Kitchen Exhaust and ERV Pressure Balance in a Tight Multifamily Building
A 12-unit, three-story multifamily building in Duluth, Minnesota (climate zone 7A), is targeting Phius CORE. Each apartment has a balanced ERV rated at 60 cfm low speed and 90 cfm high speed. Kitchen range hoods are 180 cfm and vent directly outside; bathroom exhaust fans are 50 cfm. The envelope is very tight, with a preliminary blower-door result of 0.45 ACH50. The contractor installed the ERVs with flex duct and no make-up air for the range hoods. During commissioning, a second-floor range hood pulls the apartment to -35 Pa relative to outdoors; the ERV supply airflow drops noticeably, and the floor drain trap gurgles. The building has atmospheric gas water heaters in closets within the apartments. Outdoor design temperature is -22°C. The owner wants to remove the bathroom exhaust fans to save money because 'the ERV already ventilates the bathroom.'
1. Diagnose the pressure and ventilation problem, including why the ERV is not a substitute for kitchen exhaust and why the bathroom exhaust matters.
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The range hood is a 180 cfm intermittent exhaust in a 0.45 ACH50 apartment with no dedicated make-up air. When it runs, it depressurizes the unit to -35 Pa, which is enough to backdraft an atmospheric water heater, pull soil gases or garage contaminants into the unit, and disrupt the ERV balance. The ERV cannot substitute for a kitchen hood because it is a balanced, low-capacity ventilation device; it does not capture cooking moisture, grease, odors, or combustion products at the source, and its supply airflow is reduced by the negative pressure. The bathroom exhaust is also not redundant: it provides local source control for shower moisture and odors, and it can run intermittently when the ERV is at low speed. Removing it would leave the bathroom dependent on a distant ERV exhaust grille and would likely increase moisture risk. The flex duct may also be adding resistance and leakage, making the ERV less balanced than assumed. The gurgling trap confirms pressure transients are reaching the drainage system, which is a health and safety issue, not just a comfort issue.
2. Recommend a compliant, practical design and commissioning sequence.
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Provide dedicated make-up air for each range hood, tempered or at least interlocked so the hood cannot operate without it, and size the transfer path so the apartment does not depressurize beyond a safe limit. Alternatively, use a recirculating hood with a high-capture filter if the project can justify it, but that does not remove moisture and must be evaluated against the ventilation strategy. Keep the bathroom exhaust fans, install backdraft dampers, and interlock them with the ERV or use a boost mode. Replace unnecessary flex duct with smooth, sealed duct and balance the ERV to the design airflow at low and high speed. Commission with a pressure test: measure apartment pressure with the range hood, bathroom fan, and ERV operating in combination, and verify that the water heater closet does not go negative. Insulate the ERV supply and exhaust ducts against freezing, and confirm the frost-control strategy at -22°C. Provide written operating instructions and label the controls so occupants do not run the hood and bathroom fan in a way that defeats the ERV.
3. What Phius QA evidence would you require before accepting the ventilation system?
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Require a commissioning report with measured airflow at each ERV, each bathroom exhaust, and each range hood, including the make-up air path. The report should show apartment pressure relative to outdoors under worst-case simultaneous exhaust operation, and it should confirm that combustion appliances do not backdraft. Require duct-leakage testing or at least pressure-pan checks on the ERV ducts, and photographic evidence of sealed penetrations and insulated ducts. The blower-door result should be final, not preliminary, and the ventilation system should be tested at that airtightness. Require the ERV balance report, frost-control settings, and a maintenance schedule for filters and cores. For Phius certification, the QA file should show that the installed system matches the modeled ventilation rates and that local exhaust is provided where required. If the bathroom exhaust is removed, the project should expect a design review challenge; the certifier should require a written justification and evidence that moisture control is still adequate.