What is a magnetic filter?
A magnetic filter is fitted on the return pipe, usually just under the boiler, so all the water coming back from the radiators passes through it. A strong magnet inside pulls magnetite out of the flow, and many filters also have a mesh or chamber that traps non-magnetic debris such as scale and bits of solder.
Over time the captured sludge builds up around the magnet. When the filter is cleaned, the magnet is removed and the debris is flushed out, so you can actually see what has been kept away from your boiler.
Why sludge matters
Modern boilers have narrow waterways in their heat exchangers. Sludge settling in them causes noise, hot spots and reduced efficiency, and it can block pumps and diverter valves. Out in the system, it collects along the bottom of radiators and leaves them cold low down. Our guide to a radiator cold at the bottom explains that symptom.
Do you need one?
Many boiler manufacturers expect a system filter to be fitted with a new boiler, and some make it a condition of their warranty, so check the boiler's paperwork. Current guidance for installing a new boiler also calls for the system to be cleaned and protected.
On an existing boiler, a filter is worth considering if the bleed water is dark, if radiators have cold patches at the bottom, or if the system has already been cleaned and you want to keep it that way.
What fitting involves
The heating is drained down partially, a section of the return pipe is cut out and the filter is fitted with isolation valves so it can be serviced without draining again. The system is refilled, vented and checked for leaks, and inhibitor is topped up. The filter needs enough space below and around it to be opened, so the position is chosen with future cleaning in mind.
Filter, cleanse or power flush?
A filter catches sludge as it circulates, but it cannot pull out deposits that have already settled hard in radiators. In a badly sludged system, the existing debris usually needs cleaning out first, either with a chemical cleanse or a power flush, and the filter then keeps the system clean afterwards.
Think of the filter as protection, not a cure. It is most effective on a system that starts clean.
Inhibitor and the filter
Inhibitor is a chemical added to the heating water to slow corrosion and scale. It works alongside the filter, not instead of it. Many filters have a cap or dosing point that makes it easy to add inhibitor after cleaning. Using the right amount for the size of the system matters, and the product instructions say how much.
Looking after it
Most manufacturers recommend cleaning a magnetic filter at least once a year, and a boiler service is the natural time to do it. A filter that is never cleaned eventually fills up and stops doing its job. Our boiler servicing page explains what else happens on that visit.
Ask about a filter
Call or WhatsApp 07429 632804, or use the quote form. Tell Chris your boiler model and whether a filter is already fitted. More about heating systems is on our central heating page.
