How to bleed your radiators properly (and what to do if it doesn't fix it)

Cold radiators are the single most common heating complaint I get. About a third of the time, the answer is "bleed them". The other two-thirds, bleeding will do nothing because the problem is something else. Here is how to do it properly, and how to tell when you are dealing with a different problem.

When bleeding works

Air gets into your heating system over time, especially after:

• Topping up the boiler pressure
• Working on a radiator (moving, replacing, painting behind)
• Power cuts that reset the heating
• A general slow accumulation over years in older systems

When air collects in a radiator, it rises to the top and stops the hot water from filling that section. The radiator goes cold at the top while staying warm at the bottom. Bleeding releases the trapped air and lets the water fill the radiator properly.

Symptoms that point to bleeding being the fix:

• Radiator cold at the top, warm at the bottom
• Gurgling sounds when the heating runs
• Some radiators heat properly, others do not
• Recently topped up boiler pressure

Symptoms that point to other problems:

• Whole radiator cold (top, middle, and bottom)
• Radiator warm at the top, cold at the bottom (sludge problem, see separate post)
• Water leaking from radiator joints
• Boiler losing pressure repeatedly

What you need

• A radiator key (about £2 from any DIY shop). Some modern radiators have a slotted screw that takes a flat-head screwdriver instead.
• A small cloth or container to catch drips
• 10 to 15 minutes for a typical 6 to 8 radiator house

The proper sequence

1. Turn the heating off

Important. If the heating is running, the system is pressurised and bleeding is messy. Turn the boiler off at the front panel and wait 15 to 30 minutes for the system to cool slightly.

2. Identify which radiators need bleeding

Walk around the house with the heating off (still warm from earlier). Touch each radiator. If a radiator is cold at the top while warm at the bottom, it needs bleeding.

If you are bleeding the whole house as a precaution, do it in this order: ground floor first, then upstairs, then top floor. Air rises, so working bottom to top is right.

3. Locate the bleed valve

Top corner of each radiator, usually on the opposite side from the thermostatic radiator valve (TRV). A small square or hexagonal nut, or a slotted screw on more modern designs.

4. Place your cloth or container under the valve

A few drops of water will come out. Sometimes more, sometimes a small spray. Have something to catch it.

5. Open the valve slowly

Turn the radiator key or screwdriver anti-clockwise, slowly, by quarter turns. You should hear a hiss as air escapes. This may take 5 to 30 seconds depending on how much air is in the radiator.

6. Wait for water

When the hissing stops and water starts to dribble out (slow, clean water), the air is gone. Close the valve immediately by turning clockwise.

Do not over-tighten. Hand-tight with the key is enough. Forcing it can damage the valve.

7. Check the boiler pressure

Bleeding releases water as well as air, so the system pressure drops. After bleeding all the radiators, check the pressure gauge on the front of the boiler. If it is below 1 bar, top up using the filling loop until it reads 1.2 to 1.5 bar.

8. Turn the heating back on

Run the system for 30 minutes and check that all the radiators are now hot from top to bottom. If any are still cold at the top, bleed again (sometimes one bleed is not enough).

How often to bleed

A healthy heating system should not need bleeding more than once a year, often less. If you find yourself bleeding the same radiator every few months, you have a problem that bleeding is not solving:

• Air keeps getting in. Usually means a small leak somewhere in the system. The leak draws air in when the system cools and contracts.
• Failed expansion vessel inside the boiler. Symptoms include needing to top up pressure repeatedly as well as bleed often. Plumber visit needed.
• Magnetic filter overdue for cleaning. Sludge can produce small amounts of gas as it reacts with system metal. Filter clean usually clears it.

When the radiator is not getting hot at all

Bleeding does not help if water is not flowing through the radiator at all. Check:

Is the TRV stuck?

The thermostatic radiator valve (the one with the numbered dial) can stick closed, especially if it has not been turned in years. Take the head off (it usually unscrews) and look at the pin underneath. It should pop in and out freely. If it is stuck in the closed position, gently free it with a pair of pliers.

Is the lockshield closed?

The lockshield valve at the other end of the radiator (covered with a small plastic cap) controls flow balance across the system. If it has been fully closed, no water flows. Open it 1 to 2 full turns to start. Re-balance properly afterwards if needed.

Is the radiator full of sludge?

If a radiator is cold at the bottom and warm at the top (the opposite of the air problem), the bottom is full of sludge. Bleeding will not help. The radiator needs flushing or replacement.

To check: turn off the boiler, close both valves on the radiator, drain it via the bleed valve, lift it off the brackets, take it outside and rinse it through with a hose. Black water coming out confirms sludge. After flushing, refit, refill, and balance.

For multiple sludgy radiators, a power flush of the whole system is often more efficient than radiator-by-radiator cleaning.

Common mistakes

Bleeding with the heating on

Hot pressurised water can spray out of an open bleed valve. Burns and mess. Always bleed with the boiler off.

Forgetting to top up pressure

If you bleed several radiators and do not top up, the boiler may lock out on low pressure when you switch it back on.

Bleeding too often

Bleeding regularly does not help. It only helps when there is air in the system. Bleeding a system with no air does nothing useful and can introduce more problems if you over-tighten the valves.

Not closing the valve fast enough

When water starts coming out, close the valve immediately. Letting it run for ages dumps system pressure and creates more work topping back up.

Bleeding upstairs first

Air rises. Bleed downstairs first, then upstairs. Doing it in the wrong order means you bleed upstairs, then air from downstairs rises up and you have to redo the upstairs ones.

When to call a plumber

A few situations where DIY bleeding is not the answer:

• You have bled the radiator twice and it is still cold at the top
• The bleed valve will not turn (corroded or seized)
• Water that comes out is dirty black or rusty (sludge problem, not air)
• Pressure keeps dropping despite topping up (you have a leak)
• Boiler is locking out with a fault code

For these, a plumber visit is the right call. Cost in 2026 London for radiator diagnosis: £80 to £150 for the visit, plus repair quoted on top.

What I look for on a "cold radiator" call

When customers call about cold radiators, my standard process:

1. Check the pressure
2. Check the boiler is firing properly
3. Touch every radiator and note which are cold (top, bottom, fully)
4. Check the TRV and lockshield on each problem radiator
5. Bleed where needed
6. Balance the whole system if the cold spots indicate a flow distribution issue
7. Recommend flushing or filter installation if sludge is the underlying problem

A typical visit takes 1 to 2 hours and resolves the issue 90 percent of the time. If the underlying problem is sludge or a leak, I quote the bigger work separately and you decide.

WhatsApp me if you have already bled and the problem persists. A photo of the cold radiator and a description of which radiators are affected helps me come prepared.