MVHR in an older UK home; the honest retrofit guide

MVHR was designed for the new-build envelope. Airtightness is verified at handover. Duct routes are planned before the plasterboard goes up. An older house offers neither of those luxuries, which is why a thoughtful retrofit takes weeks of planning before any kit is ordered.

The good news is that the underlying physics is still on your side. A balanced supply and extract system, properly commissioned, will deliver cleaner air, lower humidity in wet rooms, and meaningful heat recovery through winter. That holds true even in a Victorian terrace, provided the envelope is brought into shape first.

MVHR efficiency is a direct function of how much warm air you actually recover. An envelope leaking heat through gaps in skirting, floorboards, loft hatches and old service penetrations will quietly waste most of what the unit recovers. Draught-proofing, loft sealing and floor-edge work are non-negotiable before commissioning.

A pragmatic target is an air permeability under five cubic metres per hour per square metre at fifty pascals. Many retrofits land between three and five with focused sealing alone, which is enough for MVHR to start paying back.

Duct routing decides whether a retrofit is feasible at all. The unit itself is small, but each supply and extract terminal needs a continuous run of one-hundred-and-twenty-five millimetre duct back to the central manifold, with as few bends as possible.

Most successful retrofits piggyback on existing dead space; a redundant chimney breast, the void above a stair, the gap behind a stud wall in a previously refurbished bathroom. When that space does not exist, dropped ceilings in a single corridor or a bulkhead along a landing will usually do the job without losing room height.

An MVHR unit that has been installed but not commissioned is, in practice, a noisy fan moving the wrong volume of air to the wrong rooms. Commissioning balances each terminal against the design flow rate and verifies that supply and extract match within an acceptable tolerance.

Insist on a written commissioning sheet that lists measured flow rates at every grille, the supply and extract totals, and the specific fan power. Without that sheet you do not know whether the system meets Approved Document F, and neither does anyone else.

A well-commissioned system runs continuously at a low background rate and boosts automatically when a wet room senses humidity. The filters need attention twice a year and the heat exchanger benefits from a careful clean once a year. Beyond that, the system should fade into the background and simply do its job.

Summer brings a separate question. Most units offer a bypass that stops recovering heat once outdoor air is cooler than indoor air. That helps a great deal during a heatwave. It is not a substitute for shading or active cooling in a south-facing room. Treat MVHR as a winter and shoulder-season tool with a useful summer bypass, rather than a cooling system.