A practical checklist for designing a mechanical ventilation system in a commercial building. Not a textbook — a working reference you can use during design.
The checklist
- Confirm building use and occupancy (Part F Table 6.2, CIBSE Guide A)
- Determine fresh air rates (L/s per person or per m², or ACH-based)
- Confirm extract requirements for WCs, kitchens, server rooms
- Calculate total supply and extract air volumes
- Check balance (supply vs extract — positive or negative pressure strategy)
- Size main and branch ducts (velocity method or equal friction)
- Select standard duct sizes (DW144 for circular, nearest 25mm for rectangular)
- Calculate system pressure drop (straight runs + fittings + components)
- Select AHU / fan (duty point on fan curve with margin)
- Check SFP against Part L limits
- Check noise — NR target for each served space
- Confirm duct attenuation requirements
- Document design intent and assumptions
Common mistakes
Undersizing return air paths, forgetting kitchen make-up air, ignoring pressure drop from filters at dirty condition, selecting fans at peak efficiency without margin.