Ventilation Commissioning: Low Airflow, Poor Temperature Ratio and a Kitchen Hood
A 6-unit passive-house apartment building has a central balanced ventilation system with a claimed heat-recovery efficiency of 80% and specific fan power of 0.35 Wh/m³. At commissioning, total supply airflow is 780 m³/h against a design of 900 m³/h; one unit has 95 m³/h instead of 150 m³/h. Duct static pressure at the fan is 190 Pa, and the unit's measured temperature ratio is 78% when balanced but 62% when the unbalanced unit is corrected by throttling. The filters were installed before the final construction clean. Residents report noisy supply valves, stuffy bedrooms in summer, and a kitchen hood that extracts 60 L/s without a make-up air path. The summer bypass damper is accessible but not labelled. The certifier needs the ventilation commissioning evidence for certification.
1. Diagnose the likely causes of the airflow shortfall and the drop in measured heat-recovery efficiency, and state what to fix first.
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The total airflow is 780/900 = 87% of design, and the worst unit is 95/150 = 63% of design. That pattern points to distribution and resistance problems, not a single fan setting: dirty filters installed before final clean, duct leakage, crushed or disconnected flexible ducts, closed dampers, or valves throttled during an incomplete balance. The temperature ratio falling from 78% to 62% when the unbalanced unit is throttled is a classic sign of imbalance: supply and extract flows are not matched, so some air bypasses the heat exchanger or recirculates internally. The first step is not to increase fan speed, which would raise noise and fan power while hiding the fault. Replace filters, inspect and leak-test the ductwork, check each terminal, then balance the system at design airflow with doors open and closed as appropriate. Re-measure the temperature ratio only after the system is balanced and the filters are clean.
2. Is the measured specific fan power acceptable, and what should the designer check about pressure and certification evidence?
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The measured 0.35 Wh/m³ is below the Passive House ventilation target of about 0.45 Wh/m³, but it was measured at only 780 m³/h, not at the design 900 m³/h. Fan power rises with airflow and pressure, so the designer must confirm the specific fan power at design conditions. A duct static pressure of 190 Pa at the fan is high for a well-designed low-pressure system; it suggests undersized ducts, restrictive filters or a tortuous layout. If reaching 900 m³/h pushes specific fan power above 0.45 Wh/m³, the ductwork or unit needs redesign, not a waiver. The commissioning evidence should include design and measured airflow per terminal, fan speed and power, external static pressure, filter pressure drop, temperature ratio at balanced flow, and the balance report. The certifier also needs the unit's certified heat-recovery efficiency and the as-installed configuration, because the 80% claim is not proven by a single unbalanced reading.
3. What should be done about the summer bypass and the kitchen hood to protect summer comfort and certification?
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The bypass damper must be labelled, functionally tested and included in the commissioning record; an accessible but unlabelled damper is not evidence that summer bypass works. The designer should verify its automatic control set-point and confirm that it opens when outdoor air is cooler than indoor air, without creating a condensation or frost problem in winter. The 60 L/s kitchen hood with no make-up air is a more serious issue: it can depressurize the dwelling, reverse the ventilation, draw humid air into the structure and degrade the heat-recovery performance. Provide a dedicated make-up air path or use a recirculating hood with a grease filter, and check that the ventilation remains balanced when the hood operates. For summer comfort, the bypass alone may not solve the stuffy bedrooms; the team should verify the PHPP summer model, the actual shading, and whether night purge ventilation is available and operable. The certifier should receive a commissioning sheet showing bypass operation, hood make-up provision and post-balance airflow.