How Does Air Get Into Radiators?
What causes air in your radiators?
Air gets into radiators through leaks, during installation, or from the natural expansion and contraction of your pipework as the system heats and cools. It's usually harmless in small amounts, but if air keeps coming back after bleeding, that's a sign of a leak, a faulty expansion vessel, or internal corrosion, not normal wear.
Air entering the water heating system through pipe leaks can be the most troublesome cause of air in your radiators. These leaks can be caused by corrosion, damage to pipe joints, or a faulty radiator valve. If air keeps returning after bleeding the radiators, it may be worth having the system checked by a professional, particularly if you also notice other signs of a boiler problem.
Why Are My Radiators Full of Air?
Most radiators will have a small amount of air in them, especially over time, but even during the course of the heating system or radiator installation, some small bubbles of air may creep into the system. They can be created when your radiators are initially filled with water, and a little air may remain in the system.
Where Does The Air Come From in My Radiators?
It can be caused by leaks in your heating system, boiler or radiator.
Also, a natural side effect of the heating and cooling of water is expansion and contraction of the metal elements involved in the central heating system, such as the boiler itself, the heating pipes around your property or the radiators themselves, including the metal valves that the water has to pass through to get around the system, of course.
This expansion and contraction can force tiny air bubbles into the heating system or radiators. These small bubbles rise to form larger pockets of trapped air, which will displace the water at the top of your radiators, and since air doesn't conduct heat as well as water, you might notice these air pockets as cold areas at the top of your radiators. Due to the way air rises within the water, you may find that the radiators higher in your property suffer from the air in them the most.
What Causes Air in Radiators and Your Central Heating System?
Air enters radiators and central heating systems in one of four ways: it's left over from installation or refilling, it's drawn in through a leak, it's forced in by the natural expansion and contraction of the pipework, or it's produced internally by corrosion inside old pipework.
Tiny bubbles do not cause any harm in the short term; however, you are still advised to remove them. Otherwise, you run the risk of the heating system oxidising over a longer period of time. This can lead to rust inside the central heating system, which can create blockages in the central heating system, and also lead to further leaks or damage to the boiler.
If the air in your heating system is caused by leaks, then, as well as the heating system reducing effectiveness, you may also have water leaking into your premises, either from the boiler itself, the radiator, a valve or possibly even from a pipe, some of which may not be visible to you, as these may be embedded in your wall or floor of your property.
| Cause | How It Happens | One-Off or Recurring? |
|---|---|---|
| Initial fill air | Air left over from installation or refilling the system | One-off — usually clears after bleeding |
| Expansion & contraction | Heating and cooling cycles can introduce tiny air bubbles into the system | Gradual — normal over time |
| Pipe, joint or valve leak | Air can enter as water escapes through a leak | Recurring until the leak is fixed |
| Corrosion (hydrogen gas) | Rust reacting with water inside older pipework can produce hydrogen gas | Recurring — often worsens over time |
| Faulty expansion vessel | The vessel cannot properly absorb pressure changes, which can contribute to air entering the system | Recurring |
What are the Symptoms of Air in a Radiator?
You may notice over time that your radiators are not heating as well as they should, and particularly if the top of the radiator is colder than the bottom, then it’s likely you may have air in your radiators or air being passed through your central heating system.
Trapped air in your central heating system may also cause your radiators to make noises, like bubbling, gurgling and rattling. You may notice this particularly when the heating first turns on, but it can happen at any time.
This may suggest that you need to bleed the radiators, and also look for the cause of the air entering your heating system.
Does Air in My Radiators Cause Low Pressure?
If the air in your radiators was caused by a water leak and air creeping into the system as a result, then the reduction of water in the system overall would be noticed as a reduction in pressure at the boiler pressure gauge.
Also, if you bled your radiators recently to remove some air from the system, you may have also have lost some pressure. That's because, when you bleed a radiator, the air is released, which lowers the pressure in your overall central heating system, and again this pressure drop would be shown at the boiler gauge.
Air in the heating system does not cause a significant loss in pressure; however, as shown above, both the cause and effect of the air in the system definitely can.
Is Air in a Radiator or Boiler Dangerous?
Any sounds that indicate air in the central heating system and reaching the boiler, especially banging or whistling sounds, should probably be looked at. They're likely to be caused by air or a build-up of material inside your central heating, some of which you can take steps below to try and resolve. Usually, this is not a dangerous situation, but if left unattended, your heating efficiency may be affected and could also lead to corrosion, and a build-up of this could cause an emergency boiler breakdown or expensive boiler repairs being required in future.
Central Heating Problems Caused by Air in The System
Air in the central heating system can cause several problems, including:
- noise, clanking or banging
- corrosion or rust
- lack of heating
- blockages
At the extreme and left untreated, rust build-up could clog up the central heating system, leading to a failure of the boiler, or leaks to occur due to the pressure.
How Do I Get the Air Out of My Radiator?
If you want to try to resolve this yourself, then first turn off your heating, and ensure you have a valve key to loosen the valve at the top of your radiators. This is called “bleeding the radiators”. You can also ask your plumber to do this as part of your boiler servicing.
How to Bleed Your Radiators
Once the heating has been turned off, use your radiator valve key on the valve at the top of the radiator at the top, turn the key slightly to open the valve enough for the air to escape. It can be useful to have some tissue or cloth to hand as once the air is expelled, some water may escape also; once this happens, it’s a good indication that the air has been removed.
Note that most radiators will have the bleed valve at the top, or the side, but some heated towel rails may have the valve hidden from view, or in exceptional cases may not have one fitted, in which case you may need to consult your local plumber for help with this.
Tighten the valve again once the air has escaped, and then repeat this exercise for each radiator in your property. Often you will find more air towards the top of the house or in the radiator(s) that are higher up due to the air rising to find the highest point.
Re-pressurise your boiler by adding more water into the system and then switch the boiler back on. You can also read a more detailed explanation of
how to bleed a radiator by clicking here.
Warning: Do not over-pressurise your heating system, and be sure to check the manual for the recommended setting. Once you have run the heating for a while, you may need to repeat the process to find more air pockets.
For a practical guide to identifying and removing trapped air, see our article on
how to get trapped air out of water pipes.
Radiator Keeps Filling With Air? And Need Bleeding Every Week? Here's Why
If a radiator keeps filling with air even after bleeding, or you keep getting air in your central heating system generally, the cause is almost always one of three things: a pinhole leak somewhere in the pipework, a failing expansion vessel that can't absorb pressure changes properly, or internal corrosion producing hydrogen gas inside old pipework. This is where Assett Plumbing can help you, call us to check your system for leaks and prevent an emergency plumbing situation.
That last cause surprises a lot of people, but it's a genuine chemical reaction. In older steel pipework and radiators, rust reacting with water produces hydrogen gas, which behaves exactly like trapped air and needs bleeding out in the same way. If you're bleeding the same radiator repeatedly and there's no sign of a physical leak, corrosion producing hydrogen gas is a common culprit, and it's usually a sign the system needs a power flush to clear built-up sludge and rust. At Assett Plumbing, we can help identify the underlying cause and recommend the right solution for your heating system.
You can also learn more about
power flush costs and what can affect the overall price.
Should I Bleed All My Radiators?
If you only have an issue with one single radiator, then you may be able to only bleed that one; however, it's best practice to bleed all the radiators to remove all air from all the radiators.
What Problems Does Air in Radiators Cause?
If you don't bleed the radiators in your home or business heating system, it can lead to several potential issues:
- Uneven Heating: Air can get trapped inside the radiator, causing it to have cold spots and reducing its overall heating efficiency. As a result, the room may not be heated evenly, making some areas feel colder than others.
- Reduced Heating Efficiency: When air is trapped inside the radiator, it acts as an insulator, preventing the hot water from fully heating the radiator's surface. This reduces the heat output of the radiator and makes your heating system less efficient, potentially leading to higher energy bills.
- Boiler Overworking: If some radiators in your heating system are not working efficiently due to trapped air, your boiler may have to work harder to compensate for the heat loss. This can put unnecessary strain on the boiler, leading to increased wear and tear, and potentially causing it to break down more frequently.
- Noise and Gurgling Sounds: When air is trapped in the radiator, it can create gurgling or bubbling noises as the hot water tries to circulate around the air bubbles. This can be annoying and disruptive, especially if the radiators are in bedrooms or other quiet areas of the house.
- Corrosion and Rust: Trapped air can cause corrosion inside the radiator. The combination of air, water, and metal can lead to rusting, which can damage the radiator and reduce its lifespan.
- Increased Maintenance Costs: Over time, the inefficiency and strain on your heating system can lead to increased maintenance and repair costs.
How Do I Stop Air in My Radiators?
If you are having a new heating system fitted, a boiler replacement or a new radiator added, then ensure the plumber does a power flush of the system to remove any corrosion and old or dirty water from the system. Once fitted, sealed and flushed all air should be released from the system at that point.
Finding air in the system weeks or months after fitting a new heating system should be investigated, as this may indicate a leak in the heating system or within the boiler.
Prevention of Air in Your Radiators
If you have just moved into new premises, be sure to keep an eye on the water pressure at the boiler pressure gauge to make sure it retains the pressure, and if you see this dropping over time, please call Assett Plumbing to investigate the cause and get it fixed before it leads to greater issues or heating system failure.
And for landlords, Assett Plumbing can do a full check of your heating system and issue a landlord gas safety certificate before you rent the property.
Frequently Asked Questions
What causes air in radiators?
Air gets into radiators through leaks, during installation or refilling, the natural expansion and contraction of pipework, or internal corrosion that produces hydrogen gas in older systems.
Is air in a radiator dangerous?
Not usually in small amounts. However, banging, whistling or gurgling sounds that suggest air is reaching the boiler should be checked. If left untreated, the underlying issue can contribute to corrosion and potentially costly boiler repairs.
Why does my radiator keep filling with air?
This usually indicates a pinhole leak, a failing expansion vessel, or internal corrosion producing hydrogen gas rather than normal, one-off trapped air. If bleeding does not solve the problem permanently, it is worth having your heating system checked.
Is it normal to have air in a radiator after bleeding?
A small amount of air can return over time and may be normal. However, if the same radiator needs bleeding repeatedly within days or weeks, it could indicate a leak or another system fault.
Why do I keep getting air in my central heating system?
Repeated air is usually caused by a leak somewhere in the system, a faulty expansion vessel, or corrosion inside older pipework producing hydrogen gas. A qualified heating professional can check the system and identify the underlying cause.
Does air in my radiators cause low pressure?
It can. If air has entered because of a leak, you may also notice a drop in pressure on the boiler gauge. Bleeding a radiator releases a small amount of water and can cause a slight pressure drop, which is usually normal and can be topped up if needed.
Should I bleed all my radiators or just the cold one?
If only one radiator is affected, bleeding that radiator is usually enough. However, bleeding all radiators periodically can help remove trapped air throughout the central heating system.
What happens if I don't bleed my radiators?
Trapped air can cause uneven heating, reduced efficiency, higher energy bills, extra strain on the heating system, and noisy radiators. Over time, persistent air and the underlying cause can also contribute to corrosion.
Call Assett Plumbing today for help or advice regarding air in your radiators or heating system. Contact Keith on
01604 372395.
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