Many facilities use energy recovery ventilators (ERVs) to provide proper ventilation for their buildings. These units can provide both increased ventilation via 100% outdoor air and energy savings by transferring heat from the return airstream to the supply airstream in the winter or vice versa in the summer. There are many ways of transferring heat including fixed plate heat exchangers, energy recovery wheels, coil energy recovery, and heat pipes. Any ERV that includes a crossing of airstreams for heat recovery, such as fixed plate heat exchangers and energy recovery wheels, are prone to some degree of leakage between airstreams.
Reducing contamination becomes particularly important during an airborne infectious disease outbreak such as the COVID-19 pandemic. Although ASHRAE guidance indicates that well-designed and well-maintained air-to-air energy recovery systems should remain operating during the COVID-19 pandemic to ensure as much outdoor air as practical, there are precautions that need to be taken based on the type of system in place.
Analyze the type of ERV system; is it 100% outdoor air or is there intentional return air back to the unit? If there is intentional return air, try to convert to 100% outdoor air if possible. The return air damper(s) within the unit should be closed and temporarily sealed to prevent leakage. The system will need to be rebalanced so that the static pressure differential remains correct for the exchanger type.
If the system is 100% outdoor air, the strategy will be based on both the type of heat recovery being used and what the fan configuration is. Plate exchangers have lower exhaust air transfer rates than energy recovery wheels. Energy recovery wheel units will need to be reviewed so that the static pressure in the supply side of the exchanger is at least 0.5” greater than the static pressure in the return side entering the wheel. This will ensure that any seal leakage will move from the clean to the dirty airstream and any carryover will be insignificant.
Fan configuration of the energy recovery wheel system also plays a part in the direction of exhaust air transfer within the unit. ERVs with both a supply fan downstream and a return fan upstream of the wheel should not be used since the seal leakage will always go from the dirty to the clean airstream. It is advised that these units should be shut down and alternate temporary means of ventilation should be provided until a properly configured ERV can be installed.
Almost all ERV systems installed will have a fan on the exhaust air stream and a fan on either the outdoor air or supply air streams and both can be effective in limiting exhaust air transfer provided the minimum pressure differential exists between the return and supply airstreams.
When the ventilation system design includes any air recirculation, enhanced filtration of the supply air entering the building is important whether an ERV is in place or not and should be provided, if practical, for all system configurations.
Deanna Adkison, EIT