In my last article, I discussed ACCA Manual J, the industry standard for calculating design heating and cooling loads. These design loads represent the amounts of heating and cooling (in Btu) that a house requires to remain comfortable at very cold and very hot temperatures. Load calculations provide the basis for selecting heating and cooling equipment. To achieve optimal comfort and efficiency, the equipment selected must be neither too large nor too small.
Manual J considers heat losses and gains through opaque surfaces like walls and ceilings, through windows, skylights, and glass doors, through infiltration (air leakage) and mechanical ventilation, and, for cooling loads only, heat gains from people and appliances. It also adds losses and gains from ductwork passing through unconditioned spaces.
In this article, I’ll dig deeper into how Manual J calculates infiltration loads. Load calculation software makes it easy to treat infiltration as a “black box”; the user makes a few clicks, and the software does the rest. But infiltration is a critical component of heat losses and gains; in older houses and cold climates, it is often the single largest source of winter heat loss. Errors in infiltration inputs can lead to oversizing or undersizing. Understanding the processes driving infiltration—and how these translate into heating and cooling Btu—can help HVAC designers deliver consistently good results.
In winter, outdoor air leaking into a house needs to be heated to indoor temperature. In summer, outdoor air leaking in needs to be cooled and, in humid climates, dehumidified. (Because it’s the air leaking in that must be conditioned by the HVAC equipment, Manual J focuses on infiltration. The air coming into the house is balanced by an equal quantity of air leaving the house through exfiltration.)
For air to move into or out…
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