Why a schedule beats one “big overhaul”
Fouling of an MVHR unit is not a seasonal phenomenon: the fresh-air filter clogs progressively, condensate works all winter, kitchen extract grilles collect grease in summer as much as in winter. A single annual overhaul lets the system drift for six months with airflow in decline. Four twenty-minute visits, placed at the change of seasons, keep the installation permanently compliant for a third of the time of a complete service — and each visit targets the critical point of that moment.
September-October: preparing for winter
- Filters: one full fresh set (fresh-air side at the unit, coarse pre-filter if fitted). On a heat-recovery unit, this is the only moment to change both sides without risking frost during the intervention.
- Condensate purge: trap or collector under the heat exchanger, water seal restored. In already cold weather, frozen water in the collector is the number one cause of failure from October to March.
- Insulation: check the sleeve on duct runs in unheated zones; a lifted collar edge produces running water all season long.
- Sanitary gap under the kitchen hood: tissue test — the draught must hold with windows closed.
December-January: the cold-weather check
A short, opportunistic visit: listen to the unit at peak heating, verify the heat-recovery unit is not frosting in freezing weather (if it is: check recorded air temperatures on the control panel and the defrost cycle trigger), and check for persistent misting in the bedroom at 7 a.m. — morning mist is the sign the installation under-delivers exactly when demand is highest.
March-April: the post-heating review
- Airflow readings room by room with an anemometer, compared against the commissioning report: a drift of more than 15 % on one grille indicates a local clog or a tired module.
- Dust-off of extract grilles (removable grids, hygrostatic modules wiped dry — never immersed).
- In collective systems: purge the riser-foot traps with the building manager (common-area maintenance).
June-July: the summer inspection
The only time of year you can open the unit in comfort and test the summer bypass on a heat-recovery unit (damper open, supplied air should approach outdoor temperature). Take the chance to inspect the roof terminal — cowl, backdraft shutter, wasp nest — and check duct tension, slackened by attic heat.
The checklist, in five lines
- Filters changed, date written in marker on the unit casing.
- Condensate purged, water seal restored.
- Airflows measured at the five points (kitchen, bathroom, WC, two bedrooms), deviations under 15 %.
- Grilles and air inlets clean, dampers free-moving.
- Roof terminal checked, ducts taut, insulation joints sealed.
These five lines fit on a laminated card kept in the technical cupboard. The original commissioning report, stored with it, is the benchmark you compare against — without it, you maintain blind.
Validate your login