Cooler radiators in one room

[Toastandmarmiteandtea]

Any ideas what could be causing all three radiators in my top floor bedroom to cool when the room doesn’t feel as warm as the rest of the house?
The radiators get really hot at first but after a while, seem cooler and the room never feels very warm. I wondered about the TRVs but surely they can’t all be faulty.

Might just need bleeding, have you tried that?

Yes, they don’t need bleeding and they get really hot at first but seem to cool down after a while almost as if the trvs have reached the right temperature. The trvs are turned up to max. However the room isn’t as warm as the rest of the house and the radiators are hotter elsewhere. Maybe the trvs do all need replacing.

Try putting an indoor thermometer in the rooms and measure the temperature.

I would guess it’s just that the radiator thermostat thinks it has reached its temp.

Counter-intuitive , but try turning one radiator off and see if the others stay hot.

Whatever the outcomes, armed with more knowledge, call your heating engineer and explain it to them.

Thank you! Some good tips. I will try these. Boiler being serviced in a few weeks so can speak to him then.

You need to balance your radiators

Do you have all of your other radiators, or even some, on maximum? If so, try turning them down and see if that helps.

When radiators need bleeding, it is because they have air (gas, actually) in them. It floats on top of the water, and because the tops of the radiators have no hot water in them, you can easily feel with your hand that the tops are cold.

If that is not the case, they do not need bleeding and it will not help.

I agree it sounds more like they need balancing. To check that, turn off the hot radiators at the knob. Do the cold ones now get hot?

I had a look, but my old post on "how to balance radiatirs" seem to have perished. I will have a go at writing a other.

In the meantime, here is one on diagnosing heating faults (the old thread is ten years old so may perish)

"hot across the top, cools when turned down
may be badly sludged

heats all over, cools when turned down
correct behaviour

hot across the bottom, cools when turned down
is full of air, or badly sludged.

warm extreme bottom right, cold elsewhere.
insufficient flow. Turn off ALL the others and see what happens. May improve if all rads are balanced

right becomes hotter when other radiators are turned off and there is some heat across the bottom, but the top stays cold
air, or sludge, also poor flow

hot across the bottom, no noticeable change when other radiators are turned down
Air or sludge

hot all over, cools when turned down
Correct

So it seems to me that you have a sludge problem. The air you can easily bleed out, and the balancing can be addressed later.
I would recommend starting with a simple chemical clean. If you are fond of DIY plumbing it need only cost £20 for chemicals, plus two half-days of tinkering. You will need to find a downstairs drain cock, and (probably) a small feed and expansion tank in the loft. Or a plumber or heating engineer can do it. You do not need to pay Gas Engineer prices because it is not gas work.
If you are fond of plumbing and strong, you can also take the rads into the garden and squirt a hosepipe through them.
I would also recommend fitting a magnetic system filter, to capture loosened sediment, on the Return pipe to the boiler. It is not a difficult plumbing job. Preferably fit it before doing the clean as it will stop sludge moving into the boiler.
A chemical clean is not very intensive, but it will certainly do nothing but good. If more work is needed afterwards, it will not have made things worse."

Balancing post now created

https://www.mumsnet.com/talk/property/5245071-diagnosis-of-radiator-problems?reply=141094800

And, @Toastandmarmiteandtea I don't know that you have sludge. That's just some text that I copied and pasted without editing. Sorry.

Thank you. They do get really hot at first so I don’t think there’s a balancing problem? They just seem to then cool down more than the radiators downstairs after a while even though the room doesn’t feel as warm as it should. So presuming the trvs? But I would be more inclined to think that it was the trvs if it was just one of them rather than all three radiators.

It won't be the TRVs.

It might be that the rest of the house has warmed up and the boiler has stopped. You can check this by looking at the boiler to see if it is still firing, and what its temperature is.

Modern boilers modulate the flame size according to demand and you will be able to hear them getting quieter when firing at low power. If you have very fancy controls it might know that the house is nearly warm and has reduced power.

Left to themselves, the TRVs in the other rooms would stop taking hot water as each room reached its temperature, leaving more flow to go to the cold rooms.

However. Modern circulation pumps can detect when not much flow is needed, and reduce their pumping speed. I suppose it's possible that the route to your cold rooms is long and arduous, or the pipes pass through an unheated space like a loft and are not insulated adequately and lose heat. I found it difficult to deal with and manually set mine to a fixed speed. If you get an experienced local heating engineer in he might spot something I haven't thought of.