Rule of Thumb HVAC Sizing

Frequently designers and contractors "guestimate" the size of HVAC units by figuring 1 Ton (12,000 Btuh) of air conditioning will cover 400 square feet (Sq-Ft) of building area.  This ratio is vastly overused, and often leads to undersized HVAC units in our climate zone (the Sacramento Valley).

ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) has put together a table using national average data showing the Sq-Ft/Ton as follows:

Sq-Ft/Ton	High	Average	Low
Residential	600	500	380
Office	360	280	190

However, in the Sacramento Area we are in a "desert" climate zone with design temperatures around 101 degrees and daily temperature swings around 35 degrees.  When detailed heating and cooling load calculations are performed on a building in our area, the Sq-Ft/Ton usually falls around the "Average" column in the above table for Office buildings, and in the "Low" column for Residential buildings.

Factors to consider when figuring the Sq-FT/Ton ratio include:

q Climate conditions (design temperatures).

q Expansive use of glass – particularly in the South and West orientations.

q High ceilings – increasing the conditioned volume of the space.

q Outside air requirements – especially important in high occupant load areas like conference rooms and classrooms. Even residential structures are starting to take this into consideration.

q Heat generating equipment – e.g. computers, copiers, laser printers, big screen TV’s, etc.

q Lighting – especially the extensive use of incandescent and metal halide lights. Fluorescent lights are more efficient and “burn” cooler – however, their ballasts generate a fair amount of heat.

It is always best to generate a detailed heating and cooling load calculation for the building or space in question. However, if you do feel the need to use a Sq-Ft/Ton average to estimate the heating and cooling loads, be sure to reference a reputable industry standard (such as ASHRAE) using conservative values.