Bleed radiator? Step-by-step guide, and what if it doesn't get warm

In a central heating system, hot water flows from the boiler to the radiators and back again. A small amount of air always enters that water: when topping up, after work on the pipes, or because oxygen is released from the water. That air collects at the highest point, at the top of the radiator. The hot water can then no longer reach that spot.

When should you bleed a radiator?

• The radiator is warm at the bottom, but cold at the top.
• You hear gurgling, sloshing or ticking in the radiator or the pipes.
• The radiator doesn't get warm at all, while the others do get warm.
• At the start of the heating season, or after work has been carried out on the central heating system.
• After you have topped up water.

What do you need?

A radiator key (a small square key, available at DIY stores; on some radiators a flat screwdriver fits), a cloth and a bowl or cup to catch water.

Bleeding a radiator: step-by-step plan

1. Turn the heating up briefly. Turn the thermostat up for fifteen minutes and open all radiator valves, so the water circulates and the air is pushed upwards.
2. Then turn off the heating. Turn the thermostat down or put the boiler on standby, so the pump stops. Wait a few minutes: when the water is still, the air rises to the highest point. Don't bleed while the pump is running; the system will actually draw in air in some places.
3. Start downstairs. Work from the lowest floor upwards. On each floor it makes little difference which radiator you start with.
4. Open the bleed valve. This is at the top on the side of the radiator. Hold the cloth and the bowl underneath and turn the valve a quarter to half a turn open with the key. Careful: the water can be hot.
5. Let the air escape. You'll hear hissing. As soon as a jet of water without air bubbles comes out, close the bleed valve again. Not too tight, or you'll damage it.
6. Repeat this for every radiator that needs it, and then on the floor above.
7. Check the water pressure and top up if necessary. Bleeding removes water from the system, so the pressure drops. Check the pressure gauge or the boiler display.

After bleeding: top up the pressure

For most central heating systems in a home, the correct pressure when the system is cold is between approximately 1.5 and 2 bar. Check your boiler's manual to see what applies to your appliance. If the pressure is too low, top up the water:

1. Switch the boiler off or to standby.
2. Connect the filling hose to the filling valve of the system (under the boiler) and to a tap, usually in the kitchen or by the washing machine. First run some water through the hose, so there's no air in it.
3. First open the water tap, then slowly open the filling valve. Keep an eye on the pressure gauge.
4. Close both taps as soon as the correct pressure is reached, and remove the hose. Switch the boiler back on.

Don't top up too much. If the pressure becomes too high when heating up, the pressure relief valve will open and water will leak out. If you have to top up frequently, the system is losing water somewhere. Read about this in boiler losing pressure and in the article about the expansion vessel.

Radiator not getting warm, even after bleeding?

Then the cause lies elsewhere. The most common:

• The pin of the thermostatic valve is stuck. After summer, the pin under the thermostat knob sometimes gets stuck. Unscrew the knob and gently press the pin in a few times. If it doesn't spring back, it's stuck. Carefully move it back and forth with pliers.
• The radiator valve or the lockshield valve is closed. Check that the valve is open. The lockshield valve on the other side of the radiator is usually set by the installer; don't turn it without noting how it was set.
• The water pressure is too low. Radiators on the top floor in particular then receive too little water.
• The system is not properly balanced. If radiators close to the boiler heat up quickly while those further away stay lukewarm, the water is taking the shortest route. By adjusting the lockshield valves, the water is distributed more evenly.
• Dirt or sludge in the radiator. If the radiator is cold at the bottom and warm at the top, there's often sediment at the bottom.
• A problem with the pump or the boiler. If all radiators aren't getting warm, it's more likely the boiler, pump or thermostat. Then read about a boiler breakdown.

Safety

The water in the radiator can be very hot: keep your hands away from the opening and keep children away. Do not force a stuck bleed valve; if it breaks off, the radiator will leak. Never open the boiler yourself.

Do you smell gas? Then do not touch the boiler, do not use light switches or open flames, open windows and doors, leave the property and call the national gas and electricity fault number: 0800-9009.

Radiator stays cold? Here's how we tackle it

If a radiator stays cold, even after bleeding and with sufficient pressure, it's usually not due to air. The most common causes are a stuck pin in the thermostatic head, a closed lockshield valve or a system that is not properly balanced, causing the hot water to flow mainly to the nearest radiators.

We check the thermostatic valve and the lockshield valve, free a stuck pin or replace the internals, and balance the system so that all radiators heat up. If a radiator is leaking or corroded through, we replace it.

Also read central heating boiler losing pressure if you have to top up frequently after bleeding.