MVHR in airtight retrofits: where most installs go wrong
A mechanical ventilation system that is heard is a system that gets switched off — and a switched-off MVHR unit is worse than none at all.
Airtightness first, ventilation second
MVHR only makes sense in a building envelope that has been sealed deliberately. Below roughly 3 m³/h·m² at 50 Pa the recovered heat begins to justify the fan energy; above it, uncontrolled infiltration short-circuits the system and the client pays for both.
In retrofit work we test before we design. The result changes duct layout, unit selection and the case for summer bypass.
Acoustics are a design output, not an afterthought
Air velocity in the duct is the single biggest driver of regenerated noise. We design branch velocities below 3 m/s and main runs below 5 m/s, use rigid ducting with swept bends rather than flexible offcuts, and place attenuators on both supply and extract sides of the unit.
Cross-talk between rooms is handled with acoustic-lined plenums and offset terminals, not with hope.
Commissioning and the numbers that prove it
A system is not finished when it runs. Each terminal is measured, balanced against the design schedule and re-measured, then the whole set is recorded on the commissioning sheet issued to the client.
Boost rates for kitchens and bathrooms are checked separately, along with the summer bypass changeover and the frost protection strategy.
Maintenance you can actually do
Filters need changing two to four times a year depending on location. If the unit is buried behind a loft hatch above a wardrobe, that will not happen. We place plant where a person can stand, with clear filter access and labelled isolation.
Planning a system where these details matter?
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