We’re mid-way through a back-to-brick retrofit of a 1930s-style semi in York and need to settle ventilation in the next week or two. The electrician is at first fix and the window order is being finalised. I’d appreciate a technical sanity check.

The house

• Semi-detached, three floors: ground, first, and a new rear dormer loft conversion. Plus a rear extension.

• Roughly 175–195 m² treated floor area (architect’s figure 194 m²; my own measurement nearer 175).

• Four bedrooms plus a first-floor office, which I’m treating as a fifth bedroom for ventilation.

• Fabric: existing cavity walls filled and internally insulated to about 0.22; extension and dormer to current Part L; insulated slab; all new windows at 1.4. The pitched roof is insulated at rafter level, so the eaves are inside the warm envelope; the dormer’s flat roof is a cold, ventilated roof.

• Airtightness: aiming high (taped membranes and sealed penetrations), but untested so far.

• Heating: air source heat pump with a design load of about 4.5–5 kW; UFH on the ground floor and radiators upstairs. No open-flued appliances. The kitchen has a gas hob; the utility has an induction hob.

• Wet rooms: kitchen/dining, utility, ground-floor WC (no shower), family bathroom, two en-suites.

The MVHR question
MVHR was ruled out earlier because a structural steel beam in the ground-floor ceiling prevents ducting across it. But most of the first-floor boards are now up, and there’s eaves space inside the envelope for a unit. Is radial MVHR realistic here, feeding the ground-floor rooms down through the first-floor void either side of the steel? Or does a beam like this normally kill it? I’d take MVHR if it’s viable. It’s worth roughly 850 W off the design heat loss, and everyone who has it seems to rate the comfort.

The fallback: continuous mechanical extract (System 3)

• Upstairs: a central MEV unit in the second-floor eaves, extracting from the three bathrooms, discharging through the roof or the dormer cheek.

• Ground floor: three through-wall dMEV units, in the kitchen/dining room, the utility and the WC.

• Cooking: ducted cooker hoods in the kitchen and utility, in addition to the continuous extract.

• Background ventilators: through-wall rather than trickle vents in the window frames, one per habitable room (two in the open-plan kitchen/dining room), none in the wet rooms. Acoustic models in the bedrooms if possible.

• Boost rates: 13 L/s for the kitchen and utility, 8 L/s for each bathroom, 6 L/s for the WC, 56 L/s in total.

Questions

1. Whole-dwelling rate under ADF 2021 Volume 1. By bedroom count I get 43 L/s for five bedrooms. Is there also a floor-area minimum (I’ve seen 0.3 L/s per m² mentioned) that would govern at this size? If so, the continuous rates across the extract points need to add up to around 52–58 L/s, which is more than my boost total. How do people normally split that between a central MEV unit and dMEV units?

2. Background ventilator area. I’d assumed 4,000 mm² per habitable room. What’s the correct equivalent area for a continuous-extract system under ADF 2021?

3. Products. Any recommendations for a quiet central MEV unit (one en-suite opens straight off a bedroom) and decent dMEV units? Sound levels at the continuous setting matter more to me than boost capacity.

4. Detailing. What’s the best way to sleeve and seal through-wall vents and duct penetrations through internal wall insulation, using PIR with a vapour control layer, without compromising airtightness or creating a condensation risk?

5. Ducting and controls. What duct type and size would you use for the central MEV runs in the eaves? Is a humidity-sensing boost plus a light-switch overrun the right control set, or is there something better?

6. Gas hob. Any issues with a gas hob in a reasonably airtight house on continuous extract? Should the hood be ducted or recirculating?

Plans are attached (floor plans, elevations, and the ground-floor structural layout showing the steel). Thanks in advance. This forum has been invaluable in getting this far.

26-37-20-001 S1 Proposed Floor Plans.pdfE1030 - 01 - Foundation GA.pdf

You can sometimes make decent size holes in the web of a beam. Find out the MVHR duct requirements and ask your SE to accommodate?