Damp and moisture meters
The MVHR filters are clogged and the unit was not run. What ventilation rate has the survey not measured?
The survey has not measured any ventilation rate. The MVHR filters are clogged, and the unit was not run, so there is no reading of how much air the system moves. A mechanical ventilation unit with heat recovery only does its job when air can pass the filters and the fans are operating. Clogged filters are a blockage you can see. They are not a measured rate. The report records the dirty filters and the fact the unit stayed off. It has not balanced the system, and it has not said what the rooms receive.
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?
MVHR pulls air out of wet rooms and supplies air to others, passing it through filters and a heat exchanger. If the filters are blinded, the fans cannot move the air they were set up for, and if the unit was not run the surveyor has no observation of it working. Commissioning figures, where they exist, live on a certificate from the installer. The home survey did not reproduce them. Dirty filters are maintenance and a reason the rate is unknown. They are not the rate.
Ask the installer or a ventilation engineer for the commissioning sheet, and have the filters changed and the unit run under their instruction if you need a rate. Tell the solicitor the survey found clogged filters and did not measure ventilation. Do not treat a switched-off unit as a pass or a fail of the whole house. The report has not tested air quality. Cleaning filters is ordinary upkeep. It is not a figure the survey already took.
What should you ask your surveyor?
- Were the MVHR filters clogged on the day?
- Did you leave the unit unrun and the rate unmeasured?
- Is any commissioning figure outside this report?
What are the typical next steps?
- Treat the clogged filters as seen, and the rate as unmeasured.
- Ask for the commissioning sheet from the installer.
- Do not read a silent unit as a measured ventilation result.
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 the windows are openable, has the missing rate been replaced?
No. Openable windows are not the MVHR rate. The survey did not run the unit and did not measure how much air it moves. Clogged filters are why a rate could not be taken from a working system. Background ventilation through windows is a different provision. It does not supply the missing figure for this machine.
Does a new filter on its own give you the design rate?
Not without the unit being run and checked. A clean filter removes the blockage the survey saw. It does not by itself produce a measured ventilation rate, and the survey did not take one. Commissioning, if it was done, is on a separate paper. Changing filters maintains the system. Do not call that the survey's measurement.