A Power Exhaust is an active, motorized fan system designed to remove return air from the building at controlled volumes. Unlike passive barometric dampers, Power Exhaust systems:

• Actively move air out of the building
• Respond to changes in outside air intake
• Prevent excessive positive building pressure
• Help maintain balanced indoor conditions

When installed properly, a Power Exhaust system supports optimal indoor comfort and energy performance.
However, improper installation or poor setup can create negative pressure, which can draw in unconditioned air through leaky windows, doors, and wall penetrations—causing drafts, hot spots, or humidity problems.

Constant Volume Power Exhaust

A Constant Volume Power Exhaust operates at a fixed speed. It is typically activated when the economizer’s outside air damper reaches a certain position.
Key points:

• Activation is linked to the economizer damper position
• Adjustment is set in the field
• Skilled air balancing is recommended
• Excess air is removed at one consistent rate

While effective in many situations, constant-speed operation may not address varying building conditions or multiple airflow influences.

Modulating Power Exhaust

A Modulating Power Exhaust provides a more precise and responsive solution.
This system uses:

• A pressure transducer (to measure indoor vs. outdoor pressure)
• A Variable Frequency Drive (VFD) (to control motor speed)

The transducer continuously monitors pressure differences and sends signals to the VFD, which adjusts the Power Exhaust fan speed as needed.
Modulating Power Exhaust systems:

• Increase fan speed when building pressure rises
• Reduce fan speed when pressure stabilizes
• Improve energy efficiency
• Maintain optimal indoor conditions under varying loads
• Offer superior accuracy compared to fixed-speed systems

This setup is ideal for buildings with fluctuating occupancy levels, multiple exhaust fans, or advanced ventilation strategies.

Final Thoughts

Properly balancing outside air intake and exhaust airflow is essential for maintaining indoor air quality and occupant comfort. Power Exhaust systems—especially when paired with economizers—help prevent pressurization issues and maintain safe, consistent ventilation.
Whether reviewing specifications, evaluating retrofit needs, or solving building pressure issues, HVAC professionals must consider:

• Outside air requirements
• Building pressure goals
• Equipment compatibility
• Airflow control strategies

MicroMetl remains committed to supporting industry professionals with guidance and technical insight related to rooftop ventilation, economizers, and airflow management.