Bleeding radiators: a step by step guide

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‘Bleeding radiators’ is when you let out any air that has become trapped inside.

Radiators are the commonest way of spreading hot water from boilers around properties in the UK, but maintaining your radiator is a crucial yet oft-forgotten step.

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Like all aspects of your heating system radiators have to be maintained to ensure they’re working correctly, but what actually happens to stop radiators working?

Trapped air or gas prevents hot water from heating your radiator fully. The good news is, bleeding radiators is a simple job that you can do yourself, and it can make a real difference to how energy-efficient the heating in your home is.

While this isn’t a hard thing to do, please follow these instructions with caution. If you don’t feel confident about what you’re doing, stop and get advice from a qualified heating engineer.

Step 1: Turn your heating on and get your radiators hot

Turn on the heating so that all radiators in your home come on - you may need to turn up the temperature on your thermostat and individual radiator temperature controls to make sure they all come on.

Remember to wait until your radiators are fully heated before moving on to step two. You need to build up the pressure inside the radiator to be able to force the air out.

Step 2: Find out which radiators need bleeding

Once your radiators are all hot, go and check each one individually to see if all parts of the radiator are warming up. Be careful - radiators can get very hot and you don’t want to burn yourself.

Cool spots, particularly towards the top of the radiator, mean that there could be air or gas trapped and therefore you'll need to bleed that radiator.

Air or gas is more likely to get trapped in the highest radiator in the home. It can also sometimes get trapped in the coil that runs through the hot water cylinder, as that can often be the highest point in the system.

Once you’ve found your cool spots it’s time to move onto step three and bleed them.

Step 3: Bleed the radiators

Before you bleed any radiators make sure your central heating is switched off. This is reversing the process identified in step one and will allow you to handle the radiators without burning yourself or soaking your floor.

Bleeding radiators usually requires a radiator key, but with more modern radiators you can use a flat-blade screwdriver. Often finding the radiator key is the hardest part so it’s a good idea to keep it near one of your radiators or near the boiler. Otherwise you can buy a radiator key from any hardward or DIY store for a few pounds.

At the top of the radiator at one end there will be a valve. You can attach the radiator key to the square bit in the centre or put the end of the screwdriver into the groove.

How to bleed a radiator

Hold the key or screwdriver with a cloth, and have another cloth ready to catch any drips, then slowly turn the radiator key or screwdriver anti-clockwise – if gas is escaping you’ll hear a hissing sound.

Once there is no more gas, liquid will come out and the valve will need to be closed quickly. With the more modern screwdriver operated escape valve, liquid is likely to emerge as a jet rather than a dribble.

Step 4: Check the pressure

Check the pressure by having a look at the gauge on your boiler. If the pressure is too low, you’ll need to ‘top up’. You can do this using the lever or tap on your boiler, known as the filling loop.

Afterwards, you may want to run another ‘hot test’ to check that your efforts have been successful. Simply turn your heating on, wait for all the radiators to heat up and check for any cool spots.

That’s it! Congratulations, now you know just how simple bleeding radiators can be!

Automatic radiator valves

Depending on what type of radiator you have it may be possible to automate the process of bleeding your radiators by using an auto vent.

Auto vents like the Aladdin are automatic radiator bleeders that attach to your valves. The vent lets out air gradually, increasing the effectiveness of oyur radiator and meaning your boiler doesn’t have to work so hard.

Remember you will need one auto vent per radiator.

How else can you improve your heating?

There are plenty of other gadgets and gizmos to improve the effectiveness of your heating. Radiator insulation foil for example is a simple way of ensuring your radiators heat your room, not your walls.

Insulation foil sits behind your radiator and reflects the heat they generate back into the room. They are widely available in hardware shops and you should be able to install them yourself.

A slightly more expensive option but still under the £25 mark is the radiator booster. This is essential a stretched fan unit that sits on tops of your radiators and pushes the warm air generated around your room. While the radiator booster will use electricity to run, they will save you more on your heating bills.

You should also make sure the heat being generated isn’t being wasted by keeping your home and rooms well insulated. The easiest thing you can do is check for draughts around windows and doors. You can also draught-proof things like chimneys and letterboxes, anywhere that heat could escape really.

While draught-proofing is relatively cheap and easy to install yourself you could save even more on heating by spending a bit more money. Loft and wall insulation are great ways to save on heating costs in the long run, but both require a significant upfront commitment of money.

To learn more and to see whether your home is suitable you can read our guides to loft insulation and our comprehensive guide to wall insulation in the insulation section of our site.

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A new energy-efficient boiler could save you £310 a year.

Get a quote