Damp and moisture meters
Does a positive-input ventilation unit prove the damp comment in the survey was wrong?
No. A fan or a positive-input unit moves air. It does not identify whether the moisture came from rain, the ground, a leak or condensation, and it does not repair a gutter or a damp-proof course.
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?
Sellers often fit a ventilation unit after a damp quote. The unit may reduce condensation. It does not rebuild pointing, clear a cavity tray or lower a path. A surveyor who sees the unit should not treat it as evidence that the walls are dry. They also should not ignore it: it is part of how the house is being run.
Approved Document F is the building-regulations document for ventilation of new work. It is not a certificate that this unit was the right repair for this house. Ask what defect the unit was meant to cure, and whether the surveyor could see that the cause had actually been removed.
What should you ask your surveyor?
- What was the unit intended to deal with, and is that written down anywhere?
- Are the stains still there, or only the fan?
- Did you see a separate building defect, such as a gutter or a high path, that a fan cannot fix?
What are the typical next steps?
- Ask the seller for any damp report that led to the unit, and read it against the survey.
- Do not switch the unit off to “see what happens” in the week before exchange and call that a test.
- If stains remain, treat the unit as unfinished business, not as a clean bill of health.
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
Can the surveyor test the unit?
They may be able to see whether it runs. That is not a test of the damp, and it is not a ventilation design.
Should you budget a new damp-proof course because a fan is fitted?
No. The fan is not evidence for or against a course. Ask what is actually wet, and why.