Outdoor Air Correction Factor (OACF)

Outdoor Air Correction Factor - Fan ArrangementsFor those unfamiliar with the concept, the Outdoor Air Correction Factor (OACF) is the additional amount of outdoor air required in order to deliver the required amount of fresh air to the space for the purposes of dilution. It is defined by AHRI 1060 as the ratio of outdoor air to supply air and affects all air-to-air energy recovery devices to a greater or lesser degree depending on a number of factors, including the physical properties of how the exchanger is constructed and the differential pressures between airstreams.

It is a simple concept that has profound implications when sizing and selecting energy recovery equipment. If OACF is ignored in the selection process, then the prescribed amount of fresh air will not be supplied to the space and poor indoor air quality may result—in addition to creating numerous potential issues with balancing and commissioning. If OACF is accounted for, and a device with high values for OACF (i.e. greater than 1.10) is selected, there are significant impacts to the energy consumption of the equipment that will not show up if the designer is only looking at effectiveness values. Equipment aspect ratios may need to be increased to account for higher face velocities through filters and preheaters; fans and motors may need to be upsized on both the supply and return airstreams depending on the location of these components within the cabinet. In the case of multiple exchangers in series, even coil face areas or the second energy recovery device may need to be upsized to account for the extra air.

Unfortunately, many people focus on the Exhaust Air Transfer Ratio (EATR) when considering energy recovery technologies for different applications, as this is a common concept and has long been promoted in the industry as being extremely critical. EATR has historically been expressed as carryover or cross-contamination.

While some critical applications may require very low (or 0%) EATR, such as laboratory exhaust or explosion proof applications, it is important to select the correct technology that can meet these requirements effectively and efficiently. This is particularly important, because there is a relationship between EATR and OACF such that as EATR decreases, OACF increases. Therefore, maintaining artificially low EATR values when they are not required can have a significant energy penalty on the overall energy consumption of the building.

Outdoor Air Correction Factor and Exhaust Air Transfer Ratio
Estimated Annual Increase in Fan Operating Cost

OACF = 1.02
OACF = 1.08
OACF = 1.15
San Francisco, CA ($0.13 kWh)
$210
$839
$1,573
Miami, FL ($0.11/kWh)
$177
$710
 $1,331
Burlington, VT ($0.12/kWh)
$194
$775
 $1,452
Seattle, WA ($0.06/kWh)
$97
$387
 $726

For applications with Class 2 air and Class 3 air, the requirements are a maximum of 10% and 5% EATR respectively. With those goals in mind, Venmar CES has developed a high performance, high pressure contact seal to minimize the amount of additional outdoor air required when using an air-to-air energy recovery device, while maintaining target levels of EATR to conform to the recirculation limitations for different classes of air as outlined in ASHRAE Standard 62.1. Limiting (or in some cases eliminating) this additional amount of outside air, was an important problem to solve as it has a large impact on the total amount of energy consumed by a typical dedicated outdoor air system.