Radiator cold at the top but warm at the bottom

Why air collects at the top

Central heating water carries small amounts of dissolved air and gas. As it circulates and heats up, some of that comes out of solution and rises to the highest points in the system, which are usually the tops of radiators, especially upstairs. A little air after a system has been drained or worked on is normal. A radiator that fills with air over and over again is not.

This is the opposite pattern to a radiator that is cold at the bottom, which usually points to sludge and debris settling in the lower channels. It is also different from a radiator that is cold all over, which is more likely to be a valve, balancing or circulation problem. Getting the pattern right saves money, because each has a different fix.

How to bleed a radiator safely

Turn the heating off and let the radiators cool for at least half an hour. Hot water can spray out of the bleed valve and the system is under pressure. Put a cloth under the bleed valve, which is the small square fitting at one top corner of the radiator, and fit a radiator key.

Turn the key slowly anticlockwise, about a quarter turn. You should hear air hissing out. When water starts to appear in a steady trickle rather than spitting, close the valve again, firmly but without forcing it. Work through the other radiators the same way if they also feel cold at the top, starting with those on the lowest floor.

On a sealed system with a pressure gauge on the boiler, check the pressure after bleeding. Letting air out drops the pressure, and a combi or system boiler may stop working if it falls too low. Top it back up using the filling loop as described in your boiler manual. On an older open-vented system with a tank in the loft, the system refills itself from that tank.

Our full guide on how to bleed a radiator covers which radiator to start with, what to do without a radiator key and what dark water or a valve that will not close tells you.

If the bleed valve is seized, snapped or keeps weeping after closing, stop and get it replaced. Forcing an old valve can break it off in the radiator.

If the radiator keeps filling with air

Needing to bleed the same radiator every week or two tells you air or gas is continually entering or forming in the system. That is worth investigating, because the same causes often damage the boiler and radiators over time. Common reasons include a small leak on a sealed system, which lets pressure drop so that fresh water is repeatedly added through the filling loop, bringing new oxygen in with it.

Corrosion inside steel radiators produces hydrogen gas as the metal breaks down. If you bleed a radiator and the gas hisses out with little smell of air, or the water that follows is black, internal corrosion is likely. That black water is magnetite sludge, which also causes cold spots at the bottom of radiators, blocks pumps and shortens boiler life.

On open-vented systems, a poorly positioned pump or a problem with the feed and expansion tank can draw air into the system through the open vent. This is sometimes called pumping over, and it often shows up as constant air in the upstairs radiators and a gurgling noise in the pipes.

• Pressure that falls between bleeds suggests a leak somewhere on the system.
• Black water from the bleed valve suggests corrosion and sludge.
• Gurgling and bubbling in pipes, with a loft tank, may point to pumping over.
• Air returning only after the system has been topped up points to repeated fresh water.

What a heating engineer will check

A proper diagnosis starts with the pattern: which radiators gather air, how quickly, and whether the pressure changes. On a sealed system, the engineer will look for leaks at valves, joints and the pressure relief valve, and check the expansion vessel charge, since a flat vessel can make the pressure swing and the relief valve discharge.

If corrosion is the source, the options range from cleaning and dosing the system with inhibitor through to a power flush and fitting a magnetic filter. Where a pumping-over problem is found on an open-vented system, the answer may be alterations to the pipework near the pump and vent. The aim is to stop the air forming, so bleeding goes back to being an occasional job.

Getting the radiators working properly

Loyal Plumbers repairs radiators, valves and central heating systems across London and inside the M25, with 10 vans working 24 hours a day. If you have bled a radiator and it is cold at the top again, or the pressure keeps dropping, call us or send an enquiry. Tell us which radiators are affected, how often you are bleeding them, what the boiler pressure gauge reads and whether the water from the bleed valve is clear or dark.

That information helps us decide whether you need a simple valve repair, a leak investigation or a system clean, rather than recommending new radiators the property does not need.