Electrics and gas
The magnetic filter on the heat-pump return is packed with black sludge. Has the visit flushed the circuit or only seen the pot?
Only the pot was seen. The magnetic filter on the heat-pump return is packed with black sludge, and that is what the survey recorded. The visit did not flush the circuit, did not drain the system and did not clean the filter. Sludge in the pot shows debris has collected there. It does not measure what remains in the emitters, any buffer or the heat exchanger, and it is not a water test. The report notes a dirty magnetic filter. It has not flushed anything and has not decided the rest of the circuit is clear.
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 on the return is there to catch iron debris before it reaches the heat pump. A pot full of black sludge means a lot is on the magnet. The system may have been dirty, the pot may not have been serviced, or both. Flushing would mean cleaning the circuit, refilling and checking the water treatment, which a home survey does not do. Looking at the pot, even if the lid was open, is still only a look. The circuit itself was not cleaned.
Ask the seller for any service note that includes cleaning this filter. Your solicitor can request it. The survey has not treated a dirty pot as a failed heat pump and has not tested efficiency. Approved Document L is not a flushing certificate because sludge was seen, and this visit issues no building-regulations certificate. If an engineer should flush the system, that is a separate instruction. Tell a lender what was seen: sludge in the filter, circuit not flushed.
What should you ask your surveyor?
- Does the report say the pot was opened, or that sludge was visible another way?
- Is there a service label, and is it treated as a flush record or only as a sticker?
- Were cold spots on emitters noted, or was that not part of the visit?
What are the typical next steps?
- Record the sludge as seen and the circuit as not flushed.
- Ask the seller when the magnetic filter was last cleaned.
- If the water quality matters, instruct an engineer to flush and report. This visit did not.
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 black sludge mean the heat pump is already damaged?
Not by itself. It means the filter pot holds a lot of debris. Harm to a heat exchanger was not tested on this visit. The survey saw the pot and did not flush the circuit. An engineer can say whether the debris has affected the plant. Do not write the machine off, or clear it, from the state of the magnet alone.
Should the magnet be emptied on a second viewing?
Do not treat a viewing as a flush. Emptying the pot without dealing with the circuit only clears the sample you can see and leaves the rest unknown. The survey did not clean it. If it is to be cleaned, an engineer should do that and record the condition of the water. Wiping the magnet is not a system flush.