ASHRAE (and DOE) Get More Specific About Economizer Requirements

By Chris Edmondson

It’s time we all start economizing – and no, we’re not talking about budget. 

ASHRAE 90.1 – 2010, which officially becomes part of U.S. commercial building codes in less than one year dictates that henceforth all cooling systems shall include either an air or water economizer per Sections through  The only areas in the U.S. where this will not apply are the southernmost tip of Florida and south/southwest portions of Arizona where the respective climates do not favor economizing. 

In many areas of the country, particularly the humid southeast, we have to have an economizer to meet this new building code requirement.  A waterside economizer is frequently the logical choice because of the high humidity.We’ve looked at the new ASHRAE requirements for waterside economizing and were struck by the implications.  There is no doubt that this will dramatically impact cooling design in 3 significant ways.

Design Capacity.  Water economizer systems will have to be capable of cooling supply air by indirect evaporation and provide up to 100% of the expected system cooling load whenever outdoor air temperatures are 50oF dry bulb/45oF wet bulb or below.  There are certain exceptions to this new rule, such as systems serving computer rooms, however the main “take home” is that for the first time ever building codes will actually dictate economizer sizing practices.  Simply put, engineers will have to size waterside economizers to satisfy the building load whenever the outside temperature is 50°F Dry Bulb and 45°F or lower.

Integrated Economizer Control.  Economizer system control must be integrated with that of the mechanical cooling system and it must be capable of providing partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load.  In other words, the economizer and mechanical cooling system must be able to work concurrently, effectively sharing the building cooling load whenever possible.

 This presents certain piping challenges since economizer heat exchangers need cold water to be effective – several degrees colder than the condenser water minimum temperature requirement (typically 65-75 °F) of most chillers.  So provisions must be made to isolate the cold water supply to the chillers from that of the economizer plate and frame heat exchanger.  Additionally, engineers must design the system to keep the minimum load requiredon chillers to prevent short cycling.  This becomes complicated when the total GPM required by the system may be greater than the flow rate of a single chiller.  This may lead engineers to run multiple chillers at very low loads to maintain supply chilled water temperature; however, this is not a suitable option because the economizer has not reduced the system flow.

Economizer in Series or Parallel?  The question then becomes, “What is the best way to pipe the cooling towers, plate & frame heat exchanger, and chillers?” In most cases, for the very reasons outlined above, we believe that piping the plate & frame heat exchanger in series with the other system components is the best choice.  In most cases the best solution is to pipe the economizer to one dedicated cooling tower.  (See photo)  In this configuration chillers and economizer can run with two separate and appropriate supply temperatures.  This allows facilities to turn on the economizer cycle earlier than what would typically be possible, therefore improving the payback.

If you want to see your states current adoption status visit