Ventilation Versus Filtration in a Tight Retrofit
A nonprofit is renovating a 900-square-foot two-bedroom apartment for a family with an asthmatic child. The building envelope is being air-sealed and insulated, dropping the blower-door reading from 12 ACH50 to 3 ACH50. The HVAC contractor proposes a single bathroom exhaust fan rated 80 CFM and a kitchen range hood rated 150 CFM, both vented outdoors, and argues that the tighter envelope plus these fans will keep indoor air healthy. The family cooks daily with a gas range and burns candles most evenings. The child's bedroom has no supply register and the door is usually closed. Outdoor air is drawn in through whatever leaks remain. The budget allows either a balanced ventilation system or a larger exhaust strategy, not both.
1. Should the team choose balanced ventilation or a larger exhaust strategy? Justify with the specific conditions in this apartment.
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Balanced ventilation is the better choice here, and the reasoning rests on three conditions. First, the envelope is now tight at 3 ACH50. With so little incidental leakage, exhaust-only ventilation creates negative pressure and pulls makeup air through unpredictable paths, including the garage or crawlspace, and can backdraft the gas range. Second, there is a combustion appliance and daily gas cooking, so depressurization is a safety concern, not just a comfort one. Third, the asthmatic child's closed bedroom door with no supply register means the bedroom will not be reliably ventilated by a central exhaust fan in the bathroom. Balanced ventilation with a dedicated supply to each bedroom delivers filtered outdoor air where it is needed and keeps the apartment near neutral pressure. A larger exhaust strategy would worsen depressurization and still leave the closed bedroom under-ventilated. The kitchen hood should still be sized and used for cooking, but it should be part of a balanced design, not the whole ventilation plan. Recommend a heat-recovery ventilator with supply to both bedrooms and the living area, with the kitchen hood as a separate local exhaust.
2. The contractor argues the 80 CFM bathroom fan already meets ventilation needs. Identify two flaws in that claim.
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Flaw one is distribution. An 80 CFM bathroom fan removes air from the bathroom, not from the closed bedroom. With the bedroom door shut and no supply register, the child's room has no reliable air exchange. Ventilation rates are calculated for the whole dwelling, but delivery matters: air must reach occupied rooms. Flaw two is the pressure and source-control issue. Exhaust-only ventilation in a 3 ACH50 apartment depressurizes the unit and draws makeup air through unknown leaks, which may carry soil gas, garage fumes, or allergens, and can interfere with the gas range's draft. A single 80 CFM fan also does not provide filtration, which matters for an asthmatic occupant. The claim treats a CFM number as equivalent to a ventilation strategy. The exam tests whether you can see that rated capacity does not equal effective, distributed, filtered ventilation under real operating conditions.