How to bleed a radiator & when to do it

An introduction to radiator bleeding

In this comprehensive bestheating guide, we will walk you through the essential steps to properly bleed a radiator, ensuring optimal heating efficiency in every room of your home.

As experts in radiators and domestic heating systems, we understand the importance of radiator bleeding, and by following our detailed instructions, you can significantly improve radiator performance and efficiency.

Key points we will cover in this article, include:

• • The importance of regularly bleeding radiators to maintain peak performance and energy efficiency.

• • Step-by-step instructions on how to properly bleed a radiator, including the necessary tools and safety precautions.

• • Guidance on identifying when a radiator needs to be bled, such as if it’s not heating up evenly or making unusual noises.
  • Tips for maximising the heating output and lifespan of your radiators through proper maintenance.

Why do you need to bleed radiators?

Before we delve into the steps of bleeding a radiator, let’s understand why this process is essential.

Over time, air can accumulate inside your heating system, causing pockets of trapped air within the radiators. To simplify, think of air pockets like bubbles beneath the radiator’s surface.

This trapped air prevents hot water from circulating effectively, leading to cold spots and reduced heating efficiency.

By bleeding your radiators, or ‘popping the bubbles’, you release the trapped air, allowing hot water to flow freely and evenly throughout the system, and maximising its performance.

How do air pockets form in radiators?

A more technical analysis of how air pockets form in radiators is as follows:

1. Dissolved gases emerge from solution as water is heated up or cooled down over time.
2. Minor leaks or maintenance can introduce air into the system.
3. Corrosion can lead to the production of hydrogen gas inside steel radiators. If bleeding is needed on a regular basis, check for leaks and consider a system cleanse and inhibitor.

When should you bleed a radiator?

If you feel any cold spots

Bleeding a radiator is required if it is not working as efficiently or as effectively as you want it to.

You can tell if this is the case by feeling the surface of your radiator. In many cases, you’ll find a cold spot around the top of the radiator surface, and could find that your radiator doesn’t get hot at all.

If you experience anything like this with your radiators, the chances are that you’ll need to take steps to remove the air by bleeding the radiator. 

If your radiator is making noises

Another signal that you may have air building up and need to bleed your radiator is when you can hear clanging or prolonged groaning when you switch on your central heating.

There could also be other causes for your radiator to make noise, but generally, if you have a clunk or a clang in your heating system, its because you need to bleed your radiators.

When radiators are slow to heat up

After long summer downtime

When you’ve had no need to switch your radiators on for a significant period of time in summer, it is well worth bleeding them before the colder winter months when you need them to be at peak performance level. Similarly so, look to bleed the system after draining or refilling any parts of it. or after you’ve drained/refilled parts of the system.

Every few months

Simply as a general preventative maintenance measure, it makes sense to bleed your radiators every few months. As alluded to above, aim to bleed the system to coincide with the beginning of heating season.

Tools required for bleeding your radiator

To successfully bleed your radiator, you will need the following tools:

• • Radiator key: A radiator key is a specialised tool designed to fit into the bleed valve, allowing you to open it easily. If you don’t have a radiator key, you could also use a flat-head screwdriver. Note: Most UK bleed screws are a 5 mm square surface that a standard key will fit. Older radiators and certain heated towel rail models may use slotted or hex heads, which can be opened using a flat‑head screwdriver or Allen key as appropriate. “Universal” multi‑keys and multi‑tool cabinet keys often include a 5 mm square that will work perfectly with the vast majority of modern radiators.

• • A container: A washing up bowl or similar to place under the radiator to catch any excessive water.
  • Cloth or towel: Keep a cloth or towel handy to catch any water that may escape during the bleeding process.
  • Gloves and torch: Have gloves handy as a safety precaution, and a torch for visibility, especially if you’re bleeding your radiator at night or with little available light.
  • Boiler manual: To check the steps to take to re-pressurise your system for existing boiler models.

Step-by-step guide to bleeding a radiator

1. Choose which order to bleed your radiators

It is advisable to begin with the radiator that is situated furthest from the boiler on the ground floor, before working towards the boiler, then moving upstairs and repeating the process furthest‑to‑nearest. This will help push trapped air up and out of the system efficiently.

2. Turn off the heating system

 

Before starting the bleeding process, make sure to turn off your heating system.
This precautionary step ensures your safety and prevents any accidental burns or scalding from hot water.

3. Identify the radiator bleed valve

 

Locate the bleed valve on your radiator. The bleed valve is usually located at the top of the radiator, on one side. It resembles a small square or hexagonal nut.

4. Prepare for bleeding

Place the cloth or towel or washing up bowl below the bleed valve to catch any water that may drip out during the process. This prevents any water from staining your flooring or carpets.

5. Insert the radiator key

Once you’ve identified which type of radiator key will work, insert the key into the bleed valve and turn it anti-clockwise for a quarter of a turn initially. You should hear a hissing sound as the trapped air begins to escape. Don’t try for more than one full turn.

6. Close the valve once trapped air escapes

As the hissing sound stops and water starts to trickle out of the bleed valve, it indicates that the trapped air has been released.

Once a steady stream of water appears, close the bleed valve by turning the radiator key clockwise.

Typically, it will only take a few seconds for water to emerge, but larger radiators could take up to a minute.

7. Check for leaks

After bleeding the radiator, inspect the bleed valve for any signs of leakage.

If you notice any leaks, tighten the bleed valve gently until the leak stops – but be very careful not to overtigten, as this may damage the “O” ring and could lead to further problems.

8. Repeat the process

Repeat the bleeding process for all radiators in your home, working your way around the house in the order specified in step 1.

This will ensure that all radiators are free from trapped air and operating efficiently.

9. Re-pressurise your boiler

Bleeding radiators leads to the release of trapped air and a little water, so pressure may drop. Top up your boiler pressure level to somewhere between 1.0 and 1.5 bar when cold (check your manual for specifics; 2 bar when hot is a typical gauge). Some older models use an internal filling key, with most others use a filling loop to re-pressurise.

10. Switch the heating on and check on radiators

Turn the heating on and check your radiators are without cold spots, heating the space evenly and not making a racket. If any of those problems still persist, consult the Troubleshooting section below.

Troubleshooting tips for radiator bleeding

While bleeding radiators is a straightforward process, there might be instances where you encounter some challenges.

Here are some troubleshooting tips to address the common issues you may come across when bleeding radiators.

No hissing sound or water flow

If you don’t hear a hissing sound when you turn the radiator key or no water comes out, it is worth checking the system pressure is sufficiently high.

If it is, it could indicate that there is no trapped air in the radiator.

Check for other issues that might be affecting your heating system’s performance.

Ensure that the heating system is turned off and that the radiator is cool before attempting to bleed it.

Continuous hissing sound

If you hear a continuous hissing sound even after water starts flowing out, it may indicate that there is a significant amount of trapped air in the radiator.

Allow the radiator to bleed for a longer duration until the hissing sound stops completely (but be sure to have a bigger bucket handy to catch the drips!).

Water leaking from bleed valve

If water continues to leak from the bleed valve even after tightening it, there might be a worn-out or damaged valve.

In this case, it’s best to replace the bleed valve to prevent further leakage and ensure the efficient operation of the radiator.

You could even explore the option of an automated bleed radiator valve to save you time and prevent persistent issues.

Uneven heating after bleeding

If you notice uneven heating in your home even after bleeding all the radiators, there could be other underlying issues affecting your heating system.

Check for any sludge or debris buildup in the system and consider flushing it out to improve circulation.

Persistent cold spots

If cold spots persist in some rooms after bleeding the radiators, it might be a sign of underlying problems in the heating system.

A consistent build-up of gas can often indicate corrosion, leaks, or poor inhibition. Check for leaks, consider a chemical cleanse and a new inhibitor.

If these measures have little effect, it might be time to reach out to a professional heating technician to inspect and diagnose any potential issues.

The benefits of properly bleeding radiators

Bleeding your radiators on a regular basis offers several key home heating benefits.

Enhanced efficiency and even heat distribution

By removing trapped air, hot water circulates freely, ensuring even heating throughout your home.

Energy savings

By eliminating needless boiler cycling and overheating, a heating system with bled radiators will consume less energy, leading to reduced utility bills and a more eco-friendly household.

Increased comfort & warmth

No more cold spots! Properly bled radiators provide consistent and comfortable warmth to every room of your home.

Lower noise levels

Bleeding radiators can bring about a welcome dismissal of irritating gurgling and knocking sounds so you can watch your favourite programmes in peace again.

Extended heating system lifespan

Removing air reduces the strain on your heating system, potentially prolonging its lifespan.

Additional maintenance tips for an efficient heating system

Bleeding your radiators is just one aspect of maintaining an efficient heating system.

Here are some additional tips to keep your heating system running optimally.

Bleed at the start of heating season

It makes perfect sense to bleed your radiators just prior to the colder winter months when you’ll need them to provide optimal performance levels.

Conduct regular inspections

Schedule regular inspections by a qualified heating technician to identify and address any issues with your heating system before they escalate.

Clean radiators regularly

Clean your radiators regularly to ensure proper airflow and prevent dust and debris from accumulating within the system.

Take a look at our radiator cleaning guide for additional help.

Keep radiators unobstructed

Whilst beneath the window is generally considered the best place to position a radiator, the presence of heavy curtains can restrict convection and block that even level of heat distribution that a bled radiator could provide.

Balance your heating system

If you notice some radiators are significantly hotter than others, consider balancing the system by adjusting the radiator valves.

This will ensure an even distribution of heat throughout your home.

For tips on doing this yourself, you could check out our guide on balancing your home’s heating.

You may need a power flush

Debris can very quickly build up in a heating system.

Consider power flushing to remove any sludge or debris buildup that could be hindering the system’s performance.

Keep an eye on boiler pressure

The recommended boiler pressure is generally between 1.0-1.5 bar when cold, and 2.0 when hot, but consult your boiler manual for more specific guidance to achieve the very best performance from your designer radiators.

Keep your heating system in top shape with bestheating

You should now be equipped with the knowledge to properly bleed your radiators and maintain an efficient heating system.

By following the step-by-step guide and troubleshooting tips provided in this article, you can ensure consistent and comfortable warmth in your home throughout the colder months and beyond.

Remember, regular maintenance is the key to extending the lifespan of your heating system and optimising its performance.

Take the time to care for your system, and it will reward you with reliable and cost-effective heating for years to come.

If you want to learn about how to bleed a heated towel rail, visit our guide. 

Be sure to consult the bestheating Advice Centre for any more helpful heating guidance, and explore our home heating collection online if it is time to upgrade your radiator.

And let us know your favourite heating hacks in the comments below, or via Instagram, Facebook or X.

Bleeding a radiator – Frequently asked questions