock perhaps.....I am sure there are some pro plumbers here who will give free advice.....
See here for previous threads on the same problem http://www.theanswerb...1=radiator+hot+at+top
It does sound like an airblock, have you bled the radiator?
Question Author
Radiator has been bled, def no air, and that would cause rad to be cold at top, not bottom

Try getting the flow up a bit. Are the rest balanced?
Try turning a few off to test, then see what happens.
Reading the OP again, you already had problems and draining the rad and new valve(s) didn't cure it? Am I reading it right?

Was the rad just drained, or removed and flushed out?

Sounds like there might be a blockage in the rad.
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Rad was just drained, would have thought if it was a blockage, could just affect one of the panels, but both are the same.

Other rads have been quickly balanced, plan to complete it at the weekend
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One valve wad replaced when rad was refitted in Aug, other one is an old one
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Last night I turned all other rads off, no change and both valves are fully open
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Also, if it were a blockage in the rad, would the return valve also not be hot?
Assuming one of the valves is a TRV (thermostatic rad valve) .......... I don't suppose there's a chance it's been fitted on the wrong side?
I always use "reversible" valves ......... i.e. they can be fitted on either the flow or the return. There are still valves around that are one-way .......... (usually with an arrow on them). These can only be fitted on the flow side............... ???
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thanks, no, neither of the valves are thermostatic :-(

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What baffles me, is why the flow valve is too hot to touch, but the return one is only just warmer than room temp - I would have thought, if it were a blockage in the rad, that the return valve would also be hot - or am I misunderstanding the problem?

If you tried what I suggested and reported back with result .. it might help you.
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Radiators are balanced now - no joy.
Flow was on max already.
Turned off other rads, no change.
Return valve now replaced-slight improvement.
Now have some heat lower on rad, heat at top of the rad, now present 2/3s way down rad all along.
Return valve warm to touch, bout same as bottom of rad.

OK ...
For sure then .. it is a flow problem at that rad.
1. There is a block in the rad. or ...
2. There is swarf/copper debris in/near valve inlets/outlet. or ...
3. Valve in not functioning correctly.
To thoroughly check will mean a drain-down .. sorry. I suppose you could shut valves, remove rad to check flow .. and then open each valve in turn into a large bucket to check the flow.

Returns stay cold unless there is another rad before this one, and that is returning water past this rad. Then they may heat a little.
a rad that is cold at the bottom is partially sludged. the fact that you get fairly clean water at the top is similar to most rads and it is the last inch at the bottom that is black/brown.turn off the valves and drain the rad down, take it off the wall and carry it into the garden near a drain. tip it up at an angle, lower end on a piece of wood to protect paintwork and flush it with your garden hose through the will be surprised at the colour of crap that comes may need a few gentle taps with a rubber mallet to dislodge the crap. put it back on and try it out but you need to be aware that dirt/sludge that has caused this will be in the other rads to a certain degree.
has the system been fitted with inhibitor. it may be that you need a chemical clean with sentinel or fernox products
Thanks Terence.One of my rads has the same problem and it all started after a plumber put inhibitor in the system.Im not confident enough to do what you say though-probably flood the house.I'll get a plumber to do it, if I can find one!
Cleaner/Descaler will loosen/dissolve deposits and may create problems if not used properly.
Inhibitor will not.
So what exactly does inhibitor do? It was suggested because our pump kept jamming. Pump now OK but one radiator cold.

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