Sludge, inhibitor and filters

1 What sludge is

Baxi defines sludge as a black substance made up of dissolved and rusted metal, formed when water in the system reacts with metal in radiators and pipes. It is heavier than water, so it settles¹. Left alone, it can block pipes and damage the boiler pump, radiators and valves, and the narrow pathways of the heat exchanger⁵. A radiator cold at the bottom is the best-known symptom.

2 Inhibitor

Ideal describes inhibitor as a chemical that slows the reactions producing iron oxide². Worcester Bosch's Greenstar 4000 manual gives a reason to investigate repeated top-ups. They point to possible leaks, and they reduce the concentration of inhibitor protection, which can lead to corrosion⁶. That is one reason pressure that keeps dropping deserves attention.

Dosing is not covered here. Baxi says your installer should check the water and the level of chemicals at the annual boiler service⁷.

3 Magnetic filters

Baxi explains that a magnetic filter is fitted in the pipework close to where water returns to the boiler, and a magnet attracts rust fragments before they enter it³. A full filter cannot catch any more debris, though water still passes through, so it needs emptying. Baxi says your installer can clean it at the annual service⁸.

4 What the rules say

In England, para 8.8 of Approved Document L says circuits should be thoroughly cleaned and flushed before a new heating appliance is installed. A suitable chemical inhibitor should be added, and systems commissioned to BS 7593⁴. Baxi's page says the current rules also mean fitting an in-line filter⁹, and treats that as part of BS 7593 best practice. The word filter does not appear in the Approved Document L text read, and BS 7593 itself was not read, so the filter requirement is not confirmed here. Ask your installer what they will fit, and see boiler building regulations.

The rules for Wales, Scotland and Northern Ireland on inhibitors were not checked for this entry.

5 Warranty and cleaning

Baxi warns that boiler repairs caused by sludge are not covered by its warranty⁵. Other makers' terms were not checked. Vaillant says it does not recommend users power flush a system, since it needs chemicals and a dedicated machine, and it is best left to a trained engineer¹⁰.