How to Bleed a Radiator in London: Step-by-Step with Pressure Guidance
Cold spots at the top of radiators are almost always trapped air. This step-by-step guide covers the full process, what to do when system pressure drops afterwards, and when to call a heating engineer.
Why Radiators Develop Air Locks
Air enters a central heating system through several routes: during initial filling, through micro-leaks at pump seals or radiator valves, or when the system pressure drops low enough to draw air into the expansion vessel. In London's older housing stock — Victorian and Edwardian terraces with original pipework or post-war conversions — air accumulation is more common because pipework often lacks consistent falls toward the drain-off point, and manual air vents may not have been serviced in years.
The symptom is reliable: a radiator that is hot at the bottom but cold at the top, or cold in an irregular patch at the upper section. The fix is straightforward but requires attention to system pressure before and after the procedure.
Equipment You Need
- A radiator bleed key (standard square-head key available from any hardware shop; some modern radiators use a flathead screwdriver)
- A small cloth or container to catch water
- A pressure gauge reading on your boiler — check before you start
Step-by-Step Process
- Turn the heating on and let the system reach operating temperature. Run it for 15 to 20 minutes to allow any trapped air to migrate to the highest points in the system.
- Turn the heating off and wait 15 minutes for circulation to stop. Bleeding a pressurised system while the pump is running can force more air in.
- Note the system pressure on the boiler pressure gauge. A correctly pressurised cold system should read between 1.0 and 1.5 bar. The warm-system pressure will be slightly higher, typically 1.5 to 2.0 bar. Record this reading.
- Locate the bleed valve on the radiator — it is the small square nipple at the top of one end, usually on the opposite end from the lockshield valve.
- Place your cloth beneath the bleed valve to catch any water that escapes.
- Insert the bleed key and turn anti-clockwise by a quarter to half a turn only. You will hear a hissing sound as air escapes. Hold the cloth against the valve.
- Wait until water begins to dribble from the valve — this indicates all air has been purged from that radiator. Close the valve immediately by turning clockwise. Do not overtighten.
- Repeat for all radiators that showed cold spots, working methodically from the ground floor up, as air tends to accumulate at the highest points in the system.
What to Do When System Pressure Drops
Bleeding releases air that was occupying space in the system, which will cause the pressure gauge reading to fall. A drop of 0.2 to 0.3 bar across a full bleed of several radiators is normal. If pressure falls below 1.0 bar the system needs represurising via the filling loop before being switched back on — most modern boilers will show a pressure fault or lock out at low pressure.
To repressure: attach the filling loop (usually a braided flexi hose connecting the mains cold supply to the heating system, located beneath the boiler), open both valves slowly, and watch the pressure gauge rise. Stop at 1.2 to 1.5 bar. Close both valves and remove the filling loop if it is a temporary connection type.
If pressure drops rapidly — more than 0.5 bar within hours of represurising — there is either a system leak or a faulty expansion vessel requiring engineer attention.
How Often Should You Bleed Radiators in a London Home
For a well-maintained system with an inhibitor dosed correctly, annual bleeding at the start of each heating season is sufficient for most London properties. If you find yourself needing to bleed repeatedly or if the same radiators keep developing cold spots within weeks, the underlying cause needs investigation: a failing auto air vent, a pump seal drawing air, or low inhibitor levels causing internal corrosion and hydrogen generation.
When to Call a Heating Engineer
Arrange a professional visit when:
- Pressure drops consistently after represurising, suggesting a leak
- Bleeding produces discoloured water (brown or black), indicating system sludge
- Multiple radiators need bleeding every few weeks despite inhibitor top-up
- The boiler loses pressure overnight even when no bleeding has been done
- You cannot locate the bleed valve or it appears seized and will not turn without force