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High Velocity Cowls

The Health & Safety Executive (HSE) booklet HSG 258 now specifies in section 226:- Avoid rain caps or other devices that reduce upward vertical velocity. NEVER use the ‘Chinese hat’ or other devices that direct the discharge downward.

We can quickly and economically replace all existing ‘Chinese hat’ style rain caps with the now preferable Jet Cowl. This will ensure that your system is fully compliant to current HSE guidelines and performs to its maximum efficiency.

The Jet Cowl allows the discharged air to exit the stack in a vertical direction whilst still providing rain protection. This is unlike the older types of rain caps which defect the discharged air in a downward direction, possibly allowing re-entry to the building, contrary to HSG 258 section 220.

What do I need to know?

  • Ensure stack discharge penetrates the surrounding ‘boundary layer’ around the building
  • Stacks should discharge at least 3m above the highest point of the building
  • Exit velocities should be >15m/s or 1.5 times wind speed to minimise ‘downwash’
  • Chinese hat style rain caps must not be used
  • Keep exhaust points away from air intake points