Radiators Cold at the Top? How to Bleed Them and When It Means Something Worse

Radiators cold at the top but warm at the bottom have air trapped in them, and bleeding them is a ten-minute job you can do yourself. The part most people miss is what comes after: on a sealed system, letting that air out drops the boiler pressure, and if it falls far enough the boiler will stop making hot water as well.

So this guide does the whole job: how to tell air from the other causes of a cold radiator, a safe step-by-step bleed, the pressure check afterwards, and the question our engineers get asked most at this time of year across Deal, Dover, Sandwich and Thanet: why does the same radiator need bleeding again every week?

Why is my radiator cold at the top?

Air is lighter than water, so wherever it gets into a heating system it rises to the highest point it can reach. Inside a radiator that is the top few inches of the panel. Hot water still flows along the bottom, but the air pocket above it does not carry heat, so the top stays cold.

It is worth checking the pattern before you reach for the radiator key, because the pattern tells you the cause:

• Cold at the top, warm at the bottom is air. Bleed it.
• Warm at the top, cold along the bottom is sludge, and bleeding will do nothing. That is the subject of our guide to what a power flush involves.
• Cold all over while the rest of the house is warm is usually a valve problem or a system that needs balancing, which we cover in our post on one radiator not heating up.

If several radiators are cold at the top at once, especially upstairs, that is normal after the summer. Air comes out of solution over months of the system sitting still, and the first weeks of heating in October is when it shows.

How to bleed a radiator safely

You need a radiator key, an old towel and a small container. Some newer radiators take a flat-blade screwdriver instead.

1. Turn the heating off and let it cool for at least half an hour. Bleeding with the pump running pulls more air into the system, and the water that comes out of a running system is hot enough to scald.
2. Start downstairs with the radiator furthest from the boiler and work towards it, then do upstairs, where the air usually collects.
3. Find the bleed valve. It is the small square-headed screw at one top corner of the radiator, sometimes behind a plastic cap.
4. Hold the container under it with the towel ready, then turn the key anticlockwise a quarter to half a turn. You will hear a hiss. Do not remove the valve altogether.
5. Wait for the hiss to stop and water to dribble out steadily, then close the valve clockwise. Firm is enough.
6. Move on to the next radiator and repeat.
7. Check the boiler pressure gauge when you have finished. This is the step that matters, and the next section explains why.

A little rusty water first is normal. If what comes out is thick and black, like ink, the system has a sludge problem and is worth a conversation with us on 01304 381999 before winter.

No hot water after bleeding radiators? Check the pressure

This is the call we get the day after a bleed: six radiators bled, and now the boiler has shut down with a fault code and there is no heating or hot water at all.

Nothing has broken. Air was taking up space in the system, you let it out, and the pressure fell to fill the gap. Nearly every combi and system boiler fitted in the last twenty-five years is a sealed system that will not run below a minimum pressure, so once the gauge drops under about 0.5 bar it locks out. On Glow-worm boilers, which we see a lot of locally, that shows as a low-pressure code, and our Glow-worm fault code guide lists what each one means.

The fix is to repressurise. The gauge should sit between 1 and 1.5 bar when cold. You top it up using the filling loop, usually a short braided hose with one or two small taps under the boiler, following the boiler’s own manual. Open the taps slowly, watch the gauge, and close them again as soon as it reaches about 1.2 bar. Then reset the boiler. Our guide to a boiler losing pressure has the full method and the mistakes to avoid.

If you overshoot past 2.5 bar, bleed a radiator again to let some water out. Our post on boiler pressure too high explains why leaving it high matters.

If you have an older open-vented system with a feed tank in the loft, there is no gauge and the tank tops the system up on its own, so there is nothing to do after bleeding.

Radiator needs bleeding every week? That is a fault, not a chore

A system bled properly in October should not need doing again until next autumn. If the same radiator is cold at the top again within a week or two, air is being made or drawn in faster than you can let it out, and topping up every weekend is the wrong answer. Every top-up brings fresh, oxygenated, mineral-rich mains water in, and in our hard-water part of Kent that means more limescale and more corrosion every time.

The causes we find on repair visits, in rough order of how often we see them:

• Corrosion inside the system. When the inhibitor has run out, the steel in the radiators reacts with the water and gives off hydrogen. It collects at the top of radiators exactly like air does, but it keeps coming back. If the gas from the bleed valve has no smell and the water that follows it is dark, this is the likely cause, and the answer is a clean and fresh inhibitor rather than more bleeding.
• A small leak. A weeping valve, a pinhole under the floor, or a dripping pressure relief pipe outside lets water out and air in. Look for damp under radiator valves and a drip from the copper pipe through the outside wall near the boiler.
• A failed automatic air vent. If the one on the boiler has stuck, air that should vent itself ends up in your radiators.
• On open-vented systems, the pump drawing air over the vent pipe in the loft. You will often hear it gurgling in the pipes.

None of these is something to live with. Corrosion sludge quietly wears out pumps and blocks heat exchangers, and we see the end result every winter: a boiler condemned because the system around it was never looked after.

When to call an engineer

Bleeding radiators and topping up the pressure are safe for anyone. Tracing a leak, replacing an air vent, recharging an expansion vessel or adding inhibitor is a job for a Gas Safe registered engineer. We are Gas Safe registered, number 2836, and have looked after heating across East Kent for more than 25 years.

Repairs are charged by time: £90+VAT for the first hour, which is always charged, then £45+VAT per half hour after that, plus parts. Most “it keeps needing bleeding” visits are diagnosed within the first hour. You can book one through our boiler breakdown and repairs page, or see the rates in full on our pricing page.

An annual boiler service is the better way round. It is when we check the system pressure, test the inhibitor level and clean the magnetic filter, which between them decide whether you will be bleeding radiators every week next winter. Members on a BOYLA HomeCare plan have the service included, pay no call-out fees, and paid plans get emergency response within 24 hours, Monday to Friday.

Get the whole house warm before the cold sets in

Do the bleed round now, on a mild day, while there is no urgency. Check the gauge, top it up if needed, then watch for a fortnight. If the radiators stay warm to the top, you are set for winter.

If they do not, or if you are not comfortable repressurising the boiler yourself, ring us on 01304 381999. A real person answers the phone, and we will give you an honest answer on when we can get to you.