Magnetic Filters and Inhibitor: How to Protect Central Heating

Water, air and steel radiators make iron oxide — magnetite — a fine black sludge that settles in the bottom of radiators and gets carried round to the pump and the boiler's heat exchanger. Signs of it:

• Radiators cold at the bottom and warm at the top.
• Black water when you bleed a radiator.
• A noisy, kettling boiler or a pump that keeps failing.
• Radiators that need bleeding again and again.

BS 7593 is the code of practice for treating domestic heating water. In short:

1. Clean the system with a system cleaner to loosen sludge and debris, and flush it out.
2. Protect it with an inhibitor at the dose on the bottle.
3. Filter it with an in-line filter to catch the debris that's left.
4. Test the inhibitor level every year, and top up after any work that drains water out.

Most boiler makers expect this for their warranties, so it's worth doing whenever a boiler is replaced or the system is drained.

A magnetic filter sits on the heating return pipe just before the boiler, so it catches sludge before it reaches the heat exchanger. Most have isolating valves built in, so the engineer can close it off, take out the magnet and wipe it clean at each service in a few minutes.

Match the filter to the pipe — 22mm on most homes, 28mm on larger systems — and check how it can be mounted. Heat pumps have their own filters designed for their higher flow rates.

1. Use the dose on the bottle — many 1-litre bottles treat around 100 litres of system water, roughly a typical home. Larger systems need more.
2. On a system with a magnetic filter, the easiest way in is usually through the filter's dosing point.
3. Otherwise go in through a radiator: turn the heating off, close both valves on one radiator, drain a little water, and add the inhibitor through the bleed point with an injector.
4. Open the valves, bleed the radiator, top up the system pressure and run the heating so it mixes in.

London's water is hard, and scale builds up fast in the small hot water heat exchanger of a combi, cutting the flow and making it noisy. Building regulations guidance expects scale protection on new combi installations where the water hardness is over 200ppm, which covers most of London.

A scale reducer on the cold supply to the combi is the usual answer. It's a separate job from the heating side, which is protected by the inhibitor and filter.