Ventilation and indoor air quality: ventilation concepts, heat recovery, duct design and summer comfort
Concept and heat recovery
A Passive House uses balanced mechanical ventilation: outdoor air supplied to living rooms and bedrooms, extract from kitchen, bathrooms and WC, with designed overflow paths (door gaps or transfer grilles). Design airflow is typically 0.3–0.4 air changes per hour of dwelling volume, sized from occupancy (about 30 m³/h per person) and extract-room requirements. Heat recovery efficiency must be ≥ 75 % for a PHI-certified unit and specific fan power ≤ 0.45 Wh/m³.
Ducts, balancing and summer comfort
Keep ducts short, insulate any run outside the thermal envelope, and size for low velocity (roughly 3–5 m/s in mains, 1.5–2.5 m/s in branches) to avoid noise and fan energy. Commission by measuring airflow at each valve. Summer comfort is not delivered by the ventilation unit alone: use a summer bypass, night purge, external shading and, where needed, a ground-to-air heat exchanger. Cooking extraction and humidity need separate attention.
Worked example: heat recovery efficiency
Extract air 21 °C, outdoor air 0 °C, supply air after the exchanger 4 °C. η = (21 − 4) / (21 − 0) = 17 / 21 = 0.81, i.e. 81 %, which meets the ≥ 75 % requirement. If supply measured 6 °C, η = 15 / 21 = 0.71 - the unit fails the criterion even though the air “feels warm”.
Traps
Extract-only or supply-only systems cannot reach Passive House performance; both sides must be balanced and commissioned. Duct heat gains and leaks outside the envelope count as losses. Frost protection must be designed (preheater or balanced defrost) or the unit stops in cold weather. Certified efficiency is measured at a reference airflow and may drop at half flow.