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How to bleed a radiator

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Planning & preparation

  • Over time, air can seep into your central heating system and will tend to bubble up to the top of your radiators, especially those furthest away from the boiler and those fed by downpipes. This air can affect how efficiently the radiators heat your rooms. It can also make them noisy as air moves around the system
  • When you come to release air from a radiator, water can spray out, so you’ll need to protect your paintwork and flooring. Wedge one towel between the radiator and the wall, and lay another on the floor beneath. If directional valve plugs are fitted on your radiators, always direct the bleed hole towards the floor to help you catch the water
  • Place a small bowl underneath the radiator valve and keep another towel handy to catch any water that comes out

Do it right

  • Because boiler pressure can drop after bleeding, make sure you know how to re-pressurise your boiler in advance. You’ll find this information in your boiler’s handbook. If you’re unsure about re-pressuring your system, always ask a professional.

Staying safe

  • Make sure your central heating and hot water are switched off at the boiler before starting to bleed your radiators
  • Make sure the radiator is completely cooled before you start, otherwise any water that escapes could be dangerously hot
  • This guide is designed to guide you safely through the process of bleeding radiators. If you’re in any doubt, please seek advice from a plumber and always follow your boiler manufacturer’s instructions

Aftercare

  • A couple of hours after you’ve finished, check all of the radiators you’ve bled carefully to make sure the valves aren’t leaking
  • It’s worth checking your radiators for cold spots once or twice a year to see whether they need bleeding
  • The end of summer is a great time to bleed your radiators as you can make sure your central heating system is working efficiently before the cold weather arrives

Step by step

Step 1

To check if you need to bleed a radiator, run your hands over it when the central heating is on, being careful not to burn your hands.

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Step 2

If the radiator is cooler at the top than the bottom, chances are air has built up inside which means that the hot water can not heat the whole radiator.

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Step 3

Switch your central heating and hot water off at the boiler.

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Step 4

Wait until your radiators have fully cooled before starting to bleed them. This is important as otherwise steam and boiling water may escape when the valve is released and will be dangerous.

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Step 5

Now you can release the trapped air using a radiator key.

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Step 6

To locate the valve, check the top edge of the radiator – the release valve looks like a small, square pin. A double radiator will have a valve on each panel and you will need to bleed both. Universal radiators can also have a valve on the back panel, so be sure to check for this.

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Step 7

Before releasing the trapped air, protect the surrounding area using old cloths or towels and a bowl to catch any escaping water.

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Step 8

Place the radiator key into the release valve and carefully twist it one or two times anti-clockwise. You’ll hear a hiss as the trapped air is released, which will take a few seconds.

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Step 9

Once you hear a gurgling sound or water begins to come out, you’ll need to twist the key clockwise to close the valve, being careful not to over-tighten it.

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Step 10

If the valve accidentally comes out, which can happen if it is very tight, use an adjustable wrench or a spanner to replace it.

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Step 11

Repeat the process on any other radiators that need bleeding. Check the pressure gauge on your boiler – If you need to adjust the pressure in your system, you can do this by adjusting the filling loop. You should be able to find instructions for how to do this in the manufacturer’s instructions for your heating system. If you’re unsure of how to do this, seek professional advice.

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Step 12

When the pressure in your heating system is normal, turn your heating back on, and check your radiators for cold spots. If any cold spots remain, or if you find that they come back quickly, you may have underlying problems in your heating system. This may indicate a small air leak or sludge in the system. Seek professional advice to investigate any problems further.

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