Radiator Cold at the Top? How to Bleed It and Re-Pressurise

A radiator that is warm at the bottom but cold at the top usually has air trapped in it, and bleeding it with a radiator key normally fixes the problem. If it is cold at the bottom, sludge is the likely cause, and if the whole radiator stays cold, a stuck valve or an unbalanced system is more likely. After bleeding, check your boiler pressure and top it up if it has dropped.

What the cold patch is telling you

Cold at the top, warm at the bottom

Air rises, so it collects at the top of the radiator and stops hot water filling it. You may also hear gurgling. Bleeding releases the air. If you have to bleed the same radiator every few weeks, air is getting into the system from somewhere, and an engineer should find out why.

Cold at the bottom, warm at the top

This usually points to sludge: a black, iron-oxide deposit that forms as the inside of the system corrodes. It settles in the bottom of radiators and blocks the flow. Bleeding won’t help. The radiator may need to be removed and flushed, or the whole system may need cleaning.

Whole radiator cold

• Check both valves are open. One may have been turned off during decorating.
• If it has a thermostatic radiator valve (TRV), turn it to maximum. The pin under the head can stick after the summer. With the head removed, it should move freely when pressed.
• If radiators near the boiler get hot but those furthest away stay cool, the system may need balancing, which means adjusting the lockshield valves so the flow is shared evenly.
• If several radiators are cold, the problem may be the pump, a valve, or the boiler itself.

How to bleed a radiator safely

You need a radiator key (from any hardware shop), an old towel and a small container.

1. Turn the heating off and let the radiators cool. Bleeding a hot system can release scalding water, and a running pump can draw air in.
2. Find the bleed valve. It is a small square fitting at one of the top corners of the radiator.
3. Hold the towel and container under the valve. Fit the key and turn it anticlockwise a quarter to half a turn. Don’t open it further.
4. You should hear air hissing out. Wait until it stops and water starts to come out in a steady stream.
5. Close the valve by turning the key clockwise. Don’t overtighten it.
6. Repeat on any other radiators that need it. Start with those on the lowest floor and work upwards.
7. Check the boiler pressure gauge and top up if needed, then turn the heating back on and check the radiators once they have warmed up.

Re-pressurising a combi boiler

Bleeding lets water out of the system, so the pressure on a combi or system boiler will drop. When the system is cold, the gauge typically reads around 1 to 1.5 bar, but check your boiler manual for the exact figure for your model.

To top it up, you use the filling loop. On many boilers this is a braided hose with a valve at each end under the boiler. Others have a built-in key or lever. Following your manual, open the valves slowly and watch the gauge. Close them when it reaches the recommended pressure. If you have an external hose, close both valves and disconnect it afterwards if your manual says to.

If the boiler has locked out on low pressure, it may need resetting. Your manual will show you how.

When it needs an engineer

• Sludge: a power flush or chemical clean of the system, often with a magnetic filter fitted afterwards to catch new debris.
• Stuck or leaking valves: replacing a TRV, lockshield or drain-off valve.
• Balancing: adjusting the system so every radiator heats evenly.
• Repeated pressure loss: finding a leak, a faulty pressure relief valve or an expansion vessel problem.
• No heat at all, or a boiler fault code you can’t clear.

Avoid opening the boiler casing or adjusting anything inside it yourself. Bleeding radiators and topping up the pressure through the filling loop are normal homeowner tasks. Anything beyond that, particularly anything involving gas, is a job for a qualified engineer.

Our heating repairs team covers homes and businesses across Central London, 24 hours a day, 7 days a week, with free quotations. If your radiators still aren’t heating properly, call 020 3670 7648.