Heating Cleaner Calculator — Chemical Dosing

Standard system cleaners are dosed at roughly 1 litre per 100 litres of system water (1%). A single 1-litre bottle of Sentinel X400 or Fernox F3 treats a typical 100-litre system (about 10 radiators); larger systems need 2 litres. Descalers (Sentinel X800 Jetflo / Fernox DS-40) are also ~1:100 standard, rising to 2% for heavy fouling, while biocide (Sentinel X700) is more dilute at 1:300. Always follow the specific manufacturer's dosing instructions. Use our Central Heating Volume Calculator to find your system volume.

Standard high-performance system cleaners (e.g. Sentinel X400) need a minimum of 2 hours circulation at full operating temperature with all valves open and the pump on maximum, in line with BS 7593:2019+A1:2024 — or longer until performance is restored. Badly fouled systems can have the (non-aggressive) cleaner left in for several weeks. Descaler is typically circulated for around an hour, and biocide (Sentinel X700) only needs about 30 minutes to distribute before being left dosed. Never leave any chemical in longer than its datasheet recommends.

After cleaning and flushing, the system contains fresh, untreated water which is corrosive to steel and iron components. Corrosion inhibitor forms a protective film on internal surfaces, preventing rust and magnetite formation. BS 7593:2019+A1:2024 requires inhibitor to be added to all new and cleaned systems. Use our Inhibitor Dosing Calculator for the correct dose rate.

Most major inhibitor brands (Sentinel, Fernox) include a molybdate tracer that can be tested with a simple dipstick or test kit. A fully protected system reads at least 100 ppm molybdate on the Fernox Express Inhibitor Test — follow the pass threshold printed on your specific test kit, as brands differ. Test annually at each boiler service (a BS 7593:2019+A1:2024 requirement) and re-dose if the result is below the threshold. Record the result in the service documentation.

As a rule of thumb, a basic drain-down and refill uses roughly one system volume (typically 80–150 litres). A thorough flush with multiple fill/drain cycles uses around 3–5 times the system volume (240–750 litres). These are estimates only — actual usage depends on partial drains, dead-legs and pipe runs. A power flush with a dedicated machine uses significantly more — see our Power Flush Calculator for water and chemical quantities.