Calculate the Right Size for HVAC Equipment

You’ve heard it before: When it comes to installing or replacing HVAC systems, bigger isn’t necessarily better. HVAC contractors probably also have heard that getting a system sized right depends first on figuring load calculations properly, which impacts selecting the right equipment for a home’s needs and duct sizing and distribution.

But some seasoned HVAC professionals probably wonder why they should go through the trouble and time of performing calculations when rules of thumb have served them well.

The answer is houses have changed a lot in the last 10 to 15 years while rules of thumb have stayed the same. Code requirements for new houses have risen significantly and will continue to rise for at least the next six years. In addition, more homeowners are updating their older homes with better insulation, windows and other energy-efficient items. When a home’s airtightness and insulation values rise, its peak heating and cooling loads fall. High-performing homes can have radically different HVAC needs than standard homes.

Improperly sized HVAC systems are noisy, don’t keep people comfortable and don’t maintain proper moisture control. Ultimately, the systems perform so poorly the equipment’s life is at risk.

Risks of HVAC Wrong-sizing

Heating and cooling loads are the measure of energy needed to be added or removed from a space by the HVAC system. Figuring a house’s load depends on many variables: a house’s location, the tightness of its envelope, the seasons, even the time of day. To calculate the load, use the Arlington, Va.-based Air Conditioning Contractors of America (ACCA) Manual J Version 8. The manual applies to single-family houses, low-rise condominiums and townhouses. Software is available to make these calculations much easier for the designer.

Because most calculations related to HVAC system design depend on the load calculation, it had better be right. The load calculation is used to find the right equipment size with ACCA Manual S. Then it is used to figure the proper air-distribution system and ductwork in ACCA Manuals T and D, respectively.

Although HVAC contractors use these manuals, they still tend to oversize the HVAC system to support the worst-case scenario. The problem with that is a homeowner has an HVAC system designed to meet the cooling load during that one super-hot, super-humid, full-sun day in August when he has 20 of his closest friends visiting. The system is completely wrong for the other 364 days of the year.

The problems associated with an oversized HVAC system result from short-cycling in heating and cooling modes. Peak operational efficiency and effectiveness dictate that a system run as long as possible to meet loads. That may seem counterintuitive. A good analogy is the difference between the mileage and performance of a car traveling consistently at 55 mph on the highway and one frequently stopping and starting in heavy city traffic.

Short-cycling of HVAC poses the following serious risks:

• Higher energy and operational costs because of wear and tear or even failure of the HVAC system
  
• Insufficient run time for the air in the duct to maintain a consistent temperature and condition and mix with the air in some rooms
  
• Inadequate removal of moisture from the air, leaving the home uncomfortably damp and a possible breeding ground for mold

Although HVAC contractors manipulate the numbers to prevent possible client complaints, the “padding” can actually hurt their businesses.

Chicago/Orlando Model Homes

A report, “Guide to Heating and Cooling Load Calculations in High Performance Homes,” produced for the U.S. Department of Energy’s Building America program by Pittsburgh-based IBACOS, shows that relying on rules of thumb instead of hard numbers can have dramatic consequences. The report was prepared for new construction, but the concepts apply to remodeling, as well. (View the report at Ibacos.com.)