How to Remove Airlock from Gravity Fed Heating System

Heating systems in UK homes are often taken for granted until something goes wrong. One of the most common issues in older properties that use gravity fed heating systems is the presence of airlocks. An airlock occurs when trapped air prevents hot water from circulating properly through the pipes or radiators. This can lead to cold spots in radiators, reduced efficiency, and in some cases a complete lack of heating in parts of the system. Understanding how to remove an airlock from a gravity fed heating system is essential for restoring comfort and ensuring the system operates safely and efficiently.

What is a Gravity Fed Heating System

A gravity fed heating system relies on natural convection to move water between the boiler, hot water cylinder, and radiators. The system is supplied by a feed and expansion tank usually located in the loft. Cold water is drawn into the system, heated in the boiler, and then rises naturally to circulate around the radiators and hot water cylinder. Because it relies on gravity and convection rather than pumps, this type of system is more prone to airlocks compared to modern pressurised systems.

Air can enter the system through maintenance work, small leaks, or if the feed and expansion tank runs dry. Once air becomes trapped, it forms a blockage that disrupts water flow. Knowing how to remove this trapped air is key to keeping the system running smoothly.

Recognising the Signs of an Airlock

The first indication of an airlock is often when one or more radiators remain cold even when the heating is switched on. In some cases, the top section of a radiator may feel cold while the bottom is hot, showing that air has gathered in the higher part of the system. Another sign is gurgling or bubbling noises from pipes and radiators as water struggles to circulate past trapped air. In more severe cases, hot water may not reach taps or the system may fail to heat up altogether.

Methods for Removing an Airlock

There are several approaches to dealing with an airlock in a gravity fed heating system. The most common method is bleeding the radiators. Each radiator has a small bleed valve, usually located at the top corner. Using a radiator key, the valve is turned slowly until air escapes, followed by water. This simple process often resolves minor airlocks and restores circulation.

In some situations, bleeding the radiators may not be enough. If the airlock is located within the pipework rather than a radiator, another technique involves gently forcing water through the system to dislodge the trapped air. This can sometimes be done by attaching a hosepipe between a mains water tap and the radiator pipework, but caution is essential. Introducing mains pressure water into a low-pressure gravity system can cause damage if not done correctly, so this should only be attempted by someone with a good understanding of plumbing systems.

Another practical method is to check the feed and expansion tank in the loft. If the tank has run dry, it may have drawn air into the system. Ensuring the ball valve and water supply are functioning correctly will help prevent further air from entering. Topping up the tank and allowing water to flow through the system may help flush out trapped air naturally.

Preventing Future Airlocks

While airlocks can usually be cleared with the right approach, prevention is always better than cure. Regularly checking the feed and expansion tank to make sure it has sufficient water is one of the most effective ways to reduce the risk. Carrying out routine maintenance on the system, such as bleeding radiators at the start of each heating season, also helps keep things running smoothly.

Upgrading to modern controls or even considering the installation of a circulating pump can also reduce the likelihood of airlocks in an older gravity fed system. Pumps improve water flow, making it harder for air pockets to remain trapped in the pipework.

When to Call a Professional

While some homeowners may feel confident bleeding radiators, more complex methods of removing airlocks should be left to qualified heating engineers. Incorrectly introducing water pressure, disturbing pipework, or tampering with the system could lead to leaks or damage. If the airlock persists despite bleeding radiators and checking the tank, professional help will ensure the system is safely restored to proper working order.

Conclusion

Airlocks in a gravity fed heating system can be frustrating, but they are usually straightforward to resolve once identified. Bleeding radiators, checking the feed and expansion tank, and ensuring the system is properly maintained are the most effective solutions. For persistent or complex airlocks, seeking professional support is the safest course of action. By addressing airlocks promptly and taking preventive steps, homeowners can keep their gravity fed systems running efficiently, ensuring warmth and hot water are always available when needed.