Most of the information needed for sizing cooling loads in new homes can be taken directly from house plans. Essential information includes solar gain, which is a function of window area; orientation of the house; window type and glazing features (such as low-e, low solar heat gain coefficient, gas-filled); shading from landscaping and building overhangs; and shingle and siding type and color. Load calculations should be done on a room-by-room basis so that ductwork can be sized accordingly. Use ACCA's Manual D, Residential Duct Systemsas your guide.
"So builders, remodelers and their HVAC contractors need to do more than rely on rules of thumb that suggest a standard number of square feet of living space per ton of air conditioning," Seville says. "Instead, they should match each HVAC unit to specific characteristics, including the air-infiltration rate, of each house.
"Many HVAC contractors don't understand that the system has to be right-sized to dehumidify well. They are afraid of under-sizing the system because they don't want their customers to complain about being hot in the summer. But a house simply won't be dehumidified properly if the HVAC is oversized. We've seen situations where the owners have to keep a house at 65 degrees in the summer to keep it dry.
"To properly design the HVAC system, it's critical to determine how the house will be renovated and what materials will be used," says Seville. "If you change insulation types or make the house tighter or more leaky than you planned to, you may find that the system you install is not the right size, even though you used Manual J. For example, if you have the system installed, then afterwards decide to upgrade to spray-foam insulation, the system will probably be too large, and it will not be the fault of the person that did the load calculation. This is because the house is tighter and better insulated than expected."
When the HVAC contractor understands the big picture, he can effectively evaluate the situation and recommend a range of improvements that provide the best solution for the client. Without this information, the contractor may simply suggest replacement equipment — and that equipment may not be properly sized.
