Control Apparatus № 10
Economizer Map
Each hour of the year as a point: outdoor air enthalpy
on X, return air enthalpy on Y. The 45° line is the
boundary. Below it, outdoor air is colder than return — free
cooling is possible.
The green zone is full free cooling: 100% OA can meet the
load with mechanical cooling off. The gold zone is
partial economizing — OA pre-cools the return before mechanical
cooling finishes the job.
In Practice
Houston is famously bad for economizing — the dewpoint stays high
even when temps drop. The chart reveals exactly when free cooling
is viable: winter overnight hours and shoulder-season
early mornings. Together they represent ~1,500 hours/year of
no-cost cooling at most properties.
