Positive Pressure Ventilation: A Proactive Defense Against PM2.5
In Bangkok, where AQI exceeds 150 during early-year haze events, PM2.5 still infiltrates at 0.3–1.5 ACH even with windows and doors closed. The most effective countermeasure is building overpressure — maintaining positive indoor pressure so filtered air pushes out rather than polluted air pushing in.
The Physics
A building at +2–5 Pa above atmospheric pressure causes air to flow outward through every gap. This requires supplying more filtered air than is exhausted:
Q_supply (filtered) > Q_exhaust + Q_infiltration_leakage
Typically, supply must be ≥110–120% of exhaust to sustain positive pressure.
System Components
1. ERV/HRV Adjusted for Positive Pressure Standard ERV units balance supply and exhaust flows. Select units with adjustable supply/exhaust ratio: - Zehnder ComfoAir Q350: ±30% imbalance adjustment - Mitsubishi Lossnay VL-220: BACnet-adjustable flow
2. HEPA + Activated Carbon Pre-filter Stack Incoming supply air must be filtered through: - G4 pre-filter: coarse particles >10 μm - F7 medium filter: PM10 and large PM2.5 - HEPA H13: PM2.5 removal ≥99.95% - Activated carbon stage: VOC, NO₂, O₃ adsorption For a 200 m² home, supply flow of 200–300 m³/h at 50–80 Pa is required.
3. Building Envelope Sealing Positive pressure is ineffective in a leaky envelope. Priority sealing: - Blower Door Test: measure airtightness (target ≤3 ACH50 per ASHRAE 62.2) - Weatherstripping silicone on door and window frames - Fire-rated sealant on all penetrations (pipes, conduit) - Door sweeps on all exterior doors
4. Pressure Differential Monitoring Install a Seeed SDP810 differential pressure sensor (±125 Pa, I2C) between inside and outside. Connect via ESPHome → Home Assistant to: - Alert when pressure drops below +1 Pa (excessive leakage or exhaust) - Automatically adjust supply fan speed via PID control
Smart Home Integration
During AQI alerts (>100), automate fan speed increase: supply fan to 100%, exhaust fan to 70%, maintaining a +3–5 Pa differential. This creates a protective pressure bubble during worst-case pollution events.
Cost and Expected Outcomes
A complete positive pressure system (ERV + HEPA filters + envelope sealing + monitoring) costs 50,000–95,000 THB installed. Expected outcomes include 80–90% PM2.5 reduction indoors, 40–60% reduction in infiltration versus baseline, and monthly operating costs of only 300–600 THB in electricity — far less than running multiple portable air purifiers.