Domakihub How to Bleed a Radiator and Restore Full Heat
A cold top, a gurgling pipe — here is how to fix it in under ten minutes.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 4 min read
When a radiator is warm at the bottom but cold at the top, trapped air is the culprit — and bleeding it out is a straightforward fix any homeowner, rental manager, or hotel maintenance team can do without calling a plumber. The British Gas video above shows the process clearly in real time; the steps below walk you through it with the detail you need to do it safely the first time. In Greece, where many properties switch the central heating on for the first time each autumn, bleeding radiators at the start of the season keeps the system running efficiently and reduces fuel bills.
Why Air Gets Trapped in Radiators
Central heating systems circulate hot water through a closed loop of pipes and radiators. Over time — especially after the system has been drained, refilled, or left off for several months — small pockets of air collect at the highest point inside each radiator. Because air does not conduct heat as well as water, those pockets create a cold zone at the top of the panel.
In Greek holiday homes and rental apartments that sit empty over summer, air ingress during the off-season is particularly common. Bleeding every radiator at the start of the heating season is good practice and takes only a few minutes per unit.
What You Need Before You Start
You will need a radiator bleed key (a small square-headed tool available at any hardware shop for under two euros), an old cloth or small towel to catch drips, and optionally a bowl to hold under the bleed valve. If your radiator has a flat-head slot instead of a square recess, a standard flat-blade screwdriver works too.
Check that your boiler is switched off and the system has cooled for at least 30 minutes before you begin. Working on a hot pressurised system risks scalding from the released water.
How to Bleed a Radiator: 5 Steps
- 1
Turn off the heating and let it cool
Switch off the boiler or central heating thermostat and wait at least 30 minutes. The system needs to be off so that air bubbles settle at the top of each radiator and so that hot water does not spray out when you open the valve.
- 2
Locate the bleed valve
The bleed valve is a small square or slotted fitting on one end of the radiator, near the top. It is usually on the opposite end from the thermostatic radiator valve (TRV). Hold the cloth directly below the valve before you touch it.
- 3
Open the valve slowly — a quarter turn anti-clockwise
Insert the bleed key into the square recess and turn it anti-clockwise by a quarter turn only. You will hear a hissing sound as air escapes. Do not remove the key or open the valve any further — a small opening is all that is needed.
- 4
Wait for a steady trickle of water
Hold the cloth under the valve. When the hissing stops and a small, steady trickle of water appears, all the air has been expelled. Close the valve immediately by turning the key clockwise until it is snug — do not overtighten or you may damage the valve seat.
- 5
Check the boiler pressure and re-pressurise if needed
Bleeding releases a small amount of water, which can drop the system pressure. Check the boiler pressure gauge — most systems run between 1 and 1.5 bar when cold. If the needle has fallen below 1 bar, top up via the filling loop (a braided-steel hose usually underneath the boiler) until the gauge returns to the correct range, then switch the heating back on.
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FAQ
- How often should I bleed my radiators?
- Once a year is the standard recommendation — ideally at the start of the heating season before temperatures drop. If you notice cold spots returning sooner, or hear persistent gurgling, bleed again. Properties that sit empty for months (such as holiday homes in Greece) benefit from a bleed every time the system is recommissioned.
- What if water keeps coming out but the radiator still has a cold top?
- If water flows freely but the top stays cold after the system heats up, the issue may be a faulty thermostatic valve, a build-up of sludge inside the radiator, or a circulation problem rather than trapped air. At that point it is worth calling a heating engineer to flush the system or inspect the pump.
- My bleed valve is stuck and will not turn — what should I do?
- Do not force it with excessive pressure, as the valve body can shear. Try applying a few drops of penetrating oil around the valve, wait ten minutes, and try again with the correct-sized bleed key (not pliers, which can round off the fitting). If it still will not move, call a plumber — a seized valve can usually be replaced in under an hour.