Magnetic filter clean and heating system health check

Most heating systems have steel radiators and copper pipes. Over time, oxygen in the water causes rust inside the radiators. That rust is a fine black powder, magnetite, which settles at the bottom of radiators and gets carried round to the boiler.

In the boiler it can clog the heat exchanger, wear the pump and block valves. Older London houses, with large open vented systems and a tank in the loft, often have more of it, because a little fresh water and air get drawn in over the years.

A magnetic filter sits on the return pipe to the boiler. Water passes a strong magnet that grabs the magnetite before it reaches the boiler. It only works if someone empties it.

The three are different jobs.

• Filter clean. Emptying and cleaning the magnet. A short job, usually done at the yearly service.
• Chemical clean. Adding a cleaner to the system and running it for a period so the filter and drain off collect the loosened sludge, then refilling with fresh water and inhibitor.
• Power flush. A pump machine pushes water and cleaning chemicals through each radiator in turn to shift heavy sludge. It is a bigger job, mostly for older systems with clear signs of blockage.

The health check helps decide. A system with little sludge in the filter and even radiators may need nothing more. Our post on power flush or magnetic filter goes into the signs.

Some boiler makers tie their longer warranties to a filter. Baxi says its 800 range 10 year warranty depends on the supplied magnetic filter being fitted and maintained for the length of the warranty, along with an annual service. Vaillant offers a longer guarantee on some models when a Vaillant filter is fitted by a registered installer. If your guarantee certificate mentions a filter, keep a record of each clean.

Filter cleaning on its own does not need the boiler case off. But it is usually done at the service, so the engineer can see the sludge and check the boiler at the same time.

Many homes in the areas we cover have heating systems that have grown over decades: extra radiators added during loft conversions and extensions, pipes rerouted when Victorian houses were split into flats, and old open vented tanks left in lofts. Each change lets in some air and fresh water, which means more rust.

If you have had radiators added or moved, or the system has been drained for building work, ask for a filter clean and inhibitor check at the next service. Topping up with inhibitor after any drain down slows the rust from starting again.

The signs to watch are simple. Radiators cold at the bottom, a boiler that sounds louder than it did, and dark water from a bleed valve. Each one is worth mentioning when you book.