One of the most common problems that people experience with their heating system is when their radiators keep filling with air. If you have had to bleed your radiators multiple times in a short space of time, this of course can be very frustrating, especially if you are unable to figure out why! There are a few reasons why this might be happening, and while it may seem daunting, there are some solutions to this problem that you can do yourself although there is also one that you will need a professional for. In this article, we will go over why your radiator keeps filling with air and how to prevent it from happening again.

Bathroom Radiator

How to Recognise Air in a Radiator

Having air in the system will not usually cause any damage but it will prevent your system from working as efficiently as it should. In most cases, this often means radiators not working properly and also using a lot more energy than what would normally be required.

Radiators Only Warm at the Bottom

The first and most obvious way to tell if there is air in your radiator is when the radiator only warms at the bottom. If the radiator only warms at the bottom and gets progressively colder towards the top, your heating system needs bleeding to remove any excess air that is trapped.


Another sign that you have air in your radiator is hearing sounds such as hissing or bubbling coming from your pipes. The hissing and bubbling noises occur when the water is trying to force its way past the trapped air.

Other noises that may indicate trapped air in your radiators are creaking sounds and vibrations.

Why Do My Radiators Keep Filling with Air?

I take it that you are here because you have bled your radiators multiple times and you have a radiator that keeps filling with air and is trying to figure out why. In many cases, a radiator that keeps filling with air is not an issue with the radiator itself but rather an issue with the system that delivers water to it. There are multiple reasons why this can happen, from a blockage in the pipe or a leaky seal at the base, it can even be down to a lack of water pressure or the height of the pump.

Lack of Pressure

Probably the most common reason for a radiator to regularly fill with air is the lack of water pressure.

  • When you bled your radiator last, did you bleed it all the way until you had water coming out of the bleed valve? or, did the hissing stop and you just presumed it was done?
  • If there was water, did you check the pressure on the system after bleeding? If not, check the pressure. Cold, the water pressure should be approx. 1 bar or slightly over.
  • If there was no water coming out, the radiator has not been bled properly and this is a sure sign of a lack of water pressure.

This one is a simple fix – after bleeding your system, you will need to re-pressurise the boiler using the filling loop.

Poor Maintenance or Age

If your heating system is poorly maintained or is particularly old, there could be a build-up of what you think may be air, but it could actually be a build-up of hydrogen. Hydrogen is a product of oxidization (rust) in the system. If your system has begun to corrode and the pipes have become rusty, you will also begin to see a build-up of sludge where the rust has mixed with the water. A radiator that heats up at the top but is cold at the bottom indicates sludge build-up. To see if your radiator is filling with hydrogen, there is a simple test that you can do at home and all you need is your radiator bleed key, a cup, and a match.

  • Take your radiator key and prepare to open the valve.
  • Hold the cup upside down just above the bleed valve and open it slightly until you hear the hissing of gas escaping.
  • Close the valve and quickly get a match.
  • Ignite the match under the cup.
  • If you hear a small pop, it means that the gas ignited, and your radiators keep filling with hydrogen and not air.


A leak somewhere can be a cause of air accumulation in the system although they can be a little harder to locate. You will know if there is a leak in the system somewhere if you have to continually re-pressurise your boiler.

If you suspect that you have a leak in your system, we recommend that you seek professional advice to see what can be done.

Pump Problem

Again, if your radiators keep filling with air, it could be due to a faulty pump. This problem is a little harder to figure out and often not a conclusion you will come to on your own. A professional gas-safe engineer will be able to perform a health check on your system if you are worried and let you know if your pump is the issue.

Bleeding a Radiator
Bleeding a Radiator

How to Prevent Radiators Filling with Air

Over time, regardless of what you do, air will get into any system naturally. Air bubbles are the result of the continual heating and cooling of water that takes place, and some systems are better at dealing with this than others. The best way to prevent any of the nasty problems above is to ensure good practice and regular maintenance on your system.

Annual Boiler Service

An annual service is especially important to keep an eye on your boiler, pipes and radiators and regular servicing gives you a better chance of catching any issues before they become bigger problems. We recommend you have your boiler serviced during the summer months before you are in need of your heating system. This means that when you need it, you will have it without having to worry about breakdowns – unlike those who are unprepared!

Annual Bleeding

It is good practice to bleed your heating system at least once annually. Personally, I do mine twice a year. The first time (the one I recommend everyone does) is just before you start using it for the autumn/ winter. This will ensure that everything is working optimally right away. The second time that I do it is right after I am done using the heating for the year in the Spring. I do this to make sure that there is no trapped air in the system during the period of inactivity to prevent corrosion.


If you have a radiator that keeps filling with air, there is an issue somewhere in your heating system. It may be something as simple as debris or sediment clogging up the radiator, or it could be that the system needs re-pressurising. The best way to avoid any issues with your heating system is by employing good practice techniques such as regular maintenance and installing high-quality parts when they wear out. In addition, if you are unsure about what type of problem needs fixing, you should call in a local gas-safe engineer, so your home stays warm this winter.

4 Reasons Radiators Keep Filling with Air Infographic

Plumbing Wizard Tips

“Bleed your radiators at least once a year – the best time to do it is in the autumn just before you start using it regularly again!”

“After bleeding, remember to re-pressurise, this is one of the most common mistakes that I have come across on a call-out and is an easy win!”

“Get your boiler serviced every year – not doing so could not only mean that damage goes unnoticed, but it could invalidate your warranty or cause issues with your insurer!”

“If the radiator is cold at the bottom and warm at the top, this is an indication of sludge build-up – best case, the radiator will need to be flushed – worst case, the whole system!”

Frequently Asked Questions

Why do I keep getting air in my radiators?

If you have radiators that keep filling with air, the problem could be coming from any number of sources.

  • A faulty seal on the boiler
  • Loose fittings on pipes
  • Cracked radiator valve
  • Lack of pressure
  • Corrosion (Causes Hydrogen)

Some issues can be fixed at home such as tightening valves, but others will require professional assistance if they are not readily visible.

How do I stop air from getting in my radiator?

If air continually builds up in your radiators naturally and there is no common fault – you can install a Flex vent that catches and releases the air out of the system automatically.

What if no water comes out when bleeding the radiator?

In most cases, this is caused by a lack of pressure in the system. After bleeding, re-pressurise the system and then bleed again to ensure that all of the trapped air is released.

How do I bleed my radiators to get rid of the air?

To bleed your radiators, locate the bleed valve on the top of the radiator, place a container underneath to catch any water, and use a radiator key to open the valve and release the air.

Can a dirty or blocked air vent cause my radiators to fill with air?

Yes, a dirty or blocked air vent can prevent the proper circulation of air and cause your radiators to fill with air.

Can a malfunctioning pump cause my radiators to fill with air?

Yes, a malfunctioning pump can cause air to become trapped in the system, preventing proper circulation and causing the radiators to fill with air.

What should I do if my radiators keep filling with air despite bleeding them?

If your radiators keep filling with air despite bleeding them, it may indicate a deeper issue with your heating system, such as a leak or a malfunctioning valve.

How often should I bleed my radiators?

It’s recommended to bleed your radiators at least once a year, or more frequently if you notice issues with them filling with air.

Can a lack of maintenance cause my radiators to fill with air?

Yes, a lack of regular maintenance can cause air to become trapped in the system, leading to radiators filling with air.

Can a leak in the system cause my radiators to fill with air?

Yes, a leak in the system can cause air to become trapped in the system and lead to radiators filling with air.

Can using the wrong type of radiator key damage my bleed valve?

Yes, using the wrong type of radiator key can damage the bleed valve and cause it to malfunction and potentially leak.

Can a malfunctioning thermostat cause my radiators to fill with air?

A malfunctioning thermostat can cause a number of issues with the heating system, but it’s not the direct reason for the radiators to fill with air, but it may cause the system to not work properly and create a perfect environment for the air to accumulate.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

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