Clean sludge out of a heating system with a chemical cleaner

Flush sludge from a heating system with a chemical cleaner, then refill and protect it with inhibitor.

CS-01Check your boiler manual and the cleaner bottleLook up your boiler's make and model and check the cleaner is suitable for it — some boilers, especially aluminium ones, need a specific low-corrosion cleaner. Note the dwell time on the bottle; this ranges from about an hour for a rapid cleaner to two weeks for a standard one. Work out roughly how many litres your system holds so you buy enough.

1 bottle · Central heating chemical cleaner (e.g. Fernox F3)

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1 bottle · Central heating inhibitor (e.g. Fernox F1)

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CS-02Protect the floor and gather your toolsLay dust sheets under every radiator and old towels beneath each valve. Keep a bucket handy to catch drips when you bleed radiators. Have your radiator key, gloves and goggles within reach before you start.

2 each · Dust sheets

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1 each · Bucket

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1 each · Radiator key

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CS-03Turn off the boiler and isolate it electricallySwitch the boiler off at its control panel, then turn off the fuse spur or isolator switch that feeds it so no one switches it back on while you work. Only touch the radiators and pipework — never open the boiler's casing or touch its gas or flue connections yourself.

RULES

KNOW THE RULES

Only a Gas Safe registered engineer may legally work on a boiler's gas connections or flue. This job doesn't need that, but never open the boiler casing.

CS-04Turn off the water and let the system coolTurn off your stopcock, usually under the kitchen sink, and confirm it works. Wait until every radiator feels cold before you open any valve — hot water mixed with chemical cleaner can scald.

CS-05Add the chemical cleaner to the systemChoose a radiator with a bleed valve, open it slightly with the radiator key, and pour the cleaner in slowly through a funnel and short hose. On older gravity systems you can add it via the feed and expansion tank instead. Wear gloves and goggles — the cleaner is corrosive to skin and eyes — and wipe up any drips straight away.

1 each · Funnel and short hose

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1 pair · Nitrile gloves

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1 each · Safety goggles

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CS-06Turn the heating on and circulate the cleanerRestore power to the boiler and turn the heating on full, so it reaches every radiator including any towel rail. Circulate for the time stated on the bottle — often 1 hour for a rapid cleaner, longer for a standard one. Keep the room ventilated while it runs.

CS-07Check each radiator for cold spotsWhile the cleaner circulates, feel each radiator from top to bottom. A cold patch near the bottom means trapped sludge. Use the radiator key to release trapped air from any radiator that gurgles, to help the cleaner reach it.

CS-08Turn off and drain the whole system outsideTurn the boiler off, isolate it electrically again, and turn off the stopcock. Attach a hose to the lowest drain valve and run it to an outside drain or gulley, keeping it away from soil and grass. Open the drain valve and every radiator's top bleed valve to let air in and empty the system fully.

RULES

1 each · Garden hose

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KNOW THE RULES

Don't let cleaner-laden water soak into soil or run into a surface water drain — it can harm plants and watercourses. Direct it to a foul drain where possible, such as an outside gulley connected to the sewer.

CS-09Flush the system with clean waterClose the drain valve, refill through the filling loop, then drain the system again once or twice to rinse out remaining cleaner and loosened sludge. Repeat until the water draining out runs fairly clear rather than black or brown.

CS-10Refill the system and add inhibitorRefill through the boiler's filling loop, not by running an open hose straight from a tap. Add the correct dose of inhibitor as you fill, following the bottle's instructions for your system's litre capacity.

RULES

KNOW THE RULES

The Water Supply (Water Fittings) Regulations 1999 require a backflow prevention device, such as a double-check valve, on any hose or filling loop connected to the mains, to stop contaminated system water siphoning back into the drinking supply.

CS-11Bleed every radiator and check the boiler pressureBleed each radiator's top valve in turn until water flows without air. Top up the boiler pressure to the gauge mark shown in the manual, usually between 1 and 1.5 bar, then turn the boiler back on.

CS-12Test run, check for leaks, and tidy upRun the heating for 30 minutes and check every joint and drain valve for leaks, mopping up any drips straight away. Take empty chemical bottles to your council's household chemical recycling point rather than the general bin, and wash your hands thoroughly.

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