Radiator cold at the top: how to bleed it and check boiler pressure

You put your hand on the radiator and the bottom is hot while the top stays cool. The room never quite warms up, and somewhere in the house something gurgles. It is almost always air, and getting it out takes ten minutes and a key from any hardware shop.

Written by the developer of Gable · Published 22 September 2026

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Why a radiator goes cold at the top

A hot-water radiator is meant to be full of water. Air gets into the system anyway — when it is filled or topped up, when work is done on it, and gradually as it runs. Air is far lighter than water, so it floats to the highest point it can reach and stops there. In a radiator, that is the top few inches.

Water can no longer touch that part of the panel, so the top stays cool while the bottom, still full of hot water, does not. Radiator and boiler makers give the same diagnosis: cold at the top usually means air, and the fix is to let the air out.

The reverse pattern means something else. A radiator that is hot at the top and cold along the bottom is usually blocked by sludge — corrosion debris that settles in the bottom of the panel — and bleeding will not touch it. One radiator like that can sometimes be taken off and flushed; several of them usually means a system flush, which is a job for a heating engineer.

What you need, and what to check first

• A radiator key. Sold in any hardware shop. Some bleed valves take a flat screwdriver instead — look at the square or slot in the middle of the valve before you buy.
• A cloth and a small container. What comes out at the end is water, and heating water is dirty and stains.
• The right kind of radiator. This is for hot-water radiators fed by a boiler. Steam radiators are different: they clear air through an automatic vent, and are not bled by hand with a key. If your system is steam, or you cannot tell, ask a heating professional first.
• A cold system. Turn the heating off at the programmer and give it half an hour. The water inside is hot enough to scald, and a hot system is still circulating, which pushes air around instead of letting it settle at the top.

Everything in this guide is on the water side of the system. Never open, adjust or dismantle anything gas-side — the boiler casing, the flue, or any gas pipe. That is licensed work, and in the UK it is Gas Safe registered work by law.

How to bleed a radiator, step by step

1. Turn the heating off and let the radiators cool until you can hold your hand flat on them.
2. Find the bleed valve. It is at the top of one end of the radiator: a small round fitting with a square peg or a slot in the middle.
3. Hold the cloth and container underneath, and against the wall side of the valve too — the first spurt can travel.
4. Open the valve slowly, anti-clockwise, about a quarter turn. Half a turn is as far as you ever need to go. You should hear a steady hiss as the air leaves.
5. Wait for the hiss to stop and a thin, steady dribble of water to come out instead. That is the radiator full again.
6. Close the valve clockwise, firmly but gently. Stop as soon as it seats: over-tightening is how a bleed valve gets damaged.
7. Dry the area and move to the next radiator.

Each radiator takes under a minute. If one hisses for a long time and never produces water, stop and read the boiler pressure section below before carrying on.

What order to bleed radiators in

The order matters more than most people expect, and it is the part most often done backwards. Heating guides broadly agree on the sequence:

1. Start on the lowest floor, at the radiator furthest from the boiler.
2. Work back along that floor towards the boiler, radiator by radiator.
3. Then go up a floor and repeat: furthest away first, working back.

The reasoning is that air pushed out of the downstairs pipework rises upstairs. Clear the upstairs radiators first and you will often find air back in them by the time you have finished downstairs, so you end up doing the same radiators twice.

How to check boiler pressure after bleeding

Letting air out of a sealed system lets the pressure down with it. On a combi or sealed system — the kind with a pressure gauge on the front of the boiler and no tank in the loft — check the gauge once you have finished, with the heating still off and the radiators cold.

Most domestic boilers want somewhere around 1 to 1.5 bar cold, and many gauges mark the target as a green band. Treat your own boiler manual as the authority: the manufacturer sets the exact range, and the gauge reads higher once the heating has been running and the water has expanded.

If the gauge sits below the marked range, the boiler needs topping up through its filling loop — the braided silver hose with a valve at each end, usually under the boiler. The manual gives the exact procedure for your model: in outline, open the valves slowly, watch the needle climb into the range, and close both firmly. Do not walk away while it fills, and do not overshoot; too much pressure can push water out through the pressure relief valve.

If your house has a small tank in the loft feeding the heating, you have an open-vented system. It runs at atmospheric pressure, tops itself up from that tank, and has no pressure gauge or filling loop to worry about — bleed the radiators and you are done.

One thing worth acting on: if the pressure keeps falling and you are topping the system up every few weeks, that is not normal. Something is losing water — a small leak, a weeping pressure relief valve, or a failed expansion vessel — and it wants a heating engineer rather than another top-up.

How often should you bleed radiators?

Once a year is the usual advice, done before the heating season starts rather than in the middle of the first cold snap. Bleed sooner if the symptoms show up: a cold top on any radiator, gurgling when the heating fires, or rooms that take noticeably longer to warm than they used to. A system that has just had work done on it will need it again shortly afterwards.

Gable's library carries this as a yearly job, Bleed radiators and check boiler pressure, with a window of September to October for houses in the northern hemisphere — the gap between putting the heating back on and needing it to work. Southern-hemisphere houses get the same job six months across the calendar.

Gable's library: “Free vs cold rooms and 5-10% wasted fuel all winter.”

It sits alongside the other heating jobs that belong in the same few weeks. If your house is heated by forced air instead, the equivalent autumn jobs are booking a furnace tune-up and changing the furnace filter.

When bleeding does not fix it

• Still cold at the top after bleeding. Bleed it again after a day of normal running; a large air pocket sometimes needs two goes.
• Cold at the bottom, hot at the top. Sludge, not air. See above — this is a flush, not a bleed.
• Every radiator is lukewarm. That points at the boiler, the pump or the system pressure rather than at any one radiator.
• The valve drips after you close it. Do not force it tighter. Put a container underneath and call a plumber; the valve needs replacing.
• Repeated air, month after month. Air that keeps coming back is being drawn in somewhere, and wants diagnosing.

Bleeding radiators is one of the few heating jobs that is genuinely a ten-minute homeowner task. Nearly everything past it is not. The rest of the seasonal list is in the home maintenance checklist, and if you have just moved in, the first-year guide covers what to find out about your heating before you need it.

Get the reminder in September, not in January

Tick “Boiler and radiators” when Gable asks what your house has — it is not ticked by default, because the setup checklist starts with a forced-air furnace instead — and bleeding the radiators joins your autumn list with a reminder before the window. Houses without radiators never see the job at all. No account needed.