Guide · Central heating
A radiator that's warm at the bottom but cold across the top is the most common radiator complaint there is — and the good news is it's usually a five-minute fix you can do yourself.
Air is lighter than water, so any air inside your heating system rises and collects at the highest point it can reach — the top of your radiators. Once a pocket of air sits there, hot water can't fill that part of the radiator, and the top stays cold while the bottom heats up. You might also hear gurgling or trickling sounds when the heating comes on.
Bleeding lets the trapped air out through a small valve at the top corner of the radiator. It's a safe job for most homeowners:
Letting air out of a sealed system lets pressure out too. After bleeding, check the pressure gauge on the boiler — on most sealed systems it should sit around 1 to 1.5 bar when cold. If it's dropped, top it up with the filling loop; our how to repressurise a boiler guide shows how. Skip this step and the boiler may refuse to fire.
Bleeding a radiator once in a while is normal. Needing to bleed the same one every few weeks is not — it means air is getting into the system somewhere. Common causes include a small leak, a pump drawing air in, or corrosion inside the system producing hydrogen gas. If the water that comes out is black, that's a sign of corrosion and worth having looked at.
Electric radiators don't contain water from a heating system, so they can't be bled. An electric radiator that's cold at the top usually has a failed element or a thermostat fault, and it's a job for an electrician or the manufacturer rather than a heating engineer.
The opposite pattern — warm at the top, cold along the bottom — points to sludge settling in the radiator, which bleeding won't fix. Our radiator cold at the bottom guide covers that, and the how to bleed a radiator guide goes through the bleeding process in more detail.
If bleeding doesn't solve it, or the air keeps coming back, the system needs a proper look. We handle radiator faults, system cleaning and balancing under our central heating service — send the quote form with a note of which radiators misbehave and how often.
Common questions
Clear answers before you arrange a visit.
Trapped air. Air rises to the top of the radiator and stops hot water filling it. Bleeding the radiator lets the air out.
Yes, with the heating off and the radiators cool. Turn the key a quarter turn, catch the water with a cloth, and close the valve as soon as water appears.
Repeated air usually means a small leak, a pump drawing air in, or corrosion producing gas inside the system. It's worth having the system checked.
Often, yes. Bleeding lowers the pressure on a sealed system, so check the gauge and repressurise if it's dropped below the level in your manual.
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