A few decades ago, residential air conditioning was very rare in colder areas of the U.S., and cooling load calculations were usually unnecessary. These days, however, new U.S. homes routinely include air conditioning equipment, even in Minnesota, so most U.S. builders are faced with the need to calculate cooling loads.
In my last two blogs (“How to Perform a Heat-Loss Calculation,” Part 1 and Part 2), I discussed the principles behind heat-loss calculations used to size heating equipment. In this blog, I’ll discuss the principles behind cooling-load calculations used to size air-conditioning equipment.
Although most building codes require load calculations for heating and cooling equipment installed in new homes, the requirement is widely ignored and rarely enforced. Most HVAC contractors never perform cooling load calculations; instead, they size air conditioners by rules of thumb.
The age-old rule of thumb used by most contractors was one ton of cooling equipment for every 400 square feet of conditioned space. In a concession to recent improvements in insulation levels and window specifications, some HVAC contractors have adjusted their rule of thumb, and now size air conditions at one ton per 600 square feet.
Because these rules of thumb almost always result in gross oversizing of cooling equipment, most energy experts have been battling rule-of-thumb sizing for years. However, rules of thumb have their place. Using a rule of thumb is not really the problem; the problem is that HVAC contractors are using a bad rule of thumb.
At least two well-known energy consultants, Michael Blasnik and Allison Bailes, have proposed a new rule of thumb for sizing air conditioners in homes with insulation that meets minimum code requirements: namely, one ton of cooling per 1,000 square feet. According to Blasnik, “Sizing an air conditioner using tons per square foot actually works pretty well,…