How Does Air Get Into A Hot Water Heating System?

Have you ever wondered how your home stays warm during those chilly winter nights? If you have a hot water heating system, then you know it relies on a complex network of pipes and radiators to keep your living space toasty.

But have you ever stopped to think about how air gets into that system and how it can impact its performance?

If your central heating system isn’t heating up as it should, or if you hear bubbling noises, there could be air build-up. This can make radiators less efficient, which is undesirable, especially during the winter.

It is possible for air to enter your heating system through leaks, but it may have entered during installation or else could have been forced into the system when the system expanded and contracted.

Radiators with air inside are more common than people might expect, but solving this issue is straightforward.

How Does Air Get Into A Hot Water Heating System

In this article, we’ll delve into the fascinating world of hot water heating systems and explore the ways in which air can find its way in and how it can be both a helpful and harmful factor in keeping your home warm and cozy.

What Are The Symptoms Of Air In The Heating System?

Eventually, you may notice that your radiators aren’t heating as well as they should, especially if the top of the radiator is colder than the bottom.

If this is the case, you might have air in your radiators or moving through your central heating system.

There are also instances in which sucked-in air in your central heating system can cause your radiators to make noises, such as bubbling, gurgling, and rattling.

In particular, you may notice this when the heating first comes on, though it can happen at any time. The air may enter your heating system due to a faulty radiator, which may require bleeders and corrections.

What Problems Are Caused By Air In Radiators?

What Problems Are Caused By Air In Radiators

In places like radiators where there is a lot of air, heat is reduced, and as a result, your home or business may not be heated to the best of its ability.

Your house may seem cooler now that your central heating system has been installed, even when the system is on. Even though tiny bubbles will not damage your health in the short term, removing them is still a wise move.

A more extended period of time, however, can lead to oxidation of the heating system, which can lead to rust inside the central heating system, which can create blockages in the boiler and further leaks.

As a result of leaks in your heating system, you may not only have reduced effectiveness of the system.

But you may also have water leak into your property, either from the boiler itself, the radiator, or the valve, some of which may not be visible to you because they may be embedded in the walls or floors.

Why Do I Keep Getting Air In My Heating System?

Why Do I Keep Getting Air In My Heating System

Some small bubbles of air may creep into most radiators and heating systems over time, especially during installation, but even during the course of installation, some air may make its way into the system.

Typically, they occur when your radiators are filled with water, and some air remains in the system.

Excess water vapor can cause air to build up in a central heating system. If the vapor accumulates in your radiators, it can prevent the water from flowing into them. Your central heating system can also be affected by several other causes of air:

  1. It is common for central heating systems to leak and accumulate air as a result. Your boiler may experience this if you often repressurize it.
  2. Hydrogen can accumulate within the system due to rust in the pipes or sludge build-up.
  3. If the pump is installed above the supply tank, air can accumulate in the radiators.

It is also natural for the metal parts involved in your central heating system, such as the boiler, the heating pipes around your property, or the radiators themselves, to expand and contract as a result of the heating and cooling of water.

Of course, metal valves are also necessary for the water to pass through in order to exit or enter the system.

This expansion and contraction can force tiny air bubbles into the heating system or radiators. Small bubbles gather to form larger pockets of trapped air in your radiator’s top, displacing the water.

In addition, you may find that the top of your radiators are cold because air doesn’t conduct heat as well as water. In your property, higher radiators may be most affected by air because of the way air rises within the water.

How To Get Air Out Of Your Boiler Central Heating System?

How To Get Air Out Of Your Boiler Central Heating System

A bleed from your central heating system is necessary if there is air trapped in your radiators. If you follow these steps, your radiators will soon be working again, and you will also save a lot of energy and money.

What You’ll Need To Bleed Your Radiators?

Ensure you have the following items close to hand before you start reducing the air in your central heating system:

  • A screwdriver or radiator bleed key (these can be found at most DIY stores).
  • A small container to collect the spillage of fluids.
  • A large towel to keep the surface area dry and avoid accidents.

How To Reduce Air In Central Heating Systems?

For a proper bleed of your central heating radiators, follow the simple steps below:

  1. For a minimum of 15 minutes, put the top setting on your central heating and let the radiators run at their highest temperatures.
  2. Now turn off your central heating system completely to stop air being pumped around. Let it cool down for another 15 minutes.
  3. You need to turn off the radiator valve nearest the boiler. Be sure to place a container under the bleed valve.
  4. If the valve is in the air, turn it anti-clockwise using a screwdriver or bleed key until water comes out instead of air.
  5. The first thing you’ll hear is a hissing sound, meaning the air is leaving. With time, water will also begin to flow.
  6. When opening the valve, remember not to turn it all the way, as it can be complicated to close it again.
  7. Be sure to tighten the bleed valve again after no more water is coming out.
  8. You need to repeat these steps for all your radiators. Work your way around the home, starting with the radiators closest to the boiler.
  9. Once step six has been completed, turn your central heating back on.
  10. You should verify that the water pressure inside the boiler is between 1.5 and 2 bars (in the green section). Rebalancing the pressure by topping up the boiler is necessary if the pressure is too low.

Once you’ve followed the eight simple steps above, your central heating radiators will be free of air build-up and should operate smoothly again. 

Is Air In A Boiler Dangerous?

There should probably be an inspection of any sounds that indicate air has entered the heating system and reached the boiler, especially if they are coming from the boiler or any banging or whistling sounds.

Your central heating problems are usually caused by air or material building up, which you can resolve by taking the steps below.

Although this is not a dangerous situation in most cases, it could cause your heating efficiency to decrease and corrosion, resulting in an expensive boiler breakdown or repair bills in the future.


The air that enters your system will always build up over time. Two factors will influence how often you need to flush your system with cold water.

Install an air scoop if your system does not have one, and each loop or area should have a vent. Implementing these two preventative measures will ensure maximum performance from your system.

Final Words

The air in central heating radiators problem has been identified, and you know how to fix it if you notice it in your own system.

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