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How to Bleed a Radiator

Save money and improve heating performance in 7 steps

If you’ve heard some odd sounds coming from your radiators lately, especially when they’re starting to heat up, or have noticed that the radiator is cooler in specific places, often nearer the top, there’s a good chance that they’re in need of bleeding.

Radiator bleeding is the process of letting any trapped air escape from your home’s heating system, which will increase heating performance, get rid of those rattling sounds, and ultimately save you money on your heating bill in the long run.

Why do I need to bleed a radiator?

Your heating system is a vital part of your home, providing warmth and comfort all-year round. It’s almost constantly in use, providing hot water to your home on-demand and generating heat from the radiator network spread throughout. It’s not uncommon for air to find its way into a system over time, gradually reducing heating performance and creating some unwanted rattling sounds as it travels through your system.

While this typically affects unvented systems more frequently, it can - and will - happen to every home heating system at some point. By not bleeding your radiators, you will lose heating performance in places where air is trapped, meaning you’ll have to leave the heating running for longer periods of time to warm your home to your liking. We recommend regularly checking your radiators and bleeding them as necessary. If you have to repeatedly bleed radiators then discuss this with your installer when the boiler is serviced as there may be an underlying problem.

Bleeding a radiator system in steps

To handle the job, you’ll need a couple of common tools. We’ve listed what you’ll need to bleed your radiator system below.

  • A radiator key to open the bleeding valves of your radiators. This is used to open the radiator vent valve. These are purchasable from hardware stores or from online retailers.
  • A bucket & cloth. Your radiator will leak water when the air has fully escaped, so you will want to catch it before it wets your floor.
  • A pair of gloves. These will help protect your hands from any hot pipes or surfaces.
  • To assess whether you need to bleed a radiator or not, start by turning on your heating system and allowing it to start warming up your radiators. While they’re warming, listen for any clanking or rumbling sounds - this could very well be pockets of air rushing through your heating system. They will eventually settle once the system heats up, so pay a visit to each radiator in turn to get a quick understanding.

Once they’ve sufficiently heated up, you need to feel the surface for any changes in temperature, particularly towards the top of the radiator where the air is likely to end up lodged. If it feels cooler than other parts, you know there is an air pocket there. Please take extra care here as the radiators will be hot, so be sure to use gloves.

Make sure your central heating system is turned off. This will stop your circulation pump from operating and moving heating water and air through your pipes, cooling down your radiators and making them safe to bleed. All thermostat valves need to be open.

After switching off your heating, wait approximately 1 hour (or 2 hours to be cautious) to ensure that all radiators are cold. This prevents you from getting burned in case hot water or steam leaks during the bleeding process.

This is where you’ll begin to bleed the radiator. First, locate the radiator valve, which can usually be found at the upper side of the radiator - different models will have it located in different places, however. For reference, it looks like a small, round hole with a little square inside.

Prepare the area by placing the bucket beneath the valve and keep your cloth pressed against the wall to protect from spraying water.

Slowly turn the radiator key anti-clockwise. A quarter to half a turn should be sufficient to open the valve enough for air to escape. You will hear a hissing noise as soon as air drops out of the radiator - that’s how you know you’re bleeding the radiator correctly. Do not continue opening the valve once you hear a hiss or you will risk leaking excess water out of your radiator.

Once the hissing stops and only water is exiting the valve, you have successfully bled that radiator and can close the valve. Do note that water can sometimes be discoloured from time spent in the system, this is not a cause for alarm.

Before closing the valve, be sure to let a small amount of water run out before you turn the radiator key clockwise to close the valve. This ensures you’ve gotten rid of all trapped air in that radiator. Don't close the valve too tight, otherwise you might damage it.

You’ll want to bleed all the radiators that are affected by trapped air, though it doesn’t hurt to simply bleed every radiator and ensure there is absolutely no air left in the system. It’s easy to check by opening the valve and checking for the hiss - if it doesn’t hiss, you know your radiator is good to go.

Air will naturally rise as it is lighter than water, so bleed all radiators in the ground floor before heading upstairs if you have a multi-story property. This will ensure best results and save you time having to go back and forth.

Before you switch your heating back on, be sure to check the water pressure of the system by looking at the pressure gauge or digital display on the boiler - you may get an F22 fault code (please refer to the operating instructions for guidance if you are unsure). The boiler should read between 1 to 1.5 bar of pressure when the central heating is cold.

If the pressure is too low, you need to refill the heating water. If you are unsure how to refill your boiler, watch our video on pressurising your boiler here.

Finally, check if all radiators are warming up evenly and noises have been reduced. That’s all there is to it! Do be sure to bleed your radiators regularly to ensure the best heating performance.

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