How to bleed a radiator

In this guide you’ll find answers to the following questions:

  • Why do radiators need bleeding?

  • When should I bleed my radiator?

  • What causes air to build up in the system?

  • During regular maintenance, what order should I bleed my radiators in?

  • How can I tell when my radiator needs bleeding?

  • Bleeding a radiator

  • Can I bleed my radiators automatically?

  • What else can I do to improve the performance and efficiency of my radiators?

Why do radiators need bleeding?

Over time, pockets of air can build up inside your heating system, making their way to your radiators and preventing water from running through your whole system. Not only is this inefficient, it also results in uneven heating throughout your home, including cold spots where radiators are emitting little or no heat.

When should I bleed my radiator?

Set up an annual schedule to bleed your radiators just before they’re needed – typically when the temperature starts to fall during the autumn months.

What causes air to build up in the system?

Air can be churned into the pipes with water by your heating pump, or you may introduce air yourself during regular maintenance – in this case, it enters the system via your valves.

During regular maintenance, what order should I bleed my radiators in?

Work out which radiator is furthest away from your boiler – most of the air gravitates to this end of the system. If you live in a multi-storey house it’ll be on the opposite floor to your boiler, so typically the ground floor. Work your way backwards through this floor to the radiator closest to the boiler (as in directly underneath or the closest to being directly underneath it), then repeat for the next floor. 

How can I tell when my radiator needs bleeding?

Regardless of whether you regularly bleed your radiators on an annual basis, keep an eye out for one of the following indicators that air is trapped in your system:

  • Radiator makes unusual sounds.

  • Top section of the radiator is colder than the bottom half.

  • A single radiator won’t heat up, even when the thermostat has been fully opened.

If you're testing the radiators as part of a regular maintenance schedule, follow these steps:

StepDescription
Switch on your central heating This is necessary to build up the pressure inside each radiator to help you determine if air is trapped inside.
Examine radiators After the radiators have had time to heat up, touch each one carefully at various points (including near the top) to identify any cool spots, which is where the trapped gas or air will be found.
Turn off the heating Once identified, switch off your central heating to allow you to bleed them safely.

Bleeding the radiator

StepDescription
Get your tools Ideally, you’ll have a radiator key handy – these can be had from any home or hardware store, or you can buy them online for as little as £1 – search ‘radiator bleeding key’. If you don’t have one, a flat-bladed screwdriver should suffice.
Bleed the radiator Place the radiator key or angle the screwdriver into the square valve, making sure you’re holding it with a cloth to catch any drips. Slowly turn the key into an anticlockwise direction – a quarter or half-turn should be sufficient.
Stop Any air should escape with a hiss – once it’s gone, water will start to dribble out. Close the valve quickly in the opposite direction to before.
Check system pressure Both water and air will have been removed, so if your boiler has a pressure gauge, check it. If necessary, use the tap or lever on your boiler to restore pressure, which uses mains water to top up the system. Check the manufacturer’s instructions or website if you’re not sure what to do.
Turn heating back on Give it time to come up to temperature, then examine your radiators again, and repeat the process if any cold spots remain.

Can I bleed my radiators automatically?

Check your radiator to see if it comes with an auto vent, which attaches to your radiator valve to bleed automatically at set intervals to let out any air. They ensure your boiler is spared unnecessary work as air builds up in the system.

You can also purchase add-on vents to fit to existing systems – some can even be fitted without draining your system first.

What else can I do to improve the performance and efficiency of my radiators?

There are two ways – one cheap, the other less so – that can improve your radiators’ performance: the cheap option is to buy radiator insulation foil, which you fit to the wall behind the radiator. This redirects any heat from the radiator away from the walls and back into the room to heat it up more quickly, ensuring you’re not wasting unnecessary energy. Expect to pay around £6-7 for a four-metre roll that you cut to size for each radiator in your home.

Another way to push heat from your radiator into the room is with a radiator booster, which sits on top of your radiator to redirect the heat directly back into the room. Some models require access to a plug (and a certain amount of electricity) to power a fan to actively push the hot air into the room, but you can also buy passive models that simply redirect the heat coming out of the top of the radiator back into the room. These cost around £9-£27 per unit. You’ll need to measure each radiator carefully to ensure you purchase the correctly sized unit.

Are there any other ways to cut my heating costs?

Regular maintenance is key, not only to extend the lifespan of your heating system, but also to ensure it’s running at peak efficiency. Take some or all of the following steps:

  • Get an annual service plan – make sure your boiler is serviced once a year to clean it out and ensure any problems are spotted early.

  • Don’t let small problems become big ones – act swiftly to resolve drops in pressure, small leaks and other seemingly minor issues before they become major ones.

  • Prevent the boiler from seizing up – give it a run-out during the summer months.

  • Insulate your home – focus on your attic, walls (cavity insulation in most cases) and look to draughtproof doors and windows. All of this will help reduce your heating costs and make your home more comfortable to live in.

  • Replace old windows – if your windows are over 20 years old, or single-glazed, look to upgrade them to A-rated energy efficient windows.

  • Switch to a better energy deal – if you’re still on your supplier’s standard variable tariff, the most effective thing you can do (and it won’t cost a penny) is to compare energy prices and switch to a better deal.

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Published on Wed 31 Mar 2021 03.07 GMT