Magnetite sludge: the black gunge behind most radiator faults

Where magnetite sludge usually comes from

Left unchecked, magnetite reduces radiator surface area from the inside, insulates the boiler's heat exchanger and causes localised overheating, and abrades pump seals and valve seats. It builds fastest in systems that have never had inhibitor added, that lose and regain water often through top-ups, or that mix old steel radiators with newer aluminium components without dielectric protection.

Things worth knowing about magnetite sludge

• Magnetite is black and settles as sludge, unlike white limescale
• It forms fastest in systems without a maintained chemical inhibitor
• Frequent pressure top-ups accelerate corrosion by adding fresh oxygen
• A magnetic filter captures ongoing magnetite but does not clear existing build-up
• Heavy sludge is best removed with a proper flush, not filter alone

How this plays out across our coverage area

Many of the older stone-built systems we work on around the Mendips were plumbed decades before inhibitor was routine practice, so the first flush on a system like that often produces a genuinely black bucket of water rather than a faint tint.

Questions we get asked

How do I know if my system has magnetite?

Cold spots at the bottom of radiators, discoloured water when bleeding, and a boiler that kettles are all classic signs worth having checked.

Does inhibitor stop magnetite forming completely?

It slows corrosion significantly but does not eliminate it, which is why inhibitor levels are checked and topped up at every service.

Where we deal with this

• Coleford (BA3) · 4 mi
• Shepton Mallet (BA4) · 7 mi
• Downside (BA3) · 5 mi
• Coxley (BA5) · 11 mi