Damp and moisture meters
A positive-input ventilation unit in the loft is switched off and the bedrooms smell stale. What background ventilation was not running on the day?
The background ventilation that was not running is the positive-input unit in the loft. It was switched off, so it was not pushing air into the house on the day, and the bedrooms smelled stale. A positive-input system is meant to supply a trickle of air from the loft unit into the rooms. Switched off, that supply is absent. The survey has recorded the off switch and the stale smell. It has not measured a ventilation rate, and it has not said the smell proves damp or mould. The unit simply was not running.
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?
Positive-input ventilation takes air, usually from the loft, and blows it into the landing or the rooms so stale air is displaced. It only does that while it is powered and the fan turns. An off unit is a stopped fan, not a measured failure of the fabric. Stale bedrooms fit a house that had no supply from that unit during the visit. They do not, on their own, diagnose rising damp or a missing damp course. The survey did not run the unit.
Ask an electrician or the installer why the unit is off, and whether it is meant to run continuously. A switched-off fan may be a choice, a tripped supply, or a unit that has been abandoned. The survey has not decided which. Tell the solicitor the background supply was not running and the bedrooms smelled stale. Do not replace the unit from this note alone. Getting it running, if the house relies on it, is the first check. The rate remains unmeasured.
What should you ask your surveyor?
- Was the positive-input unit switched off in the loft?
- Did you leave its ventilation rate unmeasured?
- Were the stale bedrooms recorded without a damp diagnosis?
What are the typical next steps?
- Treat the loft unit as background ventilation that was not running.
- Ask why it is switched off before replacing it.
- Do not read the stale smell as proved damp.
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
If a window was open in one bedroom, was the unit unnecessary?
An open window on the day is not the positive-input unit. That unit was switched off, so the background supply it should provide was not running. One open window does not serve the other rooms, and it was not a measured rate. The survey has not said the system is surplus. It has said the fan was off and the bedrooms smelled stale.
Does a stale smell mean the loft is wet?
Not from this note. The smell was in the bedrooms, and the finding is a positive-input unit switched off. The survey has not inspected the loft for a leak, and it has not measured moisture there. Stale air fits a fan that was not running. Do not convert that into a wet-loft diagnosis the report did not make.