Pressurising and Bleeding a Radiator

Air is lighter than water, so it collects at the highest points of the system: the tops of the radiators and the radiators on the top floor. The typical signs are:

• The radiator only heats at the bottom or does not warm up at all, even though the boiler is running
• Gurgling or bubbling in the radiator or pipes, especially when the heating first comes on
• Low pressure on the boiler gauge or a low pressure fault code on the boiler display
• The room feels cooler than the temperature you have set on the thermostat

Why does the pressure drop in the first place? When you bleed a radiator, a little water escapes with the air. Small losses also happen at seals and valves. If you find yourself topping up often, say every few weeks, there is probably a leak somewhere or the expansion vessel is not working properly. At that point topping up will not solve anything, and it is time to call a heating engineer.

One thing worth knowing: many older homes have an open-vented system with a small feed tank in the loft. These systems top themselves up and have no pressure gauge or filling loop, so only the bleeding part of this job applies. The pressurising steps below are for sealed systems, which include most combi boilers.

A heating engineer or a confident homeowner works in two stages: first let the air out, then restore the pressure. Doing it the other way round makes no sense, because the pressure would simply drop again after bleeding.

To bleed the system, the heating and pump are switched off and the system is left to stand for a while. This lets the air rise to the top and the water cool down a little. Then the bleed valve on each radiator is opened slightly. It is usually a small square valve at one of the top corners, opened with a radiator key, and on some models with a flat screwdriver. The valve stays open as long as air is hissing out. It is closed once a steady stream of water comes out with no bubbles. In a house with more than one floor, start on the ground floor and work upwards, because the highest radiators trap the most air.

Pressurising means adding water through the filling valve. On sealed systems this is often a filling loop: a short flexible hose with a valve at each end, connecting the mains cold water to the heating circuit, usually underneath the boiler. Some boilers have a built-in filling key or lever instead. The water is let in slowly while you watch the gauge. On a typical domestic boiler, a cold system usually sits roughly between 1 and 1.5 bar, and should stay within about 1 to 2 bar. The exact figure is in the boiler manufacturer's instructions. A taller house may need slightly higher pressure so that the water reaches the top radiators.

In a house or a flat with its own boiler, you can do this yourself. You do not take anything on the boiler apart; you only open valves that are meant for the user. If you live in a block of flats with communal or district heating, do not top up the pressure yourself. Ask the building management or owners' association first whether you may bleed your own radiators and how it is handled in your building.

1. Switch off the heating and the circulation pump, and wait until the radiators have cooled to at least lukewarm.
2. Check the boiler pressure gauge and note the reading.
3. Hold a cloth or a small dish under the bleed valve.
4. Turn the valve slowly with the key, about a quarter to half a turn, no more.
5. Let the air hiss out and wait until water flows without bubbles.
6. Close the valve, just snug, without forcing it.
7. Repeat on the other radiators, working from the lowest floor upwards.
8. Check the pressure gauge again.
9. If the pressure has fallen below the manufacturer's figure, open the filling valve slowly and watch the needle.
10. Close the filling valve firmly once the pressure reaches the recommended level. If it is a filling loop, disconnect it as described in the boiler manual.
11. Switch the heating back on and, after half an hour to an hour, check that the radiators heat across their whole surface.
12. Once the system is hot, check the pressure once more and repeat the bleeding if needed.

If you smell gas, do not switch anything on or off. Open windows, leave the property and call the gas emergency line or your local emergency number (112). Any work on the gas boiler itself, its burner or its flue must only be done by a qualified, registered gas engineer.

The simple case is a newer system with easy-to-reach radiators, working valves and a boiler with a filling loop and a clear gauge. Even a layperson can do this quickly. These situations take more time and experience:

• Many radiators over several floors, where you have to go round the whole house and sometimes bleed more than once
• Seized or corroded bleed valves that will not open, or leak after opening and need replacing
• Radiators behind furniture or covers that are hard to get at
• An old system with sludge, where dark water comes out of the valve and the radiator still does not heat after bleeding
• Pressure that keeps dropping, caused by a leak or a faulty expansion vessel
• A system combined with underfloor heating, where the circuits are bled at the manifold and the process is more involved

In cases like these, the job is less about bleeding and more about finding the cause. A heating engineer will check the expansion vessel, the pump and the thermostatic valves, and judge whether the system needs flushing.

• Bleeding with the heating on: the water is hot and the pump keeps pushing air around the system, so you never get it all out
• Opening the valve too far: the small valve can come right out, and water then sprays over the wall and floor
• Closing too early: close the valve only when clear water flows without bubbles, not at the first spurt
• Forgetting the pressure: after bleeding, the pressure stays low and the boiler shuts down after a while
• Overfilling: too much pressure opens the pressure relief valve, which on typical boilers usually lets go at around 3 bar, and water starts dripping from the discharge pipe outside
• A filling valve left slightly open: water keeps creeping in and the pressure rises on its own
• Topping up often without looking for the cause: every bit of fresh water brings in oxygen and minerals, which encourage rust and sludge

After bleeding and pressurising, let the system heat up and go through these points:

• All radiators heat evenly across their whole surface, including the top
• Nothing gurgles or bubbles in the radiators or pipes
• The pressure on the cold system is within the boiler manufacturer's range
• The pressure rises slightly when the system is hot, but stays well below the relief valve limit
• The bleed valves are dry and not dripping
• The filling valve is closed and the loop disconnected, if the manual requires it
• The boiler shows no fault codes
• The pressure has not dropped noticeably after a few days of use