How To Bleed A Radiator: Step By Step

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Last updated: October 31, 2023

The modern home heating system is usually trouble free and requires little maintenance from homeowners.  But occasionally air can become trapped in the system, and this may cause cold spots on one or more of the radiators. The tendency is to turn the radiator up, but this is not usually the solution and is more likely to lead to the central heating system working harder than it needs to and ultimately, to higher energy bills.

What Does Bleeding a Radiator Mean?

There are tell tale signs that bleeding a radiator is required. If one or more radiators is not heating up, or is colder at the top than the bottom, this means that the radiators need bleeding. It sounds a rather dramatic undertaking, but radiator bleeding is simply allowing the trapped air to vent from the central heating system, via a bleed valve, allowing the hot water to flow more freely.

Related Read: Radiator Cold At The Bottom? Here’s Why!

What You’ll Need to Bleed a Radiator

Bleeding a radiator is a simple task that, thankfully, does not require dozens of specialist tools. All that is needed is a small bowl or bucket, an old towel (it could well get dirty) and a radiator key, sometimes referred to as a bleeding key. The radiator key can be purchased at most hardware stores for very little cost. They are sometimes sold in sets of two which is a sensible idea to buy, as the key is quite small and easy to lose.

How to Bleed a Radiator – Step by Step

A radiator that is hotter at the bottom than at the top, has cold spots on the side, or is making a gurgling or knocking noise is likely to require bleeding. The sequence for doing this job is as follows.

  • Firstly, turn on your heating system and wait for the radiators to heat up. This allows any air in the system to be pushed into the top of the radiators.
  • Identify which radiators require bleeding by looking for cold spots or checking if the top of the radiator is colder than the bottom. Be careful as any radiator, particularly one which does not need bleeding may be very hot. You may wish to wear thin gloves as a precaution.
  • Turn off your central heating system and give it time to cool. This allows the air to settle in the top of the radiator and avoids the risk of being scalded by escaping water.
  • Put the towel or bowl in position under the radiator bleed valve to catch any water which escapes. The water can sometimes be quite dirty if it has been exposed to rust, so ensure it does not reach any carpets or upholstery.
  • Look for the radiator bleed valve at the top of the radiator. It will usually be a round hole with a square metal shaft in the centre. Insert the radiator key over this shaft. Sometimes the shaft will have a slot in it to accept a flat bladed screwdriver if a key is not available. Slowly turn the radiator key anti clockwise until you hear a hissing sound. This is the air escaping from the system. Use caution as it could still be quite hot.
  • Only a small amount of rotation is usually required, no more than half a turn. Once the hissing has stopped, water will begin to drip out or come out in a small jet. As soon as this happens, turn the key clockwise to close the valve. It only needs to be hand tight. Overtightening it can cause damage.
  • It is recommended that process is repeated on all other radiators to ensure that the heating system is completely clear of any air.
  • Before turning the heating system back on, check the boiler pressure using the gauge on your boiler. Bleeding your radiators can sometimes cause the pressure in the system to drop. The needle on the pressure gauge should point to the green sector. If it points to yellow, you will need to use the central filling loop to refill the system until the needle points to the green. This is usually a pressure between 1 and 1.5 bar. It the pressure gauge is reading in the red sector, there is too much pressure in the system and you will need to drain it down until the needle is back in the green sector, using the bleed tap. If in doubt, consult a registered heating engineer about this.
  • Turn the heating system back on, allow it to warm up and check that the issue has been sorted out.
Heating radiator

When is the Best Time to Bleed my Radiator System?

It is advised that the central heating system should be bled before the weather gets too cold. This ensures that the system is working at its maximum efficiency, just at the time when it will needed most.

Which Radiators Do You Bleed First?

In a single story building, you should first bleed the radiator which is working the least efficiently or has the most obvious cold spots, as this is likely to contain the most air. If you live in a building with extra stories, it is recommended that you begin by bleeding the radiators on the lowest floor first, working your way up to bleed the highest radiators last. This is because of the natural tendency of air to rise up through the pipework, so it will usually emerge at the highest point in the central heating system.

How Long Should It Take To Bleed A Radiator?

It generally only takes about 20 to 30 seconds from first opening the bleed valve, to seeing water emerging. The more time consuming part of the process is waiting for the radiators to cool after you turn off your heating system. This can take around an hour.

Benefits of Bleeding Your Radiators

It is important to be aware of the need to remove any air from your central heating system. Not only will your home be warmer as a result, but your boiler and radiators will be working more efficiently, using less fuel and ultimately this will save you money on your heating bill.

Related Post: Do Radiator Covers Block Heat?


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