Quick answer
Heating inhibitor helps protect a compatible central heating system from internal corrosion and scale. Test the concentration during routine servicing and re-dose when required by the product instructions or after water loss. Do not assume every system needs another bottle each year.
How often you need to add inhibitor depends on the concentration already in your system and whether treated water has been lost. Annual testing is a useful maintenance check; annual re-dosing is not automatically necessary.
Check the concentration, not just the calendar
Test inhibitor concentration as part of routine annual heating-system servicing, using a test suitable for the chemical in your system. Re-dose if the test or the chemical manufacturer’s instructions show that protection is insufficient, and after draining, flushing or water loss where required. Adding another bottle automatically every year can give the wrong concentration.
Follow the chemical, boiler and system manufacturers’ instructions. For background on annual testing, see Fernox’s inhibitor testing guidance.
When to check again
- After a radiator replacement or other work that drains treated water.
- After cleaning or flushing, before returning the system to service.
- After repeated pressure loss or topping up; find the cause of the water loss as well.
Do cold radiators prove inhibitor is missing?
No. Air, sludge, a valve problem, poor balancing or a pump issue can also cause cold areas. Have the problem diagnosed rather than adding treatment without testing.
