If your radiators are warm at the bottom but cold at the top, or not heating evenly at all, there is a good chance air is trapped in the system. Bleeding a radiator is one of the few plumbing jobs that most homeowners can safely do themselves, and it takes less than five minutes once you know how.
This guide covers exactly how to bleed a radiator, what to do if bleeding does not fix the problem, and the signs that point to something more serious going on with your central heating system.
Why Does Air Get Trapped in Radiators?
Air enters the central heating system in several ways: through small leaks in joints or valves, during top-ups after a boiler pressure drop, or simply through the natural chemistry of the water breaking down over time. As air accumulates, it rises to the top of the radiator and prevents hot water from filling that section. The result is a radiator that heats unevenly, wastes energy, and leaves rooms feeling colder than they should.
In older Liverpool properties, particularly Victorian and Edwardian terraces with well-used heating systems, bleeding is often needed at the start of every heating season.
What You Need to Bleed a Radiator
- A radiator bleed key (available from any hardware shop for around £1 to £2)
- A cloth or small towel
- A bowl or container to catch drips
That is it. No tools, no plumbing knowledge beyond this guide.
How to Bleed a Radiator Step by Step
Step 1: Turn the Heating On
Run your heating until it reaches its normal temperature. This allows you to identify which radiators have cold spots before you start. Feel along the top of each radiator with your hand. Any radiator that is noticeably cooler at the top than the bottom has trapped air and needs bleeding.
Step 2: Turn the Heating Off and Let It Cool Slightly
Switch the heating off and wait 20 to 30 minutes before bleeding. You do not want to work with water at full boiling temperature. The system will hold enough pressure for the process to work without the risk of scalding water.
Step 3: Locate the Bleed Valve
The bleed valve is a small square-headed plug at the top corner of the radiator, usually on the side opposite the thermostatic valve. This is where you insert the bleed key.
Step 4: Place the Cloth and Open the Valve
Hold your cloth or bowl under the valve to catch any water. Insert the bleed key into the valve and turn it slowly anti-clockwise, roughly a quarter to half a turn. You should hear a hissing sound immediately. That is trapped air escaping.
Do not unscrew the valve all the way. A small turn is all that is needed.
Step 5: Wait for Water to Appear
Keep the valve open until the hissing stops and water begins to trickle out steadily. Once you see a continuous stream of water with no more air mixed in, close the valve by turning the key clockwise. Do not over-tighten it.
Step 6: Repeat for All Affected Radiators
Work your way through all the radiators in the house that felt cold, starting on the ground floor and working upwards. The order matters slightly: starting downstairs ensures you are bleeding from the lowest points first.
Step 7: Check the Boiler Pressure
After bleeding, check the boiler pressure gauge. Bleeding releases water from the system, which can cause the pressure to drop. Most combi boilers need to sit between 1 and 1.5 bar. If the pressure has dropped below 1, you will need to repressurise the boiler using the filling loop underneath the unit. Check your boiler manual for exact instructions, or call Jimmy Taps if you are unsure.
Step 8: Turn the Heating Back On
Switch the heating back on and feel the radiators again once they warm up. They should now heat evenly from top to bottom.
What if Bleeding Does Not Fix the Problem?
If a radiator is still cold after bleeding, or if you need to bleed the same radiators repeatedly every few weeks, the problem is not simply trapped air. The causes could include:
Sludge Buildup
Magnetic iron oxide sludge accumulates at the bottom of radiators over time, causing cold patches at the bottom rather than the top. This is common in older Liverpool heating systems that have never been power flushed. A radiator with cold patches at the bottom but warm at the top is full of sludge, not air. Bleeding will not help. A power flush of the whole system is the effective solution.
Faulty Thermostatic Radiator Valve
If a radiator stays cold even when the rest of the system is hot, the thermostatic radiator valve (TRV) on that radiator may be stuck closed. The pin inside the valve can seize, particularly in older valves or after the heating has been off for the summer. Sometimes gently tapping the valve body with a rubber mallet frees the pin. If not, the valve needs replacing.
Pump or System Fault
If multiple radiators are cold throughout the house despite bleeding, the problem may be with the circulation pump, an airlock in the system, or a boiler fault. These require a plumber to diagnose properly.
Persistent Air Ingress
If you are bleeding the same radiators every week, air is getting into the system continuously through a small leak somewhere. Over time this also introduces oxygen which accelerates corrosion. A plumber can locate the ingress point and add a leak sealer or inhibitor to protect the system.
Bleeding Radiators: When to Call Jimmy Taps
Call a plumber when: bleeding does not improve radiator performance; you need to bleed repeatedly; the boiler pressure drops after bleeding and will not stay up; any radiator is cold across most of its surface; or you hear banging or gurgling from the pipes during heating.
Jimmy Taps covers radiator issues, power flushes, valve replacements, and full central heating diagnostics across Liverpool, Merseyside, and the Wirral. Fixed prices. Free quotes. Call 07307 331103.
Frequently Asked Questions
How often should I bleed my radiators?
Once a year, at the start of the heating season, is a good habit. Bleed more often if you notice cold spots developing.
Do I need to turn the heating off before bleeding?
Yes. Always turn the heating off and allow it to cool slightly before bleeding. Hot pressurised water can cause scalding if the valve is opened too far.
What if water does not come out when I bleed the radiator?
If no water appears and no air escapes either, the bleed valve may be blocked or the system may be very low on pressure. Check the boiler pressure gauge first. If pressure is fine but nothing comes from the valve, call a plumber.
For central heating problems across Liverpool, Merseyside and the Wirral, call Jimmy Taps on 07307 331103.