Table of Contents
- Air changes per hour is the volumetric airflow divided by the room volume, expressed per hour: ACH = (volumetric_flow_rate / room_volume) x 3600 An ACH of 6 means the room air is theoretically replaced six times in an hour.
- Classrooms: about 4-6 ACH (see school ventilation) Laboratories and fume hoods: 6-12 ACH (lab recovery) Operating theatres: 20+ ACH (hospital recovery) Offices: 2-4 ACH
- Higher ACH means more outdoor air to condition and more exhaust energy to reclaim. Spaces with high ACH and high hours of operation are exactly where an AHU-integrated recovery core pays back fastest, because the recovered energy scales with the airflow.
- Does high ACH mean better air quality? Up to a point; beyond code minimums, filtration and distribution often matter more than raw rate. Is ACH the same as outdoor-air rate? Not always - ACH can include recirculated air; the recovery benefit tracks the outdoor fraction specifically. Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team - WhatsApp +86 157 5335 5505, or email kuns913@gmail.com. Frequently Asked Questions What is ACH in ventilation? Air changes per hour is the volumetric outdoor-air supply divided by the room volume, i.e. how many times the room air is replaced each hour. Why does higher ACH increase recovery value? More outdoor air means more conditioning load to recover; at high ACH the energy carried by the exhaust is large, so even modest effectiveness saves substantial heating or cooling energy.
Air changes per hour is the volumetric airflow divided by the room volume, expressed per hour:
ACH = (volumetric_flow_rate / room_volume) x 3600
An ACH of 6 means the room air is theoretically replaced six times in an hour.
- Classrooms: about 4-6 ACH (see school ventilation)
- Laboratories and fume hoods: 6-12 ACH (lab recovery)
- Operating theatres: 20+ ACH (hospital recovery)
- Offices: 2-4 ACH
Higher ACH means more outdoor air to condition and more exhaust energy to reclaim. Spaces with high ACH and high hours of operation are exactly where an AHU-integrated recovery core pays back fastest, because the recovered energy scales with the airflow.
Does high ACH mean better air quality? Up to a point; beyond code minimums, filtration and distribution often matter more than raw rate.
Is ACH the same as outdoor-air rate? Not always - ACH can include recirculated air; the recovery benefit tracks the outdoor fraction specifically.
Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team - WhatsApp +86 157 5335 5505, or email kuns913@gmail.com.
Frequently Asked Questions
What is ACH in ventilation?
Air changes per hour is the volumetric outdoor-air supply divided by the room volume, i.e. how many times the room air is replaced each hour.
Why does higher ACH increase recovery value?
More outdoor air means more conditioning load to recover; at high ACH the energy carried by the exhaust is large, so even modest effectiveness saves substantial heating or cooling energy.