HRV Tandem Hood

Category: Building Codes

Issue/Problem: Spacing of Exterior Vents for Heat Recovery Ventilators (HRV)

Analysis:        
Clarification of the proper spacing of the exterior supply and exhaust hoods or vents for HRV’s.

Since being introduced in the mid 1970’s, HRV and ERV’s have been installed in over a million North American homes. From the beginning there was debate over a suitable location for the supply air intake to ensure a safe, healthy and reliable source of fresh air. Considerations included:

• The formation of hoar frost on the intake hood rodent screen when exhaust air saturated with moisture was re-introduced in very cold winter weather.
• Contamination from sources such as clothes dryers, central vacuums, downdraft range hoods, high efficiency gas furnaces, gas meters, driveways and any other pollutant sources that might be introduced to the house via the fresh air intake.
• The blockage of the supply and exhaust hoods by leaves, grass clippings and snow.

Through observation of installed systems it became standard practice by manufacturers to recommend the spacing of hoods be six feet apart as this was known to virtually eliminate problems with frosting. Similarly, manufacturers found that hoods placed 18” above grade avoided fouling from snow, leaves and grass. The early standards for installation of HRV/ERV’s in programs such as the R-2000 program, simply adopted these early common industry practices. However, as more appliances competed for access to the outside through the rim joist of the house, it became clear that restrictions and standards were needed on the placement of hoods generally and specifically the fresh air inlet.

Code:              

• There are now clear requirements in the National and Ontario Building Codes and the CSA/CAN F326 Residential Mechanical Ventilation Standard that make a distinction between general exhaust air and truly contaminated air such as gas appliance vents and oil fill pipes. 
• 9.32.3.12. Outdoor Intake and Exhaust Openings
  • Separate air intake and exhaut outlet openings, when located on the same wall or roof, shall be installed so as to avoid contamination of the ventilation air by the exhaust air.
  • Intake openings shall be located so as to avoid contamination of the ventilation air from other local sources such as automobile exhausts and exhaust from adjacent buildings.
  • The distance from the bottom of an air intake opening to finished ground level or to any nearer and lower permanent horizontal surface shall be not less than 450 mm or the depth of expected snow accumulation, whichever is greater.
  • The distance separating air intakes from building envelope penetrations that are potential sources of contaminants, such as gas vents or oil fill pipes, shall be not less than 900 mm.


• In Section 9.32 of the NBC and OBC there is no specific measurement for the distance between exhaust and supply vents or from supply vents to driveways or other exhaust vents, although there is a clear requirement of 900 mm from contaminant sources such as oil fill pipes and gas vents.

• There are very similar requirements in the CSA/F326 Standard referenced in the NBC & OBC as follows:

• Section 8.8.3 makes the distinction for contaminants other than general exhaust as follows:

“The distance separating outdoor air intakes from building penetrations that are potential sources of contaminants, such as gas vents or oil fill pipes ….. shall be no less than 900 mm.”

Section 8.8.1 of CSA F326 Standard reads:
"Separate air intake and exhaust openings, when located on the same wall or roof, shall be installed so as to prevent contamination of the ventilation air by the exhaust air"

• And then specifically reads:

“Concentric or immediately adjacent air intake and exhaust openings shall be constructed and installed so as to prevent cross-contamination"

• Section 5.6. of the CSA F326 Standard then gives some guidance as to an acceptable or practical level of cross-contamination between air streams as follows:

“… leakage from the exhaust air stream to any supply air stream shall not constitute more than 15% of the exhaust airflow provided by the packaged ventilator.”

• So, in fact, F326 has planned for concentric or combined hoods such as hoods like the Venmar / vanEE TandemÔ Dual Hood. These types of hoods help resolve the ever more difficult issue of finding suitable locations for the numerous vents needed in houses and the availability of suitable wall space in smaller, more affordable homes such as town homes.

Conclusion:
While the traditional “best practice” distance between individual intake and exhaust hoods recommended by manufacturers is still six feet, codes and standards have progressed to be more flexible and accommodating.

• The NBC, the OBC and the CSA F326 Residential Mechanical Ventilation Standard do not place any specific restriction on the distance between exhaust and supply vents of HRV/ERVs.
• There are limitations imposed on the distance between fresh air intakes and contaminant sources such as oil fill pipes and gas vents.
• The CSA Standard makes specific reference to the allowance of concentric or adjacent air intake and exhaust openings.
• Venmar has developed the Tandem Dual Hood with specific design features to prevent cross-contamination such as a high velocity exhaust and a low velocity intake.
• Test results demonstrate less than 5% cross-contamination in even very high head winds while the CSA F326 Standard sets a limit of no more than 15%.
• Thus the Venmar Tandem Dual Hood is in compliance with the Ontario Building Code and CSA F326 requirements.