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Effect of building airtightness and fan size on the performance of mechanical ventilation systems in new U.S. houses: a critique of ASHRAE standard 62.2-2003

Abstract

As many countries have already done, the U.S. is now undergoing a transition from homes ventilated by infiltration to homes that need mechanical ventilation to provide outdoor air when windows are closed. The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) developed, and the American National Standards Institute (ANSI) recently approved, a new residential ventilation Standard 62.2-2003, which would require many new homes to have a mechanical ventilation system. However, Standard 62.2 sizes ventilation fans in such a way that the tighter the home, and the more it needs mechanical ventilation, the less likely it is to receive the minimum ventilation rate of 0.35 air-changes/hour.

This thesis uses the multi-zone building airflow software model CONTAMW to compare hourly air-change rates in a typical new home during a typical meteorological year in the moderate climate of Houston Texas. Two building parameters are varied in this analysis: leakage area (or airtightness), and ventilation fan size. There are three leakage scenarios: typical new construction, energy-efficient new construction, and tightest construction in the U.S.—and three fan scenarios: no ventilation fan, exhaust fan sized according to Standard 62.2, and an exhaust fan sized to deliver 0.35 air-changes per hour—for a total of nine scenarios. Air-change results are analyzed for time spent under-ventilated, adequately ventilated, and over-ventilated, and for exhaust ventilation efficiency, which is defined as percent of total ventilation attributable to the fan.

Results indicate that exhaust fans sized according to Standard 62.2 improve ventilation only marginally in typical new construction, while energy-efficient or tighter homes would be significantly under-ventilated. This thesis suggests revising Standard 62.2 to: 1) size ventilation fans to ensure a minimum ventilation rate, regardless of house tightness, and 2) address the impact of tightness on ventilation system performance.

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