Title 24 Gives DCV Some Validation

11/01/2005


In the world of HVAC, demand-controlled ventilation (DCV) systems aren't in high demand. But as California's Title 24 kicks into gear that soon may change.

According to the California Energy Commission's Public Interest Energy Research (PIER) program, changes last month to Title 24, California's energy-efficiency standard, require DCV in all spaces where an HVAC single-zone system includes an economizer and where there is an occupant density greater than or equal to 25 people per 1,000 sq. ft., with some exceptions.

Additionally, the new code requires acceptance tests to ensure that the technology is properly installed.

And there is more good news for DCV. Research conducted by PIER, in association with Purdue University and the National Institute of Standards and Technology, has helped shed some light on DCV performance. Using a computer simulation tool and field tests, the three entities have created some data that helps determine the optimal conditions for DCV. The study also provides information spelling out the economic benefits of the technology, identifies potential problems and creates a set of design recommendations.

DCV systems, for the record, attempt to save energy by monitoring building occupancy to measure CO 2 levels and regulate the amount of outside air drawn in for ventilation. Part of the reason that the technology hasn't seen wide use to date is a lack of sufficient design guidelines or performance documentation to accurately determine its cost-effectiveness.

PIER, Purdue and NIST, however, tested several building types, coupling DCV systems with economizers. These included a small office building, a sit-down restaurant, a retail store and multiple school spaces, which consisted of a class wing, auditorium and library. In each case, the DCV-plus-economizer system rendered greater savings than an economizer-only system, and payback periods of two years were typical.

Field tests conducted in coastal and inland climate zones in California found that inland buildings with low occupancy rates and high ventilation loads saw the highest savings, while buildings with high occupancy rates that varied only a little didn't experience significant savings.

Some of the study's conclusions include:

  • The greatest savings and shortest payback periods occurred for buildings with variable and unpredictable occupancy levels, with high occupant densities at peak occupancy.

  • Savings vary with climate. In California, greater savings occurred in hot, inland climates than in mild, coastal climates. Potential savings from DCV are greatest in cold climates where heating dominates.

The PIER/Purdue/NIST study recommends the following design practices:

  • In buildings with an economizer cycle, allow the economizer to override the DCV system at times when the additional ventilation would provide "free" cooling.

  • Select DCV systems that are able to increase outdoor air intake before the building opens in the morning to deal with concentrations of contaminants that may build up overnight.

  • Always calibrate and maintain sensors according to manufacturer recommendations.

  • Avoid placing CO 2 sensors for ventilation control near doors, windows, air intakes or exhausts or occupants. Do not use a single sensor located in a common return to control ventilation rates for multiple spaces with different occupancies.

As awareness of the benefits of DCV grows, it is expected that use will also grow.





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