Radiator Cold at the Top but Warm at the Bottom? Here's Why

Updated July 2026 · Emergency Plumber Chelsea

Check the pattern before you do anything. Put the heating on for twenty minutes so every radiator has had time to warm through, then run your hand down each panel. Cold across the top with a warm lower half is air. Warm at the top and cold along the bottom is sludge, which bleeding will not touch; our guide to a radiator cold at the bottom covers that fault. A radiator that is cold all over, while the others in the house heat normally, is usually a stuck valve or a balancing problem, which our guide to one radiator not heating up works through.

Bleed the radiator first. Turn the heating off and give the radiators a while to cool, so you are not releasing hot water. Find the bleed valve, a small square fitting at one of the top corners, and hold a folded cloth underneath it. Open it a quarter turn with a radiator key, or a flat screwdriver on some newer valves. You should hear air hiss out. When the hissing stops and a steady dribble of water appears, close the valve again firmly but not hard. If you have several radiators to do, start on the ground floor and work up, finishing with the highest ones. Our step-by-step guide on how to bleed a radiator goes through this in more detail.

Check the boiler pressure after bleeding. Most modern Chelsea homes have a sealed system with a combi or system boiler, and every radiator you bleed lets a little pressure out. Look at the gauge on the boiler: with the system cold it should normally sit somewhere around 1 to 1.5 bar, though your boiler manual gives the exact figure. If it has dropped below that, top it up slowly through the filling loop and close the loop fully afterwards. Our guide on what pressure your boiler should be explains how to read the gauge. Older houses with a small feed and expansion tank in the loft do not have a pressure gauge to check; the tank refills the system on its own.

If nothing comes out of the bleed valve, stop and look at the pressure. Opening the valve and getting no hiss and no water usually means the system has lost its pressure, or on an older open-vented system that the feed tank in the loft is empty. On a sealed system the gauge will read close to zero, and the boiler may have locked out. Top up and try again. If the loft tank is dry, its float valve has probably stuck or the supply to it has been turned off.

Air that keeps coming back is the real fault. Bleeding a radiator once after the summer, or after a plumber has drained the system, is normal. Having to bleed the same radiators every week or two is not. Something is letting air in or producing gas, and the usual causes are below.

A sealed system that keeps losing pressure is the most common cause. If you find yourself topping up the boiler regularly, water is escaping somewhere, often from a weeping valve, a pinhole in a pipe under the floor, or the boiler's own pressure relief valve. Each time you top up, fresh water goes in carrying dissolved air, which then collects in the radiators. The cold patch at the top is the symptom; the pressure loss is the problem. Our guide to a boiler losing pressure explains how to track down where the water is going.

An automatic air vent that has failed can let air in rather than out. Most boilers and many systems have a small automatic vent, usually on or near the pump, that releases air as the system runs. When one fails it can leak, stick shut, or on some systems draw air in when the pump runs. It sits in or next to the boiler, so replacing it is an engineer's job.

On an older open-vented system, the pump can pull air in or push water over. Many Chelsea townhouses still run their heating from a small feed and expansion tank in the loft, with an open vent pipe that rises back up to it. If the pump is set too fast, or the pump, vent and cold feed were laid out badly during a past alteration, the system can draw air in through the vent or push water out of it into the tank, a fault known as pumping over. Either way the top-floor radiators keep filling with air. The fix is usually to slow the pump or alter the pipework near it, which needs someone who understands how the system was laid out.

Gas from corrosion is the other cause, and it is a sign the water inside the system is attacking the metal. As steel radiators corrode from the inside, the reaction gives off gas that collects in the top of the panel, just as air does. The clue is that bleeding releases gas time after time, and the water that follows is dark grey or black. That gas can be flammable, so never hold a lighter or flame near a bleed valve. The long-term answer is to clean the system, refill it with corrosion inhibitor, and fit a magnetic filter so the sludge has somewhere to go; our radiator power flush service covers that work.

Only the top-floor radiators cold at the top points to pressure or the loft tank. In a tall Chelsea house of four or five storeys, the radiators at the top are the highest point in the system, so any air ends up there, and they are the first to suffer when pressure drops. On a sealed system, a pressure reading that is fine on the ground floor can still leave too little push at the top of a tall house. On an open-vented system, a feed tank that is not filling properly has the same effect. If it is always the top floor, have the pressure, the tank and the pump checked together.

Heated towel rails behave the same way. A towel rail that is cold across the top rungs has air in it, and most have a small bleed valve on one of the top corners. Our guide to a towel rail not heating up covers the other faults that affect them.

In a mansion block with communal heating, tell the managing agent. Many flats in the red-brick blocks around Sloane Square and along the Embankment run from shared plant in the basement. Bleeding your own radiators is usually fine, but if they keep filling with air, the cause is likely to be in the building's system rather than your flat, and the managing agent needs to know so the plant can be looked at.

When to call an engineer. Call if bleeding does not fix the radiator, if nothing comes out of the bleed valve even after topping up, if you are topping up the boiler pressure more than once every few weeks, if the bleed water is black, or if several radiators keep going cold at the top. Emergency Plumber Chelsea finds where the air is getting in, whether that is a pressure leak, a failed vent, a pump problem or corrosion, and fixes that rather than just bleeding it again. Our central heating repairs service covers valves, pumps, vents and leaks across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham, and our boiler repair service covers faults inside the boiler itself. Call 020 3002 6667. There is no call-out fee on jobs over 3 hours.

Quick tips

• Cold at the top and warm at the bottom is air; cold at the bottom and warm at the top is sludge.
• Bleed with the heating off, ground floor first, top floor last.
• Check and top up the boiler pressure after bleeding, then close the filling loop fully.
• Needing to bleed the same radiator every week or two means something is letting air in. Find the cause.
• Never hold a flame near a bleed valve. Repeated gas from a radiator can be flammable.

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