Why “ventilation by specialist” no longer works with MVHR

Many self-build clients end your appointment once the full plans application is in. MVHR is then left to the contractor, at Stage 5. That gap now carries legal weight. Here’s why “ventilation by specialist” no longer works – and how to make the case for a full appointment.

The familiar handover Your appointment as architect ends at Stage 4, and the client goes direct to a contractor.The drawings note “ventilation by specialist” – a contractor’s design portion in all but name. On site, someone asks where the MVHR goes.

Why MVHR can’t wait Mechanical ventilation with heat recovery (MVHR) needs more space than all other plant combined. It needs a suitable plant room in the right location, coordinated duct routes inside the heated thermal envelope, and penetrations through the building fabric. Coordinated at Stage 3, that’s a few lines on a drawing. Added at Stage 5, it potentially means drilling holes through structural members, unplanned bulkheads, and compromised duct routes.

Changes are cheap on drawings and expensive on site. And who gets the call when there’s a clash or the layout has to change? Often you. Revised drawings, structural engineer’s queries, and building control amendments pull you back in. That rework is rarely in anyone’s fee or budget.

It isn’t fair on the contractor either Contractors price to build, not to design. MVHR design at Stage 5 is a job they never signed up for.

The Principal Designer owns all the design Any project with more than one contractor now needs a Principal Designer. The rules took effect in England on 1 October 2023, and in Wales on 1 July 2026. The role is to plan, manage, and monitor the design work, so the building complies if built to it. That covers every discipline and element – Parts F and O included. Noting “by specialist” doesn’t move that responsibility. If a domestic client fails to appoint one, the designer in control of the design phase becomes Principal Designer by default.

Overheating is a design decision Part O applies to every new home – in England since 15 June 2022, and in Wales since 23 November 2022. Homes must limit unwanted solar gain and provide a means to remove excess heat. The simplified method only tests glazing and opening areas, not shading or mechanical ventilation. Generous glazing on the south or west elevation often breaks its limits. Then CIBSE TM59 dynamic modelling is the usual route. MVHR is normally part of the solution, so its specification feeds the model. The result then shapes the MVHR – flow rates, summer bypass, and boost. Overheating can change your glazing and your MVHR, so resolve both before fixing the window schedule. This isn’t a problem to leave to the contractor. It’s fundamental to every new home.

Making the case to your client A full appointment through Stages 5 and 6 isn’t an upsell. It’s the cheapest insurance your client can buy. One person coordinates the design team, avoiding piecemeal revisions. How we can help Solarcrest has designed MVHR for low-energy homes since 2009. We work with Principal Designers at the design stage, not after it. We provide a coordinated MVHR design in 3D (BIM/Revit ideally), a specification, and costings. We agree plant space and duct routes with you and your engineer. The contractor’s installer fits it with our oversight when needed.

Bring us in at Stage 2 or 3, before structure, fabric, glazing, and layout are fixed.

Book a free 15-minute call, and invite your client. Further information Web: solarcrest.co.uk Book a call: solarcrest.co.uk/discovery Principal Designer guide: solarcrest.co.uk/BRPD Tel: 01625 423020 Email: mvhr@solarcrest.co.uk