HVAC and Air Quality: Designing Systems for Healthy Air

Design fundamentals

A healthy HVAC system starts with the right outdoor-air rate, sized to BS EN 16798-1 Category II or better and verified at commissioning. Ductwork must be sealed (Class B minimum, ideally Class C), insulated correctly and routed to avoid contamination paths from car parks, kitchen extracts or printers.

Filtration is staged — coarse pre-filters protect fine filters which protect the building. ePM1 50% (≈ MERV 13) is the practical minimum for meaningful PM2.5 control. Add gas-phase carbon stages where outdoor pollution or process emissions warrant it.

Controls and demand response

Fixed-flow systems waste energy in low occupancy and starve fresh air in high occupancy. CO₂-driven demand-controlled ventilation modulates outdoor-air rate against measured load — the single highest-ROI controls upgrade in most office portfolios.

Bring set-points, schedules and sensor calibration into the seasonal commissioning cycle. Drifting controls are the most common cause of an originally healthy system going quietly bad. CO₂ in buildings →

Biological growth and AHU hygiene

Cooling coils, condensate trays and humidifier reservoirs are the biological hot spots. Standing water plus organic dust supports bacterial and fungal proliferation within weeks. Inspect quarterly, clean to a documented protocol and provide UV-C on cooling coils where humidity and load profiles warrant. Mould air testing →

Maintenance that actually matters

Filters changed on differential pressure rather than calendar; coils inspected and cleaned annually; drains kept clear; outdoor-air dampers verified to open under all operating modes; sensors recalibrated against reference.

Document the regime, hold the contractor to it, and audit the records. A maintenance contract that ticks boxes but never replaces a filter is worse than no contract at all because it provides false assurance.

Retrofit and upgrade pathways

Most existing UK commercial HVAC can be improved without full replacement: higher-grade filters with fan-curve verification, CO₂-based demand control, AHU recommissioning and outdoor-air audit. A staged programme typically delivers measurable IAQ improvement at 20–30% of the cost of plant renewal. Start with a ventilation assessment →