One hot pipe on radiator but radiator is cold

flyingupthere

Hi

I'd be really grateful for some advice please about a cold radiator.

When the central heating system is on normally, both of the pipes to the rad are cold and the rad is cold. However, if I shut off all of the other rads (except for a couple), one of the pipes to the cold rad gets boiling hot. However, the radiator doesn't itself warm up.

Both valves are old fashioned valves - no TRV is installed. The lockshield side (i.e. the valve on the pipe which is always cold) is fully open.

I've tried closing both valves, then bleeding the rad, and then opening each valve. When I opened the lockshield valve on the return pipe water shot out straight away with force. When I opened the valve on the flow pipe (the one getting really hot) water did come out, but not with the same force.

I'm wondering if there is a blockage in the pipe running to the flow pipe under the floorboards causing the issue (and if so what can I do about it?) or whether this could be something else.

Many thanks in advance.

ThisIsWeird

The only thing you haven't tried - or told us about - is fully opening both valves. Could you try this and report back?
The hot flow pipe is just from heat from the supply pipe under the floor convecting up the pipe 'tail' - it doesn't suggest actual 'flow'. (But does suggest no pipe blockage, so that's good).
Is your system sealed/unvented? Ie, does it have a pressure gauge? Especially if so, I would expect the bleed screw to ejaculate at roughly similar rates with each valve being opened, so the fact it doesn't could suggest the flow valve is faulty, and not opening fully. But, you haven't actually told us how far you've opened it yet - try 'fully'!
And, does it turn smoothly?

Micron

It could just be an air lock somewhere in the pipes going to or from the cold radiator.

With the heating turned off you could try fully opening both valves on the affected radiator, turn off ALL of the other radiators then turn on the heating for just one minute which hopefully might force out any air in the system, you may have to repeat this a couple times to have any effect.

Just one thing to mention, bleeding a radiator should only be done with the central heating system turned off, (just in case you didn’t know).

Albermarle

With the heating turned off you could try fully opening both valves on the affected radiator, turn off ALL of the other radiators then turn on the heating for just one minute which hopefully might force out any air in the system, you may have to repeat this a couple times to have any effect.

Maybe with the pump turned up to full speed at the same time.

In fact you could check whether the pump speed makes any difference normally.

FreeBear

flyingupthere said: I've tried closing both valves, then bleeding the rad, and then opening each valve. When I opened the lockshield valve on the return pipe water shot out straight away with force. When I opened the valve on the flow pipe (the one getting really hot) water did come out, but not with the same force.

Now that you have disturbed the lockshield, once you have the radiator getting hot, the whole system will want balancing again. Plenty of guides on the internet on how to do this, but you will need a thermometer or two to do the job properly. A wet finger is not accurate enough, nor is a non-contact IR thermometer.

flyingupthere

Thank you for all the replies!  I would have responded earlier but there seemed to be a problem with my MSE account.

Unfortunately I still haven't managed to get the radiator going!

@ThisIsWeird - I've opened both valves (i.e. the flow valve and the lockshield valve) on the radiator fully.  Sorry I should have made that clear!  The valves themselves turn reasonably smoothly, i.e. comparable to the other rads in our house of similar age, although occasionally on the flow side it can get a bit "sticky" if that makes sense when turning it the cap.  Our system is vented i.e. there is a header tank in the loft.  

@Micron - I've tried that unfortunately.  In fact, when I do it with just this radiator on, the burner light on the boiler turns off (I think because the boiler might be sensing a lack of demand?)  If I do it with this radiator plus one other radiator on, then the burner light stays on, and the pipe to the not-working radiator gets very hot.  I didn't know rads should only be bled when cold - noted thanks!

@Albermarle - Thanks for the suggestion, please see above - unfortunately I've tried this.  The pump (Grundfos UPS3) is set to full power.

@FreeBear - Thanks and noted... although fortunately all the other rads seem to be working well even when the lockshield on this not-working rad is fully open...

ThisIsWeird

Based on what you've told us, it would appear that the flow - 'control' - rad valve is faulty. Whilst it may be turning, the sealing washer could be stuck fully or partially closed.
You mentioned that when you bled the rad with the lockshield open, water spurted out the bleed screw. But when you ditto'd with the control valve open, it was much less. You have also told us that the tail pipe coming up to the control valve becomes hot, but I'm guessing not the valve itself, at least not any more than 'warm'? And not the rad at all? Ie, does the immediate part of the rad where the valve fits in become hot or warm? The tail pipe might simply be made hot from the supply pipe it's connected to.
The next Q is, how DIYish are you? To properly test these valves will, I think, require the rad to be removed. This doesn't have to be traumatic, but you need to guard against spills which can be messy.
Anyhoo, the procedure would be:
1) close both rad valves firmly. Test by opening the bleed screw - a wee squirt and then nothing should come out.
2) slip a low container like a roasting dish under one valve. Undo the large chrome nut on the rad side of the valve, and be ready to catch the trickle. Don't undo it fully, but just enough to allow the rad to drain in a controlled manner. You'll also need to slacken the bleed screw to allow air in, or else the emptying will 'glug'.
3) when completely empty - this will take a while - undo the opposite valve too, both fully. Lots of towels.
4) when you are confident the rad is empty - tho' there will still be enough inside to make a mess of your carpet if you are careless - tease the valves away from the rad a few mm to make sure they are released, and then lift the rad upwards to disengage it from its mounting brackets - it'll only be around 10mm. Then pull it away from the wall to fully remove. 
5) take the rad outside, and hose it through.
6) now you check each valve! Big pan, and open each valve in turn. Water should gush out each valve. Does it?
All the above carried out with boiler (and therefore pump) off. 

Albermarle

Quite recently I rebalanced our heating as the radiator farthest away from the boiler in a downstairs bathroom took ages to heat up.
After a few rounds of opening and shutting lockshield valves, I mainly achieved my aim.
However suddenly a radiator in an upstairs bathroom stopped getting hot, even when both valves were fully open, although like yours the inflow pipe was getting hot.
After some fiddling about/trial and error, it seemed the lockshield valve was not working properly. After opening and shutting it a few times ( and banging it ) the hot water started to flow again. Now I have not touched it again!
Maybe they can be stuck closed, even though you can turn them ???

FreeBear

ThisIsWeird said: 4) when you are confident the rad is empty - tho' there will still be enough inside to make a mess of your carpet if you are careless - tease the valves away from the rad a few mm to make sure they are released, and then lift the rad upwards to disengage it from its mounting brackets - it'll only be around 10mm. Then pull it away from the wall to fully remove.

Before lifting the radiator off the wall, it is well worth plugging the ends with a couple of these -> https://www.screwfix.com/p/flomasta-male-flanged-plug-1-2-x/98635

And while you are at it, plug the valves with a pair of these -> https://www.screwfix.com/p/flomasta-blank-nut-1-2-x/51939

The former will limit the amount of icky black sludge & water escaping while you take the radiator outside. The latter should stop any water dripping out whilst your back is turned. And before you leave Screwfix, pick up a bottle of corrosion inhibitor to add to the system when you are done.

ThisIsWeird

Good stuff, FreeBear.
I should also have suggested - once the rad is off the bracket, turn it upside down - after making sure the bleed screw is closed.

ThisIsWeird

Albermarle said:

Quite recently I rebalanced our heating as the radiator farthest away from the boiler in a downstairs bathroom took ages to heat up.
After a few rounds of opening and shutting lockshield valves, I mainly achieved my aim.
However suddenly a radiator in an upstairs bathroom stopped getting hot, even when both valves were fully open, although like yours the inflow pipe was getting hot.
After some fiddling about/trial and error, it seemed the lockshield valve was not working properly. After opening and shutting it a few times ( and banging it ) the hot water started to flow again. Now I have not touched it again!
Maybe they can be stuck closed, even though you can turn them ???

Hmm, that's a possibility - the washer could have become detached from the spindle.
Fut, you could first try - open both valves fully, get the boiler running, and shut off a few other rads - just enough to increase flow to the miscreant one. And then thump the control valve body on its side repeatedly with a lump of wood. Not 'hard', but firmly and repeatedly - thump thump thump thump - on the side just where the angle is. Keep feeling the valve body - but mind yer thumbs.