Smarter Heating & Hot Water

Inhibitor Dosing Calculator

Work out how much central heating inhibitor to add from system water volume. Confirm the in-use level with a test kit.

System volume

Estimate is a trade allowance, not measured content
System volume
100 L
Dose rate
0.5%
Inhibitor required
0.50 L
Confirm with a test kit after filling

Method

Dose (litres) = system volume × dose rate ÷ 100.
Fernox F1 is commonly 0.5 L per 100 L. Sentinel X100 is 1 L per 100 L. Always read the bottle — pack sizes and treat volumes change.
BS 7593:2019 requires the system to be dosed to the maker’s concentration, then tested. Do not mix brands in the same system.

Guidance only. Follow the inhibitor maker’s label. Over-dosing some products is wasteful; under-dosing fails BS 7593. Drain-down or a failed test means re-dose, not a one-off fill.