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How to Bleed a Radiator: A Step-by-Step Guide

bleeding a traditional style radiator
(Image credit: Luke Arthur Wells)

Learning how to bleed a radiator is a key life skill for anyone — well, at least anyone who has gas central heating. Bleeding a radiator helps to release air trapped in your central heating system that can make your radiators less efficient and create cold spots.

While many areas of plumbing are almost always best left to the professionals unless you’re a confident DIYer, bleeding a radiator is something that you can take on yourself relatively easily and that you should aim to do every year before winter rolls around so that your central heating is performing as effectively as possible. 

This is what you need to know about bleeding a radiator, and a step-by-step guide to make the process easy. 

How to Bleed a Radiator Step-by-Step 

Here’s how to bleed a radiator in 7 simple steps:

1. Prepare to Bleed the Radiators

To start, make sure that your central heating is off and that the radiators are cold. If not, you run the risk of spraying yourself with hot water when bleeding the radiator. At this point, you’ll also need to know how to re-pressurise your boiler (refer to the instruction manual if possible) and find the pressure gauge on the boiler. You’ll need to keep an eye on this during the process, so it’s a good idea to have a second pair of hands to help with this job.

the pressure gauge on a combi boiler

(Image credit: Luke Arthur Wells)

2. Protect the Floors and Walls

Pop a towel between the radiator and the wall and hold another one just underneath the bleed valve. You may want to put a bowl on the floor underneath to protect it from any further drips.

3. Find the Bleed Valve and Insert the Key

This is usually located on the top right hand side of the radiator. Insert the radiator key onto the bleed valve.

a radiator bleed valve

There is some small variation in how bleed valves look, but styles like this are the most common (Image credit: Luke Arthur Wells)

4. Release the Air from the Radiator

You’ll need to turn the key anti-clockwise to open the bleed valve. When you do this, you should hear air hissing from the valve, however, it’s not uncommon for water to squirt or drip from the valve too, a sign that you’ve cleared all the air you need to from this radiator. At this point, turn the key clockwise to close the bleed valve.

bleeding a radiator valve

Here, a multi-plumbing tool with a bleed key is being used to bleed a radiator  (Image credit: Luke Arthur Wells)

5. Keep an Eye on the Boiler Pressure

While bleeding the radiator, especially when bleeding multiple radiators across a house, and when your boiler is slightly older, you’ll need to keep an eye on your boiler’s pressure gauge. If this falls too low, into the red zone, you can cause damage to your boiler. 

If the gauge level falls outside of the ideal range, top up the boiler using the valve system — this should be relatively simple, as you need to just turn the valve on the filling loop until the pressure gauge raises back into the green zone, but refer to your boiler instructions. You’ll probably only need to do this once if at all, unless there’s a lot of air in your radiators.

a filling loop on a bosch boiler

You'll need to locate the filling loop mechanism to repressurise your boiler while bleeding radiators (Image credit: Luke Arthur Wells)

6. Continue to the Next Radiator

Repeat this step on your other radiators that need attention. You’ll only really need to bleed the radiators that you determine have air trapped inside, working from the furthest radiator away to the nearest to the boiler, starting with the ground floor.

7. Turn the Heating Back on

Once you’ve bled all the radiators you need to, turn the heating back on and check that they’re heating up correctly. If not, there may be another reason for your radiators not working that may require it to be flushed. Also, double check all the valves are properly closed and that none of them are leaking.

Watch How to Bleed a Radiator With our Video Guide

How do you Know if you Need to Bleed a Radiator? 

There are a couple of ways you can figure out whether your radiator needs bleeding. Firstly, when the heating is on, carefully put your hands on the radiator. As air rises to the top, if you have trapped air in your radiator, the radiator will be cooler, or even cold, at the top, while the bottom is warm. This is a sign your radiator needs bleeding. You’ll find this effect is even more pronounced in vertical radiators and towel rails.

Another tell-tale sign of a radiator in need of bleeding is unusual noises such as gurgling, clunking and rattling.

If you’re unsure, there’s no harm in trying to bleed your radiator, as long as you follow the steps carefully as not to de-pressurise your boiler and cause damage to your heating system. 

You may also need to bleed radiators during the process of how to balance radiators, or when draining the central heating system for installing a new boiler or replacing a radiator

What do I Need to Bleed a Radiator?

Bleeding a radiator doesn’t require many tools. The only tool you’ll really need is a radiator key, the design of which may depend on your individual radiator, but generally looks like the wind-up mechanism from an old-fashioned toy or a mini tire iron-style wrench used for car wheels. They can be bought online or at your local DIY shop for just a few pounds. 

With that, you’ll also need a couple of towels (they won’t necessarily get dirty, but use old ones anyway) and maybe some shallow receptacles such as bowls or tupperware to catch any water released while bleeding the radiator. 

How do I Bleed a Radiator Without a Key?

If you don’t have a radiator key to hand, no problem. Bleed valves are often slotted, meaning you can just use a flat head screwdriver. If not, it may have an opening for a tool like an allen key, or you can often open these valves with an adjustable wrench spanner. A radiator key gives you the most control, however, and they’re cheap so it’s worth keeping one in your toolbox. 

how to bleed a radiator without a radiator key

(Image credit: Luke Arthur Wells)

Do you Bleed a Radiator With the Heating on or off?

Always bleed a radiator with the heating off and after waiting for radiators to cool down. Once you’ve released the air in the radiator, water will start to spray or leak from the bleed valve, so you’ll want to make sure this is cold, not scolding, water. 

How Long do you Need to Bleed a Radiator for?

Not long at all. Depending on how much air is trapped in your radiator, you’ll only need to open the valve for between 5 and 30 seconds. 

What Order Should You Bleed Radiators in Your House?

The general rule for bleeding radiators is to start from the furthest radiator away from the boiler and work your way towards it. If you’re doing radiators across multiple floors, start with the ground floor first, then repeat the process upstairs. This may not always be an exact science in large, sprawling houses, but as long as you try to work to this order as much as you can, by and large you shouldn’t encounter any issues. 

How Often Should I Bleed My Radiators?

According to Toolstation (opens in new tab) spokesperson, Mark Biles, Operations Director at M & M Mechanical Services: "You should look to bleed your radiator every few weeks in the winter because cold spots can appear at the top of the radiator which is evident of air lock in the radiator. If you don't bleed them then air can travel round the system and cause issues with flow rates and potentially lead to rust over time."

Luke Arthur Wells is an award-winning interiors blogger and stylist. His blog has been one of Vuelio's top 10 interior blogs for four years running, and he recently won the Best Creative Skill category at the Amara Interior Blog Awards. Luke has worked with some of the UK's biggest brands, from John Lewis and Made to Farrow & Ball and B&Q. He's a big DIYer, and loves coming up with creative woodwork projects for his home, a Victorian terrace renovation in Essex.