Sizing air-conditioning systems has really changed from how it was done 20- 30 years ago. Back in the day, ballpark sizing was usually between 400-600 square feet per ton of cooling based on the age of the house. With the advent of modern homes, tighter thermal envelopes, increased insulation values and low e-glass windows, the only way to acquire an accurate load is to perform a computer-generated load calculation. erforming Load Calculations
When performing load calculations on an ACCA approved software, don’t be surprised that new construction homes are getting over 1,000-1,200 sqft per ton of cooling. Based on a favorable house orientation, covered porches providing window shading factors, ducts located in encapsulated attics and high-quality low e-glass windows, some homes produce loads in excess of 1,500 sqft per ton of cooling. Unfortunately, I have also seen first-hand when new homes in the Raleigh area experience high humidity and moisture/mildew issues when the installed units were grossly over-sized. Some units I surveyed were over-sized by as much as 50-75% which caused unfavorable indoor conditions within the first year. When using antiquated rules of thumb for sizing that are not allowed, over-sizing units can lead to expensive remediation efforts, lawsuits and possible disciplinary action by the licensing board. With a little time and effort upfront, an accurate load calculation can be prepared which typically results in smaller units being installed, saving the installer upfront costs while avoiding any future problems. With homeowners becoming more educated (via the internet), they are quickly realizing when steps are missed in the design process and typically look for a resolution when issues appear.
I have met many HVAC contractors who have shared with me that showing the homeowner their load calculation report during their sales presentation has increased sales significantly while producing higher quality work. As our industry continues to evolve, keeping up with technology will improve your company’s reputation as a quality contractor while reducing your risk on system installations.
