How to Bleed a Radiator
Bleeding a radiator releases trapped air from the central heating system. Air accumulates in radiators over time (from system top-ups, micro-leaks, or internal corrosion) and sits at the highest point in the radiator where hot water cannot displace it. The result is a radiator that runs cold at the top, takes longer to heat up, and wastes energy.
Bleeding is a 10-minute job that requires only a bleed key and a cloth.
Signs a Radiator Needs Bleeding
- Cold at the top, warm at the bottom: the clearest indicator. Hot water fills the lower section but air trapped above prevents full heating.
- Gurgling or trickling sounds when the heating is running.
- Noticeably slow to heat up compared with other radiators on the system.
- Gradual pressure drop on a sealed system: trapped air can mask slow pressure loss.
Important: Cold spots at the bottom of a radiator are caused by magnetite sludge settling in the radiator base, not air. Bleeding will not fix this: the system needs a chemical clean or powerflush. See the guide on flushing and protecting your central heating system.
What You Need
- Radiator bleed key (available at any hardware shop, typically under £1)
- A cloth or small bowl
- Access to the boiler pressure gauge
Step-by-Step: Bleeding a Radiator
1. Switch the heating off
Turn the heating off at the thermostat and let the system cool for 30 minutes. Bleeding a hot system risks scalding and can draw further air in through the pump.
2. Identify which radiators need bleeding
Before switching off, run the heating fully and feel across each radiator. Radiators with air pockets will have a noticeably cooler zone at the top. Make a note of which ones need attention.
3. Start with the ground floor, work upwards
Air rises to the highest points in the system. Work from the lowest radiators first: this drives air upwards and out progressively.
4. Open the bleed valve
The bleed valve is a small brass fitting at the top corner of the radiator, usually at the end opposite the TRV head. Insert the bleed key and turn anti-clockwise by approximately a quarter turn. Do not remove the valve entirely.
Hold the cloth below the valve. You will hear a hiss as air escapes. When the hissing stops and a steady drip of water appears, close the valve by turning clockwise until snug. Do not overtighten: the valve only needs firm hand pressure from the key.
5. Check the boiler pressure
Each bleed releases a small volume of water, causing pressure to drop on sealed systems. After bleeding all radiators, check the boiler pressure gauge. If it has dropped below 1.0 bar, top up using the filling loop (usually located beneath the boiler) until the gauge reads 1.0–1.5 bar.
If you are unfamiliar with the filling loop procedure, consult the boiler manual. The loop location and operation varies between manufacturers.
6. Restart the heating and recheck
Switch the heating back on and run through a full cycle. The previously cold radiators should now heat evenly from top to bottom. If cold spots remain, repeat the bleed on that radiator: occasionally a single bleed does not release all trapped air.
Inhibitor: Protect the System After Bleeding
Whenever water is added to the system (through bleeding, a pressure top-up, or any pipework work), the inhibitor concentration in the system water is diluted. Inhibitor prevents internal corrosion of the radiators, pipework, and heat exchanger.
Add Fernox F1 Central Heating Protector via a radiator bleed point: remove the TRV head, hold a cloth around the bleed valve spindle, open the bleed valve slowly, and inject the inhibitor using the dosing bottle. Close the bleed valve and re-bleed any remaining air.
An ADEY MagnaClean Professional2 Magnetic Filter fitted to the return pipe captures magnetite particles before they circulate through the boiler heat exchanger. It can be serviced annually in minutes and extends boiler life significantly.
When to Call a Heating Engineer
Do not continue bleeding as a routine fix if any of the following apply:
- Radiators need bleeding more than twice a year: indicates a system leak or failing component
- Water at the bleed valve is black or very dark: significant sludge contamination requiring a system flush
- System pressure drops repeatedly to zero: active leak present
- The boiler pressure relief valve has discharged (water stain or pipe visible outside the property): indicates overpressure, possibly a faulty expansion vessel
These are not DIY fixes. A heating engineer can pressure-test the system, identify the source of air ingress, and advise on a system flush if contamination is present.
