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MVHR Ductwork Calculator — Extract, Supply & Duct Sizing
Size MVHR ductwork to Approved Document F 2021: wet-room extract (Table 1.2), whole-dwelling design flow (Table 1.3 + 0.3 L/s/m²), then main and branch ducts by air velocity.
13 L/s extract each (Table 1.2)
8 L/s extract each
8 L/s extract each
6 L/s extract each
8 L/s extract each
Sets Table 1.3 rate: 13 + 6 × bedrooms
All floors — drives the 0.3 L/s/m² check
For indicative per-room supply only
Total straight duct length
Each adds ~1.5m equivalent length
≤3.0 m/s in mains for low noise (CIBSE Guide B)
~1.5m equivalent length per 90° bend (rule of thumb)
~1 Pa/m typical sizing target — excludes fittings
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How We Calculate This
This calculator determines MVHR ductwork requirements from Approved Document F 2021 Volume 1 extract and whole-dwelling rates, then sizes ducts for an appropriate air velocity.
The formula
Design flow = Max(Σ wet-room Table 1.2 high rates, Table 1.3 rate, 0.3 L/s/m² × floor area)
Table 1.2 high rates: kitchen 13, utility/bathroom/en-suite 8, WC 6 L/s. Table 1.3 = 13 + 6 × bedrooms (para 1.69 sets this as the MVHR minimum total rate).
Duct area = Flow rate ÷ Max velocity
Duct diameter = √(4 × Area ÷ π)
Effective length = Straight run + (Bends × ~1.5m each)
Duct-friction estimate ≈ Effective length × ~1 Pa/m
For a 3-bed house with kitchen (13), bathroom (8), en-suite (8) and WC (6) the wet-room extract sums to 35 L/s. Table 1.3 for 3 bedrooms is 31 L/s, and 0.3 L/s/m² of a 90 m² home is 27 L/s. Design flow = Max(35, 31, 27) = 35 L/s. Main duct at 3 m/s: area = 0.0117 m², diameter = 122mm → use the next standard size up (e.g. 125mm or 150mm).
The ≤3 m/s mains velocity for low noise and the ~1 Pa/m sizing target are recognised duct-design conventions (CIBSE Guide B; Passivhaus Trust MVHR Good Practice Guide), not statutory figures. The friction estimate covers straight duct only — terminals, attenuators and the heat exchanger add significant resistance, so confirm the unit's duty against the manufacturer's pressure-vs-flow curve. The ‘10 L/s per habitable room’ figure is an indicative distribution rule of thumb, not a Part F requirement.
Frequently Asked Questions
Approved Document F 2021 Volume 1 Table 1.2 sets the continuous-extract HIGH (boost) rates each wet room must meet: kitchen 13 L/s, bathroom and en-suite 8 L/s, utility room 8 L/s, sanitary accommodation/WC 6 L/s. The separate Table 1.1 figures of 30 L/s (cooker hood ducted to outside) or 60 L/s (no hood/recirculating) are INTERMITTENT-fan rates, not MVHR boost rates — do not confuse them. The whole-dwelling supply minimum is governed by Table 1.3, not by a per-room figure (see below).
Approved Document F 2021 para 1.69 sets the minimum total continuous rate of an MVHR system as the whole-dwelling ventilation rate in Table 1.3: 13 + 6 × number of bedrooms (1-bed 19, 2-bed 25, 3-bed 31, 4-bed 37, 5-bed 43 L/s; a single-habitable-room dwelling uses 13 L/s). Para 1.24 also requires at least 0.3 L/s per m² of total internal floor area. The design flow you must size the unit and main duct to is the HIGHEST of: the sum of the wet-room Table 1.2 high rates, the Table 1.3 bedroom rate, and 0.3 L/s/m². The '10 L/s per habitable room' figure is only a rough distribution rule of thumb — it is not in Part F and must never be used in place of the Table 1.3 / 0.3 L/s/m² minimum, or you risk specifying an undersized unit that fails Building Control.
Duct size depends on the air flow rate and maximum velocity. At 3 m/s (recommended maximum for noise), typical sizes are: 75mm semi-rigid for single rooms (up to 10 L/s), 100mm for two rooms, 125mm for small mains, 150mm for main trunk ducts. Oversizing ducts reduces noise and pressure drop. Semi-rigid ducts (like Zehnder ComfoFlex) are popular for their ease of installation.
Each 90° bend adds approximately 1.5 metres of equivalent straight duct length to the pressure drop calculation. Tight bends are worse than gentle sweeps. Minimise bends to keep pressure drop low — high pressure drop means the MVHR fan works harder, uses more energy, and creates more noise. Aim for fewer than 4 bends per duct run where possible.
The MVHR unit must continuously deliver the design flow rate — the highest of the summed wet-room Table 1.2 extract rates, the Table 1.3 whole-dwelling rate, and 0.3 L/s/m² — at the system's working pressure. A typical 3-bed house (Table 1.3 = 31 L/s) commonly lands at 30-45 L/s once wet rooms are summed; larger or more wet-room-heavy homes run higher. Common units: Vent-Axia Sentinel Kinetic Plus (up to 71 L/s), Zehnder ComfoAir Q350 (up to 350 m³/h), Blauberg MVHR (up to 90 L/s). Always pick a unit with rated capacity comfortably above your design flow at the actual system resistance, not just the free-air maximum.
The MVHR unit should be centrally located to minimise duct runs — usually in a cupboard, utility room, or loft space. It needs access for filter changes (every 3-6 months) and annual servicing. The unit should be on a vibration-isolating mount and the duct connections should have flexible sections to prevent noise transmission. Allow 600mm clearance around the unit.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.