Filtration and Ventilation in a Tightened House
A 1,800 ft², three-bedroom house has been tightened with spray foam and new windows. It has a 3-ton heat pump, a 1 in. MERV 13 filter in a 20 in. × 25 in. return grille, and a 50 cfm bathroom exhaust fan. The kitchen has a 300 cfm range hood. A CO2 monitor in the living area reads 1,100 ppm after four people occupy the house for two hours. The filter pressure drop is 0.35 in. w.c. when loaded, versus 0.12 in. w.c. when new; the blower is on a medium speed tap. The homeowner complains of stuffiness and cooking odors lingering. A contractor proposes a higher-MERV filter and sealing the bathroom fan to reduce attic leakage. The house has no whole-house mechanical ventilation system.
1. Calculate the approximate whole-house ventilation rate needed for this house using the common residential formula of 0.03 cfm/ft² plus 7.5 cfm per bedroom-plus-one.
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The floor-area term is 0.03 × 1,800 = 54 cfm. The bedroom term uses three bedrooms plus one, or four, times 7.5 cfm: 4 × 7.5 = 30 cfm. The total required continuous whole-house ventilation is about 54 + 30 = 84 cfm. The existing 50 cfm bathroom fan cannot provide that rate, even if it runs continuously. The 300 cfm kitchen hood is an intermittent local exhaust, not a whole-house ventilation strategy, and in a tight house it may create a large negative pressure unless makeup air is provided. The 1,100 ppm CO2 reading is consistent with inadequate ventilation.
2. Evaluate the filter pressure drop and the proposed higher-MERV filter.
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The loaded filter pressure drop is 0.35 in. w.c., which is 0.23 in. w.c. higher than the new-filter value of 0.12. On a 1 in. filter, that is a large increase and can reduce airflow, especially on a medium speed tap. A higher-MERV filter in the same 1 in. frame is likely to increase pressure drop further, not improve comfort. Before increasing MERV, increase filter face area or use a deeper media filter with a lower pressure drop at the required airflow. The immediate step is to replace the loaded filter and verify airflow and static pressure. Filtration should be selected for the contaminant goal and the blower’s available static pressure, not by MERV alone.
3. Recommend a ventilation and IAQ strategy for this house.
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Install a balanced whole-house ventilation system, such as an ERV or HRV, sized near 84 cfm and commissioned with airflow measurement. Keep local exhaust for the bathroom and kitchen, and provide makeup air for the 300 cfm range hood or use a smaller hood with a pressure-controlled makeup-air path. Seal attic penetrations and the bathroom fan housing to reduce uncontrolled leakage, but do not rely on sealing alone for ventilation. Use a larger-area filter with an appropriate MERV rating that keeps pressure drop within the blower’s allowance. After commissioning, verify CO2 trends, bathroom and kitchen exhaust flows, and static pressure, and confirm the house does not become excessively depressurized when exhaust fans operate.