A central heating system relies on clean, correctly treated water to operate efficiently. Over time, corrosion, limescale and sludge can develop inside radiators, pipework and boiler components. Small internal leaks can also cause a sealed system to lose pressure.
Central heating cleaner, inhibitor and leak sealer are designed to address different problems. They are not interchangeable, so it is important to select the correct treatment.
The quick answer
- Central heating cleaner removes sludge, scale and debris already inside the system.
- Central heating inhibitor helps prevent future corrosion and limescale.
- Internal leak sealer is designed to seal small, inaccessible leaks and weeps.
In many cases, the correct process is to clean and flush the system first, then refill it with fresh water and add inhibitor. Leak sealer should only be used when there is evidence of a suitable small internal leak.
What does central heating cleaner do?
Central heating cleaner is used to loosen and remove sludge, corrosion deposits, limescale and other debris circulating within a wet heating system.
Black magnetite sludge can settle in radiators and areas with reduced water flow. This can contribute to:
- Radiators that are cold at the bottom
- Uneven heating around the property
- Noisy boilers or pumps
- Slow system circulation
- Dirty or discoloured system water
- Reduced heating efficiency
A product such as Fernox Cleaner F3 is designed for cleaning existing central heating systems. It can also be used when commissioning a new system to remove installation debris, flux residues and oils.
The cleaner must be circulated according to the manufacturer’s instructions. The system should then be drained and thoroughly flushed before being refilled.
Cleaner removes existing contamination, but it does not provide permanent protection against corrosion. Inhibitor should therefore be added after the cleaning and flushing process is complete.
What does central heating inhibitor do?
Central heating inhibitor is a preventative treatment. It remains in the system water and helps protect radiators, boilers and pipework against corrosion and limescale formation.
Products such as Fernox Protector F1 are suitable for mixed-metal systems, including systems containing aluminium. One 500ml bottle can treat up to 130 litres of system water, which is approximately 16 radiators, depending on the system design.
Inhibitor should normally be added:
- After cleaning and flushing a heating system
- When filling a newly installed system
- Following repairs that required system water to be drained
- When testing shows that the existing inhibitor concentration is too low
The inhibitor level should be checked periodically and as part of routine boiler or heating-system maintenance. Continually topping up a sealed system with fresh water can dilute the inhibitor and introduce additional oxygen, increasing the potential for corrosion.
What does central heating leak sealer do?
Internal leak sealer is designed for small, inaccessible leaks or microscopic weeps within a central heating system. These leaks may cause gradual pressure loss without leaving an obvious pool of water.
Fernox Leak Sealer F4 circulates through the system and forms a seal at areas where oxygen enters through a small leak. It is designed to remain permanently in the system and is suitable for use with common system materials, including aluminium.
Leak sealer may be appropriate when:
- A sealed system is losing pressure gradually
- No visible external leak can be located
- The suspected leak is small and inaccessible
- Draining the system for an immediate repair would be difficult
Leak sealer should not be treated as a substitute for repairing a clearly visible leak, damaged component or failed boiler part. Rapid pressure loss, significant water leakage or repeated boiler faults should be investigated by a suitably qualified heating engineer.
Can cleaner, inhibitor and leak sealer be used together?
They may be used within the same overall maintenance process, but each product has a different purpose.
A typical treatment sequence is:
- Identify any faults or visible leaks.
- Add a suitable cleaner and circulate it as instructed.
- Drain and thoroughly flush the system.
- Refill the system with clean water.
- Add the correct dose of inhibitor.
- Use an internal leak sealer only where a suitable small, inaccessible leak has been identified.
Always follow the instructions printed on the product and avoid mixing chemicals unless the manufacturers confirm that they are compatible.
Common heating-chemical mistakes
Adding cleaner without draining it afterwards
Cleaner is intended to loosen contamination so it can be removed. Unless the product instructions specifically state otherwise, the contaminated water should be drained and the system flushed after cleaning.
Forgetting inhibitor after a repair
Draining water from a heating system can remove some or all of its protective inhibitor. The system should be correctly re-dosed after refilling.
Using leak sealer on every pressure problem
Pressure loss can be caused by several issues, including a faulty expansion vessel, pressure-relief valve or another boiler component. Leak sealer will not repair a mechanical fault.
Using the wrong dosage
Too little treatment may not provide the expected result. Check the system volume, number of radiators and manufacturer’s dosage instructions before application.
Which heating chemical should you choose?
Choose a cleaner when the system contains sludge, scale or installation debris.
Choose an inhibitor when the system has been cleaned or refilled and requires ongoing protection.
Choose an internal leak sealer when a small, inaccessible internal leak is causing gradual water or pressure loss and a mechanical fault has been ruled out.
Correct water treatment can help maintain circulation, reduce corrosion and protect important boiler and heating-system components.
Browse NXTFlow Supplies for trade-quality central heating chemicals, plumbing products and system-protection essentials, with fast UK dispatch and free delivery on qualifying orders.
Important: Always follow the product manufacturer’s instructions. If you are unsure about a leak, boiler fault or the condition of your heating system, consult a suitably qualified heating engineer.
