How to bleed a radiator

Learning how to bleed a radiator is a simple, straightforward task that anyone can master, even if you’ve never done it before. Read on to discover how to release the trapped air in your radiators to keep your heating system running at maximum efficiency.

Person bleeding radiator using a key

In the guide you'll find answers to questions including:

  • Why is bleeding radiators important?

  • Which radiator should I bleed first when performing regular maintenance?

  • How to bleed radiators: a step-by-step guide

  • What is an automatic radiator bleed valve?

  • What is radiator insulation foil?

  • Beyond bleeding radiators, are there other ways to make my system more efficient?

Why is bleeding radiators important?

Bleeding radiators helps free pockets of air that become trapped inside the heating system, which in turn ensures your heating system runs more efficiently (thus helping you to reduce your energy bills). When air gets trapped in an un-vented system, water can’t flow through the full radiator. This results in cold spots where the air is trapped, and uneven heating.

What are the signs that it’s time to bleed a radiator?

Even if you’ve set up a regular maintenance schedule whereby you bleed your radiators once a year, it pays to look for the following tell-tale signs that there’s trapped air in your system:

  • Unusual sounds coming from the radiator.

  • Cold spots on the radiator, or a top section that feels cooler than the bottom half.

  • A single radiator that remains cold even after switching the heating system on and checking that its thermostat is fully open.

When is the best time to bleed a radiator system?

If you’re looking for a particular time of year to bleed your radiator system, we recommend scheduling it for when the temperature starts to drop in early autumn. This ensures your system will run efficiently throughout the winter months when you really need it. 

Why does air build up in the system? 

There are two broad causes explaining how pockets of air can work their way into your central heating system: routine maintenance, which can let air in through valves, or the action of your heating pump, which can churn air into the pipes.

Which radiator should I bleed first when performing regular maintenance?

If you’re planning to bleed all your home radiators, start with those furthest away from your boiler – in a multi-storey house this means starting on the ground floor, then working your way up. Begin by bleeding the radiator positioned furthest from the boiler on each floor, moving up to the next as each floor’s radiators are completed. 

How to bleed radiators: a step-by-step guide

The following steps will help you to both identify radiators that may need bleeding, plus resolve the issue. First, if you’re performing regular maintenance to bleed your entire radiator system, skip to step 4:

It’s important to switch off the central heating first. Otherwise, you’ll soak the floor with water or potentially burn yourself handling hot spots. Turn off the heat and let your radiators cool down.

Step 1: Turn on the heating system

This builds up the pressure inside each radiator so that you can find out whether air’s trapped inside. 

Step 2: Examine each radiator

Once the radiators have heated up, check each one by touching it carefully to identify any cool spots. These will be near the top of the radiator, which indicates the presence of trapped gas or air that will need to be released.

Step 3: Switch off central heating 

Once you’ve identified trouble spots, you’ll know which radiators need to be bled. You can then move on to begin the bleeding process.

Step 4: Bleed the radiator

It’s easiest to use a radiator key to bleed a radiator. Look for the valve at the top of the radiator. It should have a square shaped groove in the centre. Attach the radiator key to this square valve, holding your tool with a cloth to catch drips.

Where can I find a radiator-bleeding key? You’ll find radiator keys available at any home or hardware store. You can also buy them online for as little as £1 – search for ‘radiator bleeding key’.
How do I bleed a radiator without a key? If you don’t have a radiator key, you can use a flat-blade screwdriver. Angle the screwdriver tip into the valve, and then proceed the same way you would with a key.

Slowly turn the radiator key in an anti-clockwise direction. Usually, a quarter to half a turn will be enough. You want to avoid opening the valve fully, because water will gush out in a jet. As air escapes, you’ll hear a hissing sound. Once this sound stops and water starts pouring out, you’ll need to close the valve quickly. Turn the bleed valve in a clockwise direction to seal your radiator. 

Step 5: Check the system pressure

The process of bleeding your radiator releases water from the system along with air. This reduces your heating system’s pressure, so the next step should be to check your boiler’s pressure gauge. If it’s too low, use the tap or lever on your boiler to restore pressure. This works by topping up the system with mains water. However, the process can vary according to your type of boiler or heating system, so be sure to look at the manufacturer instructions or website if you’re not clear. 

Step 6: Turn the heating on again

Finally, turn the central heating back on so that you can check whether your bleeding was successful. Wait once more for the radiators to heat up, checking for unusual sounds or cool spots. If you find any, repeat as needed. 

What is an automatic radiator bleed valve?

Some radiators use an auto vent, which automates the bleeding process for greater convenience and efficiency. Automatic radiator valves or auto vents attach to the valve, letting out air at regular, gradual intervals. This prevents air from building up over time, saving your boiler a bit of work. Like radiator bleeding keys, these can be purchased to fit existing systems – look for models that can be fitted without draining your system.

What is a radiator booster?

For even greater efficiency, you could use a radiator booster. This is a fan unit that you can attach to the top of your radiator. When warm air is generated, the booster pushes it throughout the room. This type of unit does use electricity to run, but that might be offset by the savings you’ll see on your heating bill.

What is radiator insulation foil?

Radiator insulation foil is a cheap way to boost your heating system’s efficiency that requires no power. It’s basically a sheet of thin reflective foil that’s placed behind the radiator to deflect heat away from the walls and back into the room. This ingenious solution ensures your rooms are brought up to the desired temperature more quickly, which reduces consumption and – in turn – helps bring down your energy bills. This simple yet effective tool is widely available in hardware stores, costs very little (around £6-7 for a four-metre roll you cut to size for each radiator) and is easy to install yourself.

Beyond bleeding radiators, are there other ways to make my system more efficient?

A great way to lower your heating bills is to make sure your system’s free from trapped air, sludge, and other materials that could slow it down. Here are a few ways to keep your heating system in best possible condition:

  • The Energy Saving Trust recommends taking out an annual service plan.

Regular servicing by a heating engineer each year will catch problems early, preventing wear and tear and extending your boiler’s lifespan.

  • Fix small problems before they become larger ones. 

Pressure drops, small leaks and other issues may escalate, so have these investigated early. 

  • Turn the boiler on from time to time during the summer. 

This keeps the system running efficiently, preventing the boiler from seizing up

  • Insulation is one of the most important things when it comes to keeping your home warm. 

Make sure your wall and attic insulation are in good shape. Although it requires upfront costs to replace old insulation, you’ll see significant savings on your heating bills as a result. 

  • You should also draught proof your windows and doors to keep heat from escaping. 

When warm air leaks out through draughts, the heating system must work harder to keep the temperature up. 

  • Finally, it’s worth comparing energy deals to make sure you’re on the best possible rate. 

Switching suppliers is an easy, almost instantaneous way to save money on your heating bills.

Last updated: 9 November 2020