What the system needs from the building
A mechanical ventilation with heat recovery system extracts from wet rooms and supplies to habitable rooms through a balanced duct network, recovering heat from the extract air as it goes. It needs three things from the building: somewhere to put the unit, continuous routes for rigid ductwork at the designed diameters, and a reasonably airtight envelope. In a refurbishment, all three are contested.
Duct routes are a structural conversation
Radial duct runs and manifolds need depth. In a house with new steelwork, that depth has to be found alongside the frame, not after it — notching and drilling structural members is not a site decision. We set out duct routes with the structural engineer while the frame is still on paper, and agree penetrations through new steel with the engineer rather than on site.
Airtightness or nothing
Heat recovery efficiency assumes air passes through the unit rather than around the building. In a period property with retained joists, chimney breasts and suspended floors, that means a deliberate airtightness line, tapes and grommets at every penetration, and a test before the linings hide the work. If a project is not going to test, it is not going to be airtight.
Commissioning and handover
A system that has not been balanced and commissioned is a fan. Commissioning records flow rates at every terminal against the design, and the results should be handed over with the filter schedule and access details. Filters need changing, so the unit needs to be reachable without a ladder and a hard hat.
Coexisting with other systems
In high-specification refurbishments MVHR rarely arrives alone. Air source heat pumps, water-based underfloor heating and comfort cooling all compete for the same voids, risers and plant space. Coordinating them is a drawing exercise before it is an installation exercise, and it is where the programme is won.