How Much Inhibitor Do I Need?
Quick Answer
Typically 1 litre per 100 litres of system volume
Based on: standard corrosion inhibitor (e.g., Sentinel X100, Fernox F1). A typical 3-bed house system holds 80–120 litres.
System Volume Estimates
- 1–2 bed flat (6–8 radiators): ~50–80 litres
- 3-bed semi (8–12 radiators): ~80–120 litres
- 4-bed detached (12–16 radiators): ~120–160 litres
- Per radiator (average): ~8–12 litres (depending on size)
- Per metre of 15mm pipe: ~0.15 litres
How to Add Inhibitor
- Via the filling loop: Use a dosing vessel or connect directly
- Via a radiator: Close valves, drain the radiator, pour in inhibitor, refill and bleed
- Via the F&E tank: Pour into the feed and expansion tank (vented systems)
- Dosing pot: Some systems have an inline dosing pot for easy addition
When to Dose
- After a system flush/powerflush
- On a new installation (Building Regulations Part L requirement)
- After any work that drains or partly drains the system
- Annual top-up: Check concentration at each boiler service
Why It Matters
Corrosion inhibitor protects radiators, boiler heat exchangers, and pipework from internal corrosion and scale. Without it, sludge builds up, reducing efficiency and potentially causing boiler breakdowns. BS 7593 and boiler manufacturers require it for warranty compliance.
Last updated: April 2026. Always check the specific inhibitor product’s dosing rate — some concentrated formulas differ from the standard 1L/100L.