Why do I need inhibitor in my central heating system?

Central heating inhibitor protects the system from internal corrosion, limescale build-up, and sludge formation. Without it, radiators can develop cold spots, the boiler can become noisy (kettling), and components can fail prematurely. BS 7593 (2019) requires inhibitor to be added to all new and refurbished central heating systems in the UK. You need to know your total system volume first — use our Central Heating Volume Calculator if unsure.

How often should I add inhibitor?

Inhibitor should be checked annually during your boiler service. If the system has been drained for any reason (radiator replacement, leak repair), inhibitor must be re-dosed. The inhibitor concentration can be tested using a Fernox or Sentinel test kit — your heating engineer should do this during servicing.

Is Fernox F1 or Sentinel X100 better?

Both Fernox F1 and Sentinel X100 are excellent inhibitors widely used by UK heating engineers. They perform similarly in protecting against corrosion and scale. The choice often comes down to personal preference or what your boiler manufacturer recommends. Do not mix different brands in the same system.

How do I add inhibitor to a sealed system?

For sealed (pressurised) systems, the easiest method is to inject inhibitor through a radiator bleed valve using a syringe or Sentinel/Fernox rapid-dose aerosol can. Alternatively, you can connect via a filling loop after partially draining the system. The system must be at zero pressure before adding inhibitor via the filling loop.

What happens if I use too much inhibitor?

Mild over-dosing (up to 50% extra) is generally not harmful and provides extra protection. However, significant over-dosing is wasteful and unnecessary. Under-dosing is more problematic — it will not provide adequate protection against corrosion. Always aim for the correct dose rate.

TradeCalcs

Inhibitor Dosing Calculator — How Much Inhibitor Do I Need?

Calculate the correct amount of central heating inhibitor for your system. Covers Fernox F1, Sentinel X100, and generic inhibitors.

Total System Volume

Use our System Volume Calculator if unsure

Inhibitor Brand

Price Per Bottle (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our inhibitor dosing calculator uses a simple formula based on your total system water volume and the manufacturer's recommended dose rate.

The formula

Inhibitor required (litres) = System volume (litres) ÷ 100 × Dose rate

Standard dose rates

Fernox F1 Protector: 1 litre per 100 litres of system water (500ml bottle treats ~50L)
Sentinel X100: 1 litre per 100 litres of system water
Generic inhibitors: Typically 1 litre per 100 litres (check label)

If you do not know your system volume, use our Central Heating Volume Calculator to work it out first. The requirement to dose inhibitor is specified in BS 7593:2019 (Treatment of water in domestic hot water central heating systems) and is a condition of most boiler manufacturer warranties.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.