Electrics and gas
Is a magnetic filter, together with the boiler return, any evidence on sludge the canister was not opened to show, or only a note of what was visible?
No. A magnetic filter on the boiler return is not evidence of the sludge inside it, because the canister was not opened. The filter and the return show where the surveyor was looking. Sludge that would have been visible in the canister was not shown. The visit recorded a fitting. It did not capture debris, did not judge how dirty the system is, and has not decided that the filter is full or empty. The sentence is a note of what was visible, not a look inside the pot.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A magnetic filter sits on the return so particles in the heating water pass a magnet and collect in a canister. That is why installers fit them. The amount collected is only known when the canister is opened or drained into a clear vessel. A clean-looking body can hide a full pot, and a stained body can be old dirt on the outside. The surveyor saw the filter on the return. They did not open it. The magnet's job can be explained. The sludge it may be holding cannot.
If you want to know how dirty the system is, ask a heating engineer to open the canister as part of a service. The home survey has not done that, and a sticker or a tidy filter is not a substitute. Tell the solicitor the sludge was not shown. Do not describe the heating as full of debris, and do not describe it as clean. Where the boiler is gas, use a Gas Safe registered engineer for the appliance. The closed canister stays a limit, not a result.
What should you ask your surveyor?
- Was the magnetic filter on the return, or was the pipe only assumed to be the return?
- Did the canister show external staining, or did it look unmarked?
- Was any service valve present that would have allowed the pot to be opened?
What are the typical next steps?
- Treat the filter as a fitting seen from the outside.
- Do not claim the canister proved sludge or proved a clean system.
- Ask a heating engineer to open it if the dirt load matters to you.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Does having a magnetic filter mean the system was cleaned?
No. Fitting a filter is not the same as flushing the system, and this canister was not opened, so any sludge inside was not shown. The survey saw the filter on the boiler return. It has not decided the water is clean or dirty. A filter can be new, full or forgotten. The closed pot does not say which.
Can the colour of the canister be used as a sludge test?
No. Paint, old spills and factory finish all change the outside colour. The survey did not open the canister, so it did not see the magnet or the debris. External colour is part of what was visible. It is not a sample. Ask an engineer to open the pot if you need to know what is inside.