Damp and moisture meters
Has a dehumidifier left running for the viewing already cured the damp?
No. A dehumidifier running on a viewing has dried the air for that visit. It has not cured a leak, a bridged course or a cold wall. Read what is still there when you ignore the machine: salts, stains, mould, and the outside levels.
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?
These machines collect water from the air in the room they stand in. They do not rebuild a damp-proof course, close a crack in render, or stop a pipe. Sellers sometimes run them so the house feels fresher. The tank may be full, the windows may be shut, and the walls may still be marked. A surveyor notes the appliance if it is relevant and still reports the staining. They do not treat the humming as a completed repair.
A home survey is a visual inspection on one day. It does not monitor the house after the machine is switched off, and it does not guarantee concealed parts. If the report mentions damp and a dehumidifier, ask which defect remains once the air is no longer being dried artificially. Taking the machine with the seller on completion, which is common, returns the rooms to whatever moisture the building actually makes.
What should you ask your surveyor?
- Was a dehumidifier running, and did you see a full tank?
- Which stains or salts were still visible despite it?
- Does your comment depend on the machine staying in the house?
What are the typical next steps?
- Do not treat a dry-feeling room as a cured wall while a machine is running.
- Ask whether the dehumidifier is included in the sale. Assume it is not.
- Read the damp comment for the cause, not for the appliance that was masking the air.
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 an empty tank mean the air was already dry?
No. An empty tank can mean the machine was just emptied, or that it was not running for long.
Can a dehumidifier hide mould?
It can slow new growth. It does not remove staining already in the paint, and it does not reach a void it is not standing in.