How to Bleed a Radiator in London -- Complete Step-by-Step Guide
A radiator cold at the top has trapped air. Bleeding it takes under 5 minutes and costs nothing. Here's exactly how to do it.
Before You Start
What You Need to Bleed a Radiator
Everything you need costs under a fiver and is available from any hardware shop.
Radiator bleed key
£2-£5 at B&Q, Screwfix or Amazon. Fits the square brass nut on the bleed valve. One key fits virtually all UK radiators.
OR a flathead screwdriver
Some modern radiators have a slotted bleed valve rather than a square nut. Check yours before buying a key.
Old cloth or small towel
To wipe up drips and protect the floor or carpet beneath the valve.
Bowl or bucket
Holds the water that trickles out once the air has cleared. Usually only a small amount.
Pen and paper
Note which radiators needed bleeding so you can track whether the same ones keep needing it.
Total cost: £2-£5 for the bleed key if you don't already own one.
Diagnosis
How to Identify Which Radiators Need Bleeding
Not every cold radiator has trapped air. Run this check first so you don't waste time on the wrong fix.
Turn heating on and run for 15-20 minutes
Give the system enough time for heat to reach every part of each radiator before you assess them.
Feel each radiator using the back of your hand
Use the back of your hand, not your palm -- the radiators will be hot. Work your way through the house systematically.
Warm at bottom, cold at top: air trapped -- bleed it
This is the classic air lock symptom. Hot water rises but air sits above it, blocking full circulation.
Cold all over: different problem
A radiator that is completely cold usually has a stuck thermostatic radiator valve (TRV), a closed lockshield valve, or a sludge blockage. Bleeding will not help. See our guide to radiators not heating up.
Cold at bottom, warm at top: sludge build-up
Magnetite (black sludge) settles at the bottom and blocks circulation there. Bleeding releases trapped air but does not remove sludge -- a power flush is needed.
Step-by-Step
How to Bleed a Radiator -- 8 Steps
Follow these steps in order. The whole process takes under 5 minutes per radiator.
Turn on the central heating and wait
Switch the heating on and leave it running for 15-20 minutes until all radiators are warm. This allows air to rise to the top of each radiator where the bleed valve is.
Switch the heating OFF
Turn the boiler or heating controls off before you start bleeding. If the pump is running while you bleed, it can push more air into the system as fast as you release it.
Start with the radiator furthest from the boiler
Work systematically -- usually start upstairs, and begin with whichever radiator is furthest from where the boiler is located. This clears the most trapped air first.
Locate the bleed valve
The bleed valve is the small square brass nut set into the top corner of the radiator body -- usually on the left or right end. Many have a plastic cap you can pop off with a fingernail.
Place cloth and bowl underneath
Slide a folded cloth under the valve and hold a bowl directly below. Water will drip or trickle out when the air has cleared -- you don't want it on the floor or carpet.
Insert the key and turn anti-clockwise
Push the bleed key squarely onto the brass nut and turn slowly anti-clockwise (counter-clockwise). About a quarter turn is usually enough -- you do not need to fully remove the valve.
Wait for the hissing to stop
Air will escape with a hissing sound. Keep the key in position and hold the cloth ready. Do not leave the valve unattended -- the transition from air to water can happen quickly.
Close when water flows steadily
When water starts trickling steadily with no more hissing, close the valve by turning the key clockwise. Tighten firmly but do not overtighten -- the brass thread is easy to strip.
Repeat for all affected radiators
Once you have bled every radiator that needed it, turn the heating back on and check each one. Then check the boiler pressure gauge before the system fully heats up.
After Bleeding
Check Boiler Pressure After Bleeding
Releasing air from radiators reduces the amount of water in the system, which drops the pressure reading on the boiler. This is normal -- but you need to correct it.
1.0-1.5 bar
Normal
System is fine. Turn heating back on and monitor radiators.
Below 1.0 bar
Top up needed
Repressurise using the filling loop under the boiler. See our step-by-step guide.
Above 2.5 bar
Too high
Bleed a small amount of water from a radiator to reduce pressure before turning heating on.
If your pressure is below 1 bar after bleeding, read our full guide: How to Repressurise a Boiler -- Step by Step.
Maintenance
How Often Should You Bleed Radiators in London?
Once a year is normal
For most London homes, bleeding radiators once a year -- ideally in September before the heating season starts -- is enough to keep the system running efficiently.
Older iron radiators may need it more
Victorian and Edwardian properties often have cast iron radiators that are more prone to corrosion. Hydrogen gas produced by corrosion inside the radiator can require bleeding 2-3 times a year.
Every few weeks: there is a problem
If the same radiator keeps filling with air, you have a micro-leak introducing fresh air into the system, a failed automatic air vent, or active corrosion producing hydrogen gas. This needs a plumber to investigate -- it will not resolve itself.
Sign it is time: noticeably cooler at the top than last year
If a radiator that heated well last winter is now cold at the top, bleed it. It does not mean something has gone wrong -- slow air accumulation over a season is expected.
Troubleshooting
What If Bleeding Doesn't Fix the Cold Radiator?
Bleeding solves one problem: trapped air. If the radiator is still cold after bleeding, the cause is different.
Still cold all over after bleeding
Cause: TRV stuck closed, lockshield valve fully closed, or pump not circulating to this radiator
Action: Open the lockshield valve slightly, check TRV pin is not stuck
Warm at top, cold at bottom
Cause: Sludge (magnetite) settled at the bottom blocking water flow
Action: A power flush is needed to clear sludge from the system
More information →Bleed valve leaks and won't stop
Cause: The valve seat is damaged -- usually from overtightening or age
Action: Call a plumber -- valve replacement costs £60-£120 and is a 30-minute job
Still cold after bleeding AND repressurisng
Cause: Completely different underlying issue -- not air, not pressure
Action: Read our full diagnosis guide for radiators not heating up
More information →Cost Guide
Radiator Bleeding -- Tools and Costs
| Item | Cost | Where to Get |
|---|---|---|
| Radiator bleed key | £2-£5 | B&Q, Screwfix, Amazon |
| Replacement bleed valve | £5-£15 | Plumbers merchant |
| Bleed valve replacement (plumber) | £60-£120 | If valve stripped |
| Boiler repressurisation (DIY) | Free | -- |
| Plumber call-out (if DIY fails) | £60-£100 | Us |
All plumber prices include VAT. Fixed quote given before any work starts.
London Quick Tips
London homes often have 12-20 radiators across multiple floors -- set aside 30-45 minutes to bleed them all in one session.
Start at the top floor and work downwards -- air rises, so upper-floor radiators accumulate it fastest.
Victorian terrace houses: check the radiator in the loft conversion or top bedroom last -- it is often the one with the most trapped air.
After bleeding, always check the boiler pressure gauge and top up if it reads below 1.0 bar -- London homes with many radiators see a bigger pressure drop when bleeding.
Radiator Questions
Frequently Asked Questions
Bleeding Didn't Fix the Problem?
London heating engineers, same-day diagnosis. If the radiator is still cold after bleeding, there is a different underlying cause -- our engineers carry the parts to fix the most common faults on the first visit.
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