Advanced search

Radiator warm at the top and only lukewarm at the bottom

(20 Posts)
ThisOneNoThatOne Mon 30-Oct-17 20:30:16

Can any help with how I can solve this? It has a thermostatic valve that is turned to full, but never really gets that hot. It's a column radiator.

We've recently had a new heating system installed (this is a new radiator) and a power flush was carried out. Any ideas on how to fix this pls?

Melony6 Mon 30-Oct-17 20:55:12

Let the air out of the radiator. I’m sure YouTube will tell you how.

3out Mon 30-Oct-17 20:57:41

If this happens with us, it’s either because the radiator needs bled, or that the pressure is too low on the central heating (in which case, there are valves we have to turn just above the boiler until the pressure gauge rises again, and then turn the valves off again)

Gingernaut Mon 30-Oct-17 21:00:37

There should be a valve at the top of the radiator.

There's a special key to open the valve in order to bleed off the air caught inside, at the top of the radiator.

Turn the key, open the valve and let the air 'fart' out until water comes out.

Use an old rag to soak up the water and wipe up. It'll be very dirty. envy <-- not envy.

ThisOneNoThatOne Mon 30-Oct-17 21:01:04

Thanks, I've already bled it and there's no air. I thought if it needs bleeding it's cold at the top as that is where the air is? Can air get trapped at the bottom?

I wonder if it's the pressure then? 3out when you have this problem does it happen on all radiators or just one? In my case all the other radiators are fine, just but this one...

lokijet Mon 30-Oct-17 21:08:26

more likely that there is sludge/ clag at the bottom and the system may need to be flushed

ThisOneNoThatOne Mon 30-Oct-17 21:11:28

We've had a power flush and this is a brand new radiator (although this was fitted a couple of weeks after the power flush). Would another power flush help? Are they expensive? The original was was included in the price as part of the whole system change.

johnd2 Tue 31-Oct-17 00:20:48

Sorry, it's nothing to do with bleeding air. Basically either there's not enough flow or its blocked.
Try turning off all the other radiators and turning it on and see if it warms up properly.
Make sure the valves at both ends are on.
If it doesn't work still, you need to remove I from the wall and check if it's clear and if the pipes are clear. How thick are the pipes?

Humptynumpty02 Tue 31-Oct-17 09:58:51

Was your system balanced properly after you had the new rad installed?

PigletJohn Tue 31-Oct-17 11:56:05

hot at the top and cold at the bottom is insufficient flow. It doesn't need bleeding.

Turn off all the other radiators in the house. Does it now get fully hot?

If so, you just need to rebalance (instructions available)

If no, there is sadly a risk that it might have been fitted upside down. Tall column radiators have baffles to direct the flow. Contact the manufacturer for the fitting instructions, and ask what markings there are to help you work out if it was correctly fitted. Professional heating engineers should know how to fit it properly.

Mildura Tue 31-Oct-17 13:32:59

I had a similar problem recently, rad was warm at the top but cold at the bottom, it turned out the thermostatic valve was stuck. Changed the valve and now working fine.

ThisOneNoThatOne Tue 31-Oct-17 15:01:10

Thanks every for your help.

The thermostatic valve is definitely on. The one on the other side I'm not sure. One thing I have noticed is that the pipe going to the thermostatic valve is warmer than the one at the other side. Am i ok to try release the other valve slightly?! I don't want to make things worse! The pipes are about 15mm.

I've turned off all the radiators apart from the problem one. The radiator did get a lot warmer but it is still slightly cooler at the bottom.

I've looked if it could be fitted upside down, it doesn't look like it as the painted over valve is at the top.

I do have a spare thermostatic valve so I'll try and swap them over. Is it a case of draining the whole system?

Thanks again!

PigletJohn Tue 31-Oct-17 15:57:44

does your boiler have a pressure gauge on the front?

ThisOneNoThatOne Tue 31-Oct-17 16:03:37

Yes it has this

Hope you can see the photo!


PigletJohn Tue 31-Oct-17 16:35:18

then you don't need to drain the whole system. Let enough out for the pressure to fall away to 0, fully close both valves on this radiator, then you can unscrew (only) the one valve you want to remove and water will not gush out, because there is no other opening where air can go in to displace it.

Put a wide flat bowl underneath to catch anything that does come out; for added safety be ready to screw the valve back on if anything unexpected happens.


In your position I would start by just swapping the thermostatic heads over. This does not involve any water or plumbing. If you look at your spare, you will see the water valve has a pin on the top, and the thermostat pushes, or doesn't push, down on it.

When fitting or removing a head, always screw it fully upwards first to release pressure on the pin while you are working. When you remove the head, the pin will spring all the way out and the valve will be fully open. There's no need to remove the water valve if it is new, there's no reason for anything to be wrong with it. The pin is very unlikely to be jammed down but you will see if it is when you take the head off.

You mention this has been recently fitted so you should be getting the installers back to fix it.

PigletJohn Tue 31-Oct-17 16:39:36


looking at your pic

is 51C the temperature you have set on the boiler for you CH?

Too cool.

Put it up to 70C and turn off all the other radiators.

The incoming (flow) pipe of your radiator should be "too hot to hold" and the outgoing (return) pipe should be "too hot to hold for long"

which is about 70C and 50C.

ThisOneNoThatOne Tue 31-Oct-17 19:14:47

Thank you very much piglet john! That's all very informative.

I'll see how changing head works out.

And good to know that the temp is too low. I'll change that too.

Thanks again!

johnd2 Tue 31-Oct-17 22:42:23

True piglet John that would still have reasonable efficiency with as 50c return, but at this time of year 51c flow should be plenty and more efficient. We have ours on 50c now. Later in the year when it's 0c outside you might need to crank it up, but that will likely use more gas.
The more you throttle things down to get a 20c temperature difference, the more flows via the bypass, so unless you've got very big radiators it's better to keep the flow below 60c unless needed.

PigletJohn Tue 31-Oct-17 23:22:30

Yes, 60C is notionally the most economical temperature for a condensing boiler, but I'm struck by the point that this radiator seems cool at the bottom.

If the top is not very hot, that might be normal because of the temperature gradient.

As there are TRVs, we will see if turning up the flow temp makes the rad seem reasonably hot all over. The weather may still be too mild to see if it gets hot enough to bring the room to temp.

As closing the other rads did not make it fully hot, there might be a bypass or some other cause. Turning up the boiler temp, as well as closing the other rads, is likely to make this rad as hot as it is capable of getting, especially if the thermostatic head is taken off.

If the rad is not capable of getting hot enough to heat the room, then there is a problem, so this test will give us more info.

johnd2 Wed 01-Nov-17 08:52:52

Agreed piglet John, good diagnostic process.
My only comment is the the most efficient temperature for the boiler would technically be the biggest gap between flame temperature and water return temperature. So 0c. Obviously you're not going to get any useful heat from that, so it's a compromise.
Of course the most notable boost in efficiency from a condensing boiler comes as the flue gases condense in the heat exchanger, which would be a gas temperature of 99c or so at that point, and I've seen various flow/return temperatures suggested to achieve that.
But in the general case, keep it as low as possible or use weather compensation to do it automatically.

Join the discussion

Registering is free, easy, and means you can join in the discussion, watch threads, get discounts, win prizes and lots more.

Register now »

Already registered? Log in with: