How to Put Leak Sealer in Central Heating System

A central heating system is something most of us rely on every day without giving it much thought. When it works well, it quietly keeps our homes warm and our water hot, but when problems arise, even small leaks can cause disruption and concern. A leaking central heating system not only wastes water but can also lead to reduced pressure, higher bills, and long-term damage to pipes, radiators, or boilers if left unchecked. One practical solution that many homeowners and landlords consider is the use of a leak sealer. This product is designed to locate and seal small leaks within the system without the need for extensive repairs. However, it must be used correctly to be effective and to avoid causing damage.

What a leak sealer does

Leak sealers are liquid chemicals formulated to circulate through your central heating system and seal small weeps or pinhole leaks. They are especially useful for systems where a slow loss of pressure is noticeable but the source of the leak is hard to identify. Unlike visible leaks from valves or pipe joints, these issues can occur in hidden pipework under floors or inside walls, making direct repair complicated and costly. A leak sealer works by depositing particles at the site of the leak, which then form a seal once exposed to air. It is important to note that leak sealers are intended for minor leaks only. They will not be effective for larger issues such as burst pipes or faulty radiators.

When to consider using a leak sealer

A common sign that a leak sealer might help is if your boiler pressure continues to drop even after being topped up, yet you cannot find any obvious signs of water around the system. In some cases, you may notice damp patches on walls, ceilings, or floors, which could indicate a leak in hidden pipework. Leak sealers are also sometimes recommended as a temporary measure for older systems where replacing pipework may not be immediately practical. However, they should not be seen as a permanent solution for significant faults. If a system is losing pressure rapidly, it is better to call a qualified heating engineer to inspect and repair it.

Preparing the system for treatment

Before adding a leak sealer, it is important to make sure your central heating system is clean and free from blockages. Many professionals advise flushing the system beforehand to remove any sludge or debris that could prevent the sealer from circulating properly. The system should be at normal operating temperature and pressure when the sealer is introduced, as this ensures it will move through the pipework effectively. You should also ensure that the system is filled with the correct amount of inhibitor, which protects against corrosion. Combining inhibitor and leak sealer gives the system both protection and sealing ability.

How to add leak sealer through a radiator

One of the most common methods is to introduce the leak sealer directly into a radiator. To do this, turn off the heating system and allow it to cool. Choose a radiator with an accessible bleed valve, often upstairs, as this will help the sealer circulate more evenly. You will need to partially drain water from the radiator using a bleed key or small valve, then use an applicator or funnel to introduce the leak sealer. Once added, tighten the valve securely and top up the system pressure if necessary. The system can then be restarted, allowing the leak sealer to circulate. It may take a few hours of operation before the sealer fully locates and plugs the leak.

Using a dosing point or filling loop

Some systems, especially sealed systems and combi boilers, may have a dosing point designed for adding chemicals. This is usually found on the filling loop or filter housing. Using this method can be simpler and avoids the need to remove radiator valves. A cartridge or bottle of leak sealer can be connected and injected directly into the system. Always follow the manufacturer’s instructions carefully, as incorrect application may lead to blockages or system inefficiency.

Aftercare and monitoring

Once the leak sealer is in the system, it is important to keep an eye on boiler pressure over the following days and weeks. A successful application should stabilise the pressure and reduce or stop any signs of leaking. If the problem continues, it may suggest the leak is too large or in a location that the sealer cannot effectively reach. In such cases, professional repair will be required. It is also good practice to book a service after using leak sealer, to ensure that the system is still operating efficiently and safely.

Risks and considerations

While leak sealers can be useful, they are not without risk. Overuse of the product or adding it to a system that is already heavily sludged can lead to blockages, especially in narrow pipework or boiler heat exchangers. Some manufacturers caution against using leak sealers in combination with certain high-efficiency boilers, so it is always wise to check your boiler manual before proceeding. Leak sealers should also be used in the recommended dosage, as adding more than necessary does not make the seal stronger and can harm the system.

When to call a professional

Although adding a leak sealer is a job some confident homeowners can carry out themselves, many prefer to have a qualified heating engineer do it. An engineer can also confirm whether a leak sealer is the right approach or whether a proper repair is needed. This is particularly important for landlords, who have a legal duty under the Landlord and Tenant Act to maintain heating systems in safe working order. If a leak is severe or persistent, relying on a leak sealer alone is not sufficient and could lead to greater damage.

Long-term system care

Preventing leaks in the first place is the best approach. Regular servicing, checking system pressure, bleeding radiators, and ensuring inhibitor levels are maintained can all help extend the life of your central heating. For older systems, it may be worth considering upgrades such as magnetic filters, which capture rust particles before they can damage pipework. Investing in these measures reduces the likelihood of leaks and minimises the need for chemical sealers.

Conclusion

Using a leak sealer in a central heating system can be a cost-effective and practical way of addressing small leaks that are otherwise hard to locate. When applied correctly and in the right circumstances, it can restore stability to your system and prevent further damage. However, it is not a cure-all solution and should never replace proper servicing and maintenance. By understanding when and how to use a leak sealer, and by seeking professional advice when needed, you can keep your heating system reliable, efficient, and safe.