How do I know if my radiator needs bleeding?

The classic sign is a radiator that is cold at the top but warm at the bottom — trapped air has risen and is blocking hot water from filling the upper section. Other signs are gurgling or knocking sounds, a radiator that takes far longer to warm than the others, or cold spots you can feel by hand. If the whole radiator stays cold, that is usually a valve or flow problem, not air, and bleeding will not fix it.

What do I need to bleed a radiator?

Just a radiator bleed key (a small square key that fits the bleed valve, sold cheaply at any hardware store) or, on some valves, a flat-head screwdriver. Add a cloth and a small container to catch the drips of water that follow the air. That is the whole kit. Turn the heating off and let the radiators cool first so the water inside is not scalding and the system is not actively pumping.

Which radiator should I bleed first?

In a multi-story home, start with the downstairs radiator furthest from the boiler, then work toward it, and finish with the upstairs radiators (air rises, so the top-floor ones often trap the most). Doing them in order helps push air through the system efficiently. If only one radiator has a cold top, you can bleed just that one, but if several are affected, work through them methodically.

Why does my radiator keep needing to be bled?

Repeatedly trapping air points to a deeper issue: air being drawn in through a small leak, a faulty automatic air vent, corrosion producing hydrogen gas inside the system, or a pump set too fast pulling air past a seal. Bleeding treats the symptom. If you are bleeding the same radiator every few weeks, have a heating engineer check for leaks, corrosion inhibitor levels, and pump settings — the underlying cause needs fixing.

Do I need to repressurize the boiler after bleeding?

Often, yes. Releasing air (and a little water) can drop a sealed-system combi boiler's pressure below its normal 1 to 1.5 bar range. Check the pressure gauge after bleeding; if it has fallen too low, top it up using the boiler's filling loop per the manufacturer's instructions until it reads in the normal band. Open-vented systems with a header tank refill themselves and usually need no manual repressurizing.

How to Bleed a Radiator: A 10-Minute DIY Guide

Written & researched by Derek Holt Home & Appliances Editor

Updated August 2026

Overhead view of a person adjusting a wall thermostat in a warm hallway, in soft overcast light

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Bleeding a radiator is one of the easiest home-heating fixes — plan on about 10 minutes per radiator, a couple of dollars for a bleed key, and no special skill. When a radiator is cold at the top but warm at the bottom, trapped air is blocking the hot water; opening the bleed valve lets that air escape so it heats evenly again. Turn the heating off and let it cool first, and keep a cloth handy for the water that follows. After bleeding, check the boiler pressure. There is no risk here beyond a bit of dribbled water.

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Tools and materials

A radiator bleed key (or a flat-head screwdriver on some valves).
A cloth and a small container to catch the drips.
Rubber gloves, since the water can be warm and often dirty.
If a radiator stays cold overall after bleeding, a plug-in space heater can cover a room while you sort out the flow issue.

Wide view of a person adjusting a wall thermostat in a warm hallway, in bright midday sun

Step 1: Turn off the heating and let it cool

Switch the heating off and give the radiators 20 to 30 minutes to cool. Bleeding a hot, actively pumping system risks scalding water and can draw more air in. You want the system static and the water no more than warm before you open a valve.

Step 2: Find the radiators that need it

Feel each radiator top and bottom. The ones cold at the top with a warm bottom have trapped air. Note them, and plan your order: downstairs-furthest first in a multi-radiator home, finishing with the upstairs units.

Close-up detail of the material being worked in a heating & cooling project, in soft overcast light

Step 3: Open the bleed valve

The bleed valve is the small square nipple at the top corner of the radiator. Hold your cloth and container under it, fit the bleed key, and turn it slowly counterclockwise — a quarter to half turn is plenty. You will hear air hiss out. Do not open it fully; you only need to crack it enough to release the air.

Step 4: Close it when water appears

Let the air hiss until a steady dribble of water comes out instead — that means the air is gone and the radiator is full. Immediately close the valve by turning the key clockwise until snug (do not overtighten and strip it). Wipe up the drips. Move to the next radiator in your order and repeat.

Close-up detail of the material being worked in a heating & cooling project, in warm evening lamplight

Step 5: Check the boiler pressure and test

After bleeding, check the boiler's pressure gauge. If it has dropped below the normal 1 to 1.5 bar on a sealed system, top it up with the filling loop per the manual. Turn the heating back on and, once warm, feel the radiators again — they should now be evenly hot top to bottom, with no cold cap and no gurgling.

Common mistakes

Bleeding with the heating on and hot. Turn it off and let it cool first.
Opening the valve too far. A quarter to half turn is enough; more sprays water.
Forgetting the cloth and container. Water always follows the air.
Not checking boiler pressure after. It can drop too low on sealed systems.
Overtightening the valve closed. Snug, not cranked, or you strip the brass.

When to call a heating engineer

Bleeding is fully DIY. Call a professional if a radiator stays cold after bleeding and repressurizing (a flow, valve, or sludge problem), if you have to bleed the same radiator repeatedly (a leak or corrosion issue), if the boiler pressure will not hold, or if you find a leak at a valve. Work on the boiler itself, any gas component, or a persistent system fault is a job for a qualified heating engineer.

How to Bleed a Radiator