It is well-known and common practice for HVAC engineers to overdesign or apply a fudge factor to their cooling load calculations in order to size a cooling system. A Steven Winters Associates analyst looked at the load calculations, the installed cooling system size, and actual performance data for a number of buildings. The maximum cooling load calculated was compared to the actual peak cooling system load for the year. Also analyzed was the cost to operate the cooling system.
The results showed that many buildings were way overdesigned, some by a factor of two or more. Furthermore, even on buildings where the system was properly sized, the system operated at far less than full load for 99% of the year. That may sound good, but the consequences were noted to be higher electrical costs to cool the space and lower efficiency. In one building, the cost was estimated to be six times more than if properly sized. Much of this is due to low part-load efficiencies on some types of cooling equipment.
The bottom line conclusion is that it is unnecessary to overdesign systems or to apply fudge factors to cooling loads – just use ASHRAE standards. You’ll keep everyone cool, especially the accountant that has to pay the electrical bill every month. Steve Terry, DTC HVAC & Refrigeration Instructor
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