Add inhibitor to your central heating

Protect a radiator from rust and sludge by dosing the central heating system with inhibitor in one session.

CH-01Check what inhibitor your system needsLook at your boiler manual or any sticker on the boiler for the recommended inhibitor brand. Work out roughly how much you need: most bottles cover 10 radiators per litre, so check the label's dosing table against how many radiators you have. Buy a dosing vessel if you want the easier no-drain method, or just a bleed key if you're happy to drain a little water instead.

1 1L bottle · Central heating inhibitor (e.g. Fernox F1 or Sentinel X100)

Buy:ScrewfixB&QToolstationAmazon

1 each · Radiator bleed key

Buy:ScrewfixB&QToolstationAmazon

1 each · Dosing vessel/adapter (optional)

Buy:ScrewfixB&QToolstationAmazon

CH-02Turn off the heating and let it coolSwitch the heating off at the programmer or thermostat and wait at least an hour before you start. Hot water in the radiator and pipes can scald, and you need it cool to work on safely.

CH-03Protect the floor and gather your kitLay old towels or a dust sheet under the radiator you're working on. Have a washing-up bowl ready to catch drips or drained water. Small spills are normal even with a dosing vessel.

1 set · Old towels or dust sheet

Buy:ScrewfixB&QToolstationAmazon

1 each · Washing-up bowl

Buy:ScrewfixB&QToolstationAmazon

CH-04Isolate one radiatorPick any radiator on the system - it doesn't need to be a specific one. Close the thermostatic valve (TRV) fully, then close the lockshield valve on the other end using a small spanner or the plastic cap, turning clockwise. Note how many turns it took so you can put it back exactly the same.

CH-05Add the inhibitorWith a dosing vessel: screw it onto the bleed valve point, open the valve to purge air from the vessel, pour in the correct dose, then reopen both radiator valves so the pump draws the inhibitor into the system as it circulates. Without a vessel: remove the bleed valve, drain about a cupful of water into your bowl to make space, then use a small funnel and length of tubing to pour the dose straight in through the bleed hole before refitting the valve.

1 set · Small funnel and short length of tubing (if not using a dosing vessel)

Buy:ScrewfixB&QToolstationAmazon

CH-06Bleed the radiatorOpen the bleed valve slowly with your key, holding a cloth underneath. Let it hiss until water runs out steadily with no more air, then close it firmly. This clears any air that got in while you were working.

CH-07Reopen both radiator valves fullyOpen the TRV and turn the lockshield valve back to its original position, counting the same number of turns you noted earlier. This restores normal flow through the radiator.

CH-08Turn the heating on and check for leaksSwitch the heating back on and run it for 15-20 minutes. Check the bleed valve, the lockshield, and the TRV connections for any drips. Check your boiler's pressure gauge is in the normal range shown in the manual (often 1-1.5 bar for a sealed system), and top up using the filling loop if it's dropped.

CH-09Tidy up and record what you've doneWipe up any spills and pour drained water down an outside drain, not a sink, as it can carry rust and sludge. Wash your hands after handling inhibitor, as it can irritate skin. Write the date and product used on a sticker near the boiler so you or the next person knows the system's been treated.

1 pair · Disposable gloves

Buy:ScrewfixB&QToolstationAmazon

Some links may earn us a small commission at no extra cost to you.

Save this as a project

Back to all guides