The Air Conditioning Contractors of America (ACCA) guidelines for sizing HVAC equipment, ACCA Manual J Residential Load Calculation, enables contractors to estimate heating and air conditioning loads more accurately. Using Manual J, a contractor calculates heat loss from the building through walls and ceilings, leaky ductwork, and infiltration through windows, doors, and other penetrations as well as heat gain into the building from sunlight, people, lights and appliances, doors, walls, and windows, and infiltration though wall penetrations. Design conditions for the area are also used as inputs into load calculations.
Air infiltration measurements must be estimated unless a blower door test is performed. The use of blower door test results will provide more accurate sizing calculations at a slightly higher design expense. Again, increases in design costs can be offset by decreased equipment size which lowers initial cost.
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By properly sizing HVAC equipment rather than using rules of thumb, smaller systems can often be specified and, hence, initial cost is reduced.
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Proper HVAC sizing is an essential step in the efficient operation of HVAC systems. A right-sized system will operate for long periods of time (rather than frequently cycling on and off), resulting in the optimum equipment operating efficiency.
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Proper HVAC sizing can reduce short-cycling of equipment, resulting in longer equipment life and better control over indoor environmental conditions.
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Most of the information needed for sizing cooling loads in new homes can be taken directly off house plans. Essential information includes solar gain, which is a function of window area, orientation of the house, window type and glazing (such as low-e, gas-filled), shading from landscaping and building overhangs, and shingle and siding color. In addition, infiltration levels of the house need to be determined and may require the use of a blower door test. In retrofit applications, blower door testing is more important for load calculations. Load calculations should be done on a room by room basis so that ductwork can be sized accordingly.
Independent contractors are available to perform blower door testing. Learning load calculation software is not difficult, but taking a class can help. Several software manufacturers offer technical support, as well.
There is an increased cost associated with performing Manual J calculations over simply using rules of thumb. A contractor needs more time at the house to make measurements and interview the homeowner and more time at the office to enter information and perform load analysis. Additional costs are incurred if a blower door test is conducted.
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Some jurisdictions, such as California, require load calculations.
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The benefits of properly sizing HVAC systems include satisfied and comfortable customers, lower initial and operating costs, reduced callbacks, longer equipment run times and less cycling, and proper dehumidification during the cooling season.
Load calculations cost approximately $100 or $200 per house and take between one and two hours for an average home. If a number of homes of similar plans are being calculated, costs may be lower. However, the additional cost is usually recouped immediately because the system can typically be downsized.
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