Equivalent ventilation is a key metric for evaluating different ventilation designs. The central idea behind equivalent ventilation is to use a standard continuous-ventilation rate as a basis for comparison with intermittent-ventilation systems. Intermittent ventilation should produce the same, or lower, pollutant exposure compared to the standard continuous ventilation rate. Hence, the alternative would be “equivalent” to the standard or baseline.
Implementing Equivalent Ventilation
The only current standard promoting this approach is ASHRAE Standard 62.2-2016, which provides the baseline for “equivalency”. LBNL developed this equivalency metric, based on assumptions from this ASHRAE Standard, such as the idea of a “generic contaminant” emitted at a constant rate. Maximum concentrations of this generic contaminant are a theoretical value that continuous-ventilation applications of the ASHRAE 62.2 standard will achieve.
An equivalent ventilation system is one that produces the same or lower exposure to this generic contaminant averaged over a year compared to the ASHRAE baseline. Researchers and designers compare time-controlled ventilation rates with the continuous ventilation rate from the ASHRAE standard. Researchers assign a value of one (1) to the contaminant concentration associated with this ASHRAE baseline. Intermittent ventilation must then produce a value of 1 or less to comply with the standard.
Equivalent Ventilation: Application
Like most residential ventilation requirements, equivalent ventilation only applies to chronic pollutant exposures. However, LBNL scientists adapted the calculation procedure to consider peak contaminant levels and acute exposures. These adaptations, evaluated only during times of occupancy, are particularly useful for ventilation control strategies that are occupancy-based. One important limit researchers set is that a building should never have 5 times equivalency and in practice should remain below 2.5 times equivalency, even if the building is unoccupied.
This equivalency approach can also be used with time-varying emission rates: during unoccupied times for example. SVACH is investigating this approach. Although this equivalency metric governs ventilation rather than IAQ directly, these principles and adaptations will also be useful for direct IAQ metrics.
Further Reading
“Equivalence in Ventilation and Indoor Air Quality.” HVAC&R Research (2011).