This page documents the calculation logic, regulatory references, and engineering assumptions underpinning every VentCalc output. If a Building Control officer or peer reviewer questions the numbers, point them here.
All calculations are cross-referenced to the following UK regulatory documents and guidance notes. Clause references appear on the Detailed Schedule tab of the Excel export.
Vol 1 (Dwellings) and Vol 2 (Non-domestic). Primary source for whole-dwelling ventilation rates, wet-room extract rates, and system type selection. 2021 edition applies to all work in England.
Table 6.9 (Non-domestic) provides SFP limits for central AHUs and local units. AD Part L1A / L1B (domestic) limits apply to MVHR and MEV systems. Used for all PASS/FAIL SFP checks.
Annex B (Table B.4) provides Category II outdoor air supply rates for non-residential occupancies (l/s per person + l/s per m²). Used as default commercial airflow basis.
Supplementary extract rates for commercial spaces including sanitary accommodation, catering kitchens, server rooms, and gyms. Applied where AD Part F Vol 2 does not give specific rates.
Table 1 occupancy-based rates (5 l/s per person) and minimum 3 ACH for teaching spaces. Applied automatically when a room is identified as a classroom or learning space.
For dwellings, AD Part F 2021 (Vol 1) sets two independent airflow requirements. VentCalc takes the higher of the two as the design value to ensure worst-case compliance.
The minimum continuous supply rate is based on the number of bedrooms. Part F uses occupancy as (number of bedrooms + 1) — one person per bedroom plus one additional occupant.
Occupancy basis: N + 1 persons, where N = number of bedrooms. Default person rate: 8 l/s per person (Cat II, BS EN 15251).
Minimum continuous extract rates for each wet room type. Boost (intermittent) rates apply when the fan is occupancy-triggered.
| Room Type | Continuous Extract | Boost Rate |
|---|---|---|
| Kitchen (without cooker hood) | 13 l/s | |
| Kitchen (with cooker hood, recirculating) | 13 l/s | |
| Kitchen (cooker hood with extract duct) | — | |
| Bathroom | 8 l/s | |
| WC / Cloakroom | 6 l/s | |
| Utility Room | 8 l/s |
For non-domestic buildings, VentCalc applies BS EN 16798-1 Category II rates as the primary method. Where occupancy data is not available, ACH-based rates from CIBSE Guide B2 are used as a fallback.
| Space Type | Design Rate | Regulatory Basis |
|---|---|---|
| Office / Open-plan | 10 l/s/person + 0.6 l/s/m² | BS EN 16798-1, Table B.4 Cat II |
| Meeting Room | 10 l/s per person | BS EN 16798-1, Table B.4 Cat II |
| Classroom | 5 l/s/person (min 3 ACH) | BB101 (2018), Table 1 |
| WC / Sanitary | 25 l/s per fitting (min 6 ACH) | CIBSE Guide B2, Table 3.1 |
| Kitchen / Catering | 20–60 ACH (by heat load) | CIBSE Guide B2, Table 3.2 |
| Reception / Lobby | 8 l/s per person | BS EN 16798-1, Table B.4 Cat II |
| Server / IT Room | ≥6 ACH (heat load-dependent) | CIBSE Guide B2, Section 3.5 |
| Corridor | 0.5 l/s per m² | AD Part F Vol 2, Table 1.1 |
Indicative duct sizes are calculated from the design airflow and a target velocity. Static pressure is estimated using a simplified Darcy-Weisbach approach applied to an equivalent duct run length.
Default design velocity: 4.0 m/s (low-velocity system, CIBSE Guide B2 recommendation). Range 3–5 m/s for supply and extract ductwork.
The 1.5× fittings multiplier represents a standard feasibility-stage allowance. At detailed design, the index circuit should be calculated with actual fitting pressure losses (bends, junctions, attenuators) from CIBSE pressure loss tables.
SFP is the ratio of total fan circuit power to the total air volume flow rate. Every VentCalc output applies a strict PASS/FAIL check against the relevant AD Part L limit for the system type.
Fan power at feasibility stage is estimated from the duty pressure and an assumed fan total efficiency of 55% (typical value for a belt-drive centrifugal fan at selection point). At detailed design, actual manufacturer efficiency curves must be used.
| System Type | SFP Limit | Reference |
|---|---|---|
| Intermittent extract fan (domestic) | 0.5 W/(l/s) | AD Part F Vol 1, para 4.7 |
| Mechanical extract ventilation (MEV / dMEV) | 0.5 W/(l/s) | AD Part F Vol 1, Table 4.1 |
| MVHR — domestic supply + extract with heat recovery | 1.5 W/(l/s) | AD Part F Vol 1, Table 4.2 |
| Zonal balanced supply + extract with heat recovery (non-domestic) | 2.3 W/(l/s) | AD Part L Vol 2, Table 6.9 |
| Central AHU — supply + extract, no heat recovery (non-domestic) | 2.0 W/(l/s) | AD Part L Vol 2, Table 6.9 |
| Mechanical extract only — non-domestic | 0.5 W/(l/s) per fan | AD Part L Vol 2, Table 6.9 |
| Local recirculation unit (no outside air) | N/A — not subject to SFP limit | AD Part L Vol 2 |
When a drawing is uploaded, an automated image analysis process extracts room labels, dimensions, equipment tags, and annotation text. The Confidence Level reflects certainty in that extraction — not the accuracy of the underlying calculation logic.
Values derived directly from AD Part F tables, clearly legible room labels, or manually entered data. Area measurements confirmed against drawing scale. No significant assumptions required.
Some values inferred from room type heuristics, partial text extraction, or standard default rates. Drawing legibility may be reduced (low resolution, hand-annotated, or scanned).
Significant assumptions applied. Drawing was not machine-readable, or critical information (floor area, room usage) could not be extracted with confidence. OCR accuracy below reliable threshold.
All outputs are indicative feasibility calculations produced at RIBA Stage 2–3. They are suitable for:
They are not suitable for: