Glossary · Central heating, radiators and hot water
What is central heating inhibitor?
Central heating inhibitor is a chemical added to the water in a heating system to slow the corrosion of its metal parts. Less corrosion means less rust and sludge circulating, so water and heat reach the radiators more easily. It is added after the system is cleaned and its strength should be checked regularly.
Key points
- Inhibitor slows the corrosion that creates rust and sludge, according to Worcester Bosch.
- It should be added to a clean system and replaced if the system is drained down, says Baxi.
- Worcester Bosch says the concentration should be checked every 12 months, or sooner if system water is lost.
- Vaillant's leaflet gives a pH of 6.5 to 8.5 as a good reading and says poor water quality can void a warranty.
- Topping up with plain mains water repeatedly can dilute it, according to Ideal Heating.
How inhibitor works
Over time the oxygen in a heating system reacts with the metal of its radiators and pipes and forms rust. Worcester Bosch says that inhibitor is a chemical that inhibits this corrosion process. With less corrosion there is less debris in the water, so water and heat flow easily to where they are needed.[1]Worcester Bosch, Efficient heating systems The debris is central heating sludge.
Ideal Heating adds that the reaction between water and steel also releases gas. The bubbles it forms can gradually replace water in the system and lower the pressure.[5]Ideal Heating, Why is my boiler losing pressure?
When it is added and checked
New or replacement system. Vaillant’s leaflet says legislation requires a new or replacement heating system to be chemically cleaned and the filled system treated with inhibitor, and gives BS7593 as the standard.[4]Vaillant, Love your boiler, love your system (water quality leaflet)
After cleaning. Baxi says that once the system has been cleaned it should be treated with an inhibitor to protect it.[3]Baxi, How to clean a central heating system
After draining down. Baxi says the inhibitor should be replaced, for example after a radiator is removed to decorate.[3]Baxi, How to clean a central heating system
Each year. Worcester Bosch says the concentration should be checked every 12 months, or sooner if system content is lost.[2]Worcester Bosch, Do you recommend treatment of the central heating water?
Baxi says that at a service the installer should check the water still has the right amount of inhibitor, and that the installer records the water treatment on the Benchmark checklist when a boiler is installed.[3]Baxi, How to clean a central heating system See Benchmark commissioning checklist.
Vaillant’s leaflet says the water drained during the annual service can be used for these checks, including the inhibitor level. It gives a pH between 6.5 and 8.5 as a good reading, and says that too acidic water risks corrosion. It also says poor water quality can void a warranty or guarantee.[4]Vaillant, Love your boiler, love your system (water quality leaflet)
What to look out for
Ideal Heating says that if you have regularly topped up the boiler with plain mains water, the inhibitor concentration may have become too low, and that a central heating specialist should drain the system and refill it with the correct amount.[5]Ideal Heating, Why is my boiler losing pressure? Worcester Bosch says to use suitable water treatment in accordance with the manufacturer’s instructions.[2]Worcester Bosch, Do you recommend treatment of the central heating water?
You can ask your engineer at the annual service whether the inhibitor level has been tested. See also boiler service.
Example
Anita in Bradford has a radiator taken off the wall to redecorate. A plumber drains part of the system and refills it. Because Baxi says inhibitor should be replaced after a drain-down, the plumber adds more when refilling. At the next service the engineer tests the water and notes the result.
What people get wrong
Many people think inhibitor is a one-off. It is added when the system is cleaned and then needs checking, because it can be lost whenever water is drained or topped up.
The second mistake is thinking a magnetic filter replaces it. They do different jobs. Worcester Bosch says the filter collects debris using magnets, while inhibitor slows the corrosion that makes the debris.[1]Worcester Bosch, Efficient heating systems
The bottom line
Central heating inhibitor protects the metal in a heating system from corroding. It goes into a clean system, needs replacing after a drain-down and should be checked at least every 12 months. A magnetic filter works alongside it, and neither takes the place of the other.
Common questions
What does inhibitor do?
Worcester Bosch says it inhibits the corrosion that would naturally happen in a system, such as rust from the metal in radiators. With less corrosion there is less debris in the water, so heat flows where it is needed.
How often should inhibitor be checked?
Worcester Bosch says the concentration should be checked every 12 months, or sooner if system content is lost. Vaillant says it can be checked at the annual service using the water drained from the boiler.
Do I need to add more after draining a radiator?
Yes. Baxi says that if the heating system is drained down, for example to remove a radiator for decorating, the inhibitor should be replaced.
Can I top up my boiler with tap water?
Topping up with plain mains water repeatedly can dilute the inhibitor. Ideal Heating says to have a central heating specialist drain and refill the system with the correct amount if the concentration has become too low.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester Bosch. Efficient heating systems. Checked 11 November 2026. ↩
- Worcester Bosch. Do you recommend treatment of the central heating water?. Checked 11 November 2026. ↩
- Baxi. How to clean a central heating system. Checked 11 November 2026. ↩
- Vaillant. Love your boiler, love your system (water quality leaflet). Checked 11 November 2026. ↩
- Ideal Heating. Why is my boiler losing pressure?. Checked 11 November 2026. ↩