Drives in action: fire safety in stairwell pressurization

This Fun Fact Friday is on an HVACR life safety topic – stairwell pressurization. Did you know variable speed drives (VSDs) play a very important role in stairwell pressurization?

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Background: In the event of a building fire, a fan blows fresh (outside) air into the stairwell, keeping the stairwell positively pressured. That way when occupants open the doors to the stairwell, the smoke doesn’t easily infiltrate into the stairwell – thus keeping the air in the stairwell relatively smoke-free and providing a safe escape path for occupants.

How does the drive come into play? The drive adjusts the fan speed to maintain pressure. Depending on how many doors are open, the fan needs to speed up or slow down. The more doors that are open at a given time, the faster the fan must run to maintain pressure.

Here are a couple of drive-related tips:

Leverage the Fireman’s Override mode found in drives. That way if someone had pressed “Off” on the Hand-Off-Auto (HOA) control panel, or there was some minor alarm that may have typically prevented drive operation, activation of Override mode will get the drive running that fan when it’s go-time.
Wire the pressure sensor to the drive, and leverage the drive’s internal Proportional Integral (PI) loop to maintain pressure. Note: be sure to verify this is acceptable with local safety codes, as code requirements vary based on the country.
Don’t use a VSD with bypass (full-speed-only operation). Over-pressurizing a stairwell can have deadly consequences, as exit doors open INTO a stairwell – and too much pressure makes it extremely difficult to open a door. Note: mechanical pressure relief venting would negate that issue.

Taller buildings may have multiple pressure sensors in the stairwell. The ACH580 drive accepts multiple sensor inputs and here is a YouTube video that walks you through how to commission that scenario.