How to Bleed a Radiator: Step-by-Step Guide for Beginners

Cold at the top and warm at the bottom. That is the classic signature of a radiator with air trapped inside it, and it is usually why a room never quite reaches the temperature the thermostat promised. Bleeding a radiator is one of the few household jobs that takes ten minutes, needs a single cheap tool and makes an immediate, obvious difference to your heating. It is also easy to get wrong in small ways, whether that means dripping black water onto a pale carpet or dropping the system pressure until the boiler locks out. Here is how to do it properly.

Why radiators need bleeding in the first place

Central heating water carries dissolved air. Every time the system heats up, some of that air comes out of solution, and because air is lighter than water it rises through the pipework and collects at the highest point it can reach. In most homes that is the top of an upstairs radiator, which is why the bedrooms often start misbehaving first.

The result is a pocket of air sitting where hot water should be. The radiator still feels warm at the bottom, because water is getting in, but the top stays stubbornly cool. You may also hear gurgling, ticking or a faint rushing noise, and the boiler has to work harder to bring the house up to temperature.

One useful check before you reach for a key: where the radiator is cold tells you what the problem is. Cold along the top points to air, and bleeding will fix it. Cold along the bottom, or a radiator that is warm at the top and cold at the bottom, usually means sludge and corrosion debris collecting in the base of the radiator. Bleeding will not shift that, and the system needs attention from a heating engineer.

What you will need

• A radiator bleed key. They cost very little from a hardware shop or supermarket, and some radiator valves accept a flat-head screwdriver instead. Never use a knife or scissors.
• An old towel, plus a shallow bowl, mug or jug to catch drips.
• Kitchen roll or a cloth for mopping up. The water can be dirty and black, so wear gloves if you would rather not stain your hands.
• For a sealed system or combi boiler: a quick look at the pressure gauge and knowledge of where the filling loop lives.

Before you start

Turn the heating off at the thermostat or programmer and let the system cool for at least half an hour. Radiator water can be close to 70°C and will scald you if it spits. Bleeding a hot system also means you are more likely to get a face full of water when the air runs out.

Put the towel down under the valve and have the bowl ready. The bleed valve is the small square or slotted pin at the top of the radiator at one end. Work through the house in a sensible order, downstairs first and then upstairs, and take your time. On a sealed system, every radiator you bleed drops the pressure a little, so check the gauge as you go rather than leaving it until the end.

If you have an older open-vented system with a small tank in the loft, glance inside it first. If the tank is empty or the ball valve is stuck, bleeding will simply pull more air into the system and you will be back where you started.

How to bleed a radiator, step by step

1. With the heating off and the radiator cool, hold the bowl and cloth directly under the bleed valve.
2. Insert the key and turn it anticlockwise about a quarter to half a turn. Do not unscrew it further than you need to; the valve pin only has to lift a fraction.
3. You will hear a hiss as air escapes. Let it run until the hissing stops.
4. Water will start to dribble out, spitting at first. As soon as it flows steadily, close the valve clockwise, firmly but gently. The aim is nipped up, not cranked tight.
5. Dry the area and watch for a minute. If it weeps, give the valve the tiniest extra turn. If it still drips, stop forcing it, because the valve body can snap, and call a plumber.
6. Repeat on each radiator, then check the boiler pressure before you switch the heating back on.

If nothing comes out at all when you open the valve, the radiator may simply be isolated. Check that both the thermostatic valve and the lockshield valve at the other end are open before assuming the bleed valve is blocked.

Sealed systems, combi boilers and pressure

Most modern homes run a sealed system, where the boiler keeps water at pressure. When the system is cold, that pressure usually sits somewhere around 1 to 1.5 bar, though your boiler manual is the authority for your model. Bleeding radiators releases water as well as air, so the needle will drop as you work.

If it falls below about 1 bar, top up using the filling loop: usually a braided hose with a valve at one or both ends, often under the boiler. Open the valve slowly, watch the gauge rise into range, then close it fully. Over-pressurising is worth avoiding, because the boiler's pressure relief valve may discharge and then fail to seal. If the pressure keeps falling over the following days, there is a leak or a component fault, and that is a job for a professional. Never remove the boiler casing yourself; gas work in the UK must be done by a Gas Safe registered engineer.

If the radiator is still cold afterwards

Cold at the bottom

Almost always sludge. The system needs flushing, and a powerflush by a heating engineer is the usual remedy. This is not something to attempt with a bucket and a hose.

One radiator cold while the rest are fine

The system may be out of balance, a valve may be stuck, or the pin inside a thermostatic valve may have jammed shut. Sometimes removing the head and easing the pin free does the trick, but seized valves usually need replacing.

Needing to bleed the same radiator every few weeks

Air is getting in somewhere. Common causes include a failing pump, corrosion inside the system, or a small leak pulling air in. There is also a safety point worth knowing: if the water that comes out fizzes, smells unusual or appears unusually black, corrosion may be producing hydrogen gas, which is flammable. Stop and get the system checked rather than bleeding it repeatedly.

Make it part of your autumn routine

Bleeding radiators once a year, ideally in early autumn before you start relying on the heating, keeps the whole system running efficiently and spreads heat evenly through the house. While the heating is on over the following days, walk round and feel each radiator top to bottom. If one stays cool in the same place, you will know whether it needs another bleed or a professional visit.

And if you are unsure at any point, or the boiler pressure will not hold, call a heating engineer. A ten-minute job is not worth a flooded floor or a boiler that will not fire. For anything involving the gas side of the boiler itself, always use someone on the Gas Safe register.

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