How Contractors Really Size Air Conditioning Systems

Secondary Author(s)

Parker, Danny; Klongerbo, Jon; Sonne, Jeffrey; Cummings, Jo Ellen

Report Number





HVAC; Buildings; Cooling


This paper presents results from 489 of the 5559 Florida air conditioning contractors surveyed (an 8.5% response rate) regarding equipment sizing methods in new residences. Air conditioning sizing is accomplished by using ACCA's Manual-J procedure by 33% of the respondents, software by 34.4% of the respondents, square-footage by 24.2% and other estimate procedures by about 8.4%. Those using square-footage estimates varied from 350 square-feet- per-ton to 700 square-feet-per-ton. Over a third of respondents indicated oversizing intentionally on some jobs, in order to avoid complaints, accommodate future expansions, enable quicker cooling down of homes, and to allow for lower cooling set points by homeowners.The contractors' most popular method of sizing for air distribution was an estimate based on square footage. The variation was considerable, with responses of 0.8 cfm/ft2 and 1.5 cfm/ft2 both being common. One in four respondents indicated using methods that lead to inaccurate results. Some of the respondents chided others that size on square footage while others were critical of the Manual J method. A few respondents indicated their dissatisfaction with contractors who oversize systems, while others protested that undersizing is often done in order to achieve the low bid. Quoted comments of respondents are included throughout the paper.

Date Published



Buildings - Cooling; Buildings - HVAC

This document is currently not available here.