Rule of Thumb HVAC
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:
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
to consider when figuring the Sq-FT/Ton ratio include:
Climate conditions (design temperatures).
Expansive use of glass particularly in the
South and West orientations.
High ceilings increasing the conditioned
volume of the space.
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.
Heat generating equipment e.g. computers,
copiers, laser printers, big screen TVs, etc.
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.
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.