How to Bleed Radiators — Step-by-Step Guide for London Homes
Cold spots at the top of radiators are almost always caused by trapped air. Bleeding is a simple five-minute task you can do yourself. This guide covers everything — when to bleed, what to use, and what to check afterwards.
When to Bleed Your Radiators
Bleeding a radiator releases trapped air from the heating system. Air accumulates over time — particularly after the heating has been off for summer, after system maintenance, or after topping up the boiler pressure. Signs you need to bleed:
- Cold at the top, warm at the bottom: Classic sign of trapped air — the air rises to the highest point in the radiator and prevents hot water circulating through the top section
- Gurgling sounds from radiators: Air bubbles passing through the system make a gurgling or bubbling sound
- Whole radiator cold when system is running: Could be air, but could also be a closed lockshield valve or sludge — see below
- Heating taking longer to warm up: Trapped air reduces system efficiency
What You Need
- A radiator bleed key (£1-2 from any hardware shop, fits all standard radiators)
- A cloth or small bowl (to catch a few drops of water)
- Knowledge of where your boiler pressure gauge is
Step-by-Step: How to Bleed a Radiator
- Turn the heating off and let the system cool for 15-20 minutes. Do not bleed with the system running — the pump creates pressure that makes bleeding less effective and more likely to spray water.
- Locate the bleed valve: It is the small square socket at the top corner of the radiator — usually the end opposite the supply pipe. On older radiators it may be a nipple-style fitting.
- Hold your cloth under the bleed valve.
- Insert the bleed key and turn anti-clockwise by about a quarter to half a turn — no more. You will hear a hiss as air escapes.
- Wait for the hissing to stop and for a steady dribble of water to appear. As soon as water flows, close the valve by turning the key clockwise firmly (do not overtighten).
- Repeat for all radiators — start on the ground floor and work upward; bleed the highest radiators last.
- Check boiler pressure: After bleeding, boiler pressure may have dropped. Check the pressure gauge on the boiler front — it should read 1-1.5 bar when cold. If it has dropped below 1 bar, top up via the filling loop until pressure reaches 1.2 bar.
What If the Radiator Is Still Cold After Bleeding?
If bleeding does not resolve the cold radiator:
- Check the lockshield valve is open: The lockshield is the valve cap (usually a white plastic cap) on the opposite end from the thermostatic valve. It may have been turned off during maintenance and not reopened. Remove the cap and turn the valve stem anti-clockwise to open.
- Check the thermostatic radiator valve: TRVs sometimes stick in the closed position. Turn the TRV head fully to its maximum number and wait 10 minutes.
- Sludge: If the radiator is cold throughout and other radiators are fine, magnetite sludge may have settled in that radiator. A plumber can remove and flush the individual radiator or recommend a full power flush.
Frequently asked questions
How often should I bleed my radiators in a London home?
Once a year at the start of heating season (September-October) is good practice. If you notice cold spots or hear gurgling at any point, bleed immediately regardless of schedule. In London hard water areas, scale and corrosion byproducts can displace air more frequently — biannual bleeding may be needed in older systems.
Do I need to top up the boiler after bleeding radiators?
Possibly. Each time you release water from the system by bleeding, the pressure in the heating circuit drops slightly. After bleeding all radiators, check the boiler pressure gauge. If it reads below 1 bar, add water via the filling loop until it reaches 1.2 bar. If pressure drops repeatedly without bleeding, there is a small leak in the system.
Water came out of the bleed valve but the radiator is still cold — why?
If water came out immediately with no air hiss, you either didn't need to bleed or the air is in a different part of the system. If the radiator is still cold, the issue is likely a closed lockshield valve, a stuck TRV, or sludge blockage — not trapped air.
My bleed valve is stuck and will not turn — what should I do?
Do not force it — you risk snapping the valve nipple, which requires draining the system to replace. Apply a small amount of penetrating oil (WD-40) and leave for 10 minutes. Try again with a proper radiator bleed key for maximum torque. If it still will not move, call a plumber — a seized bleed valve can be drilled out and replaced without draining the full system.