English version machine-translated from the Vietnamese original — under review.
Author: NSCA R&D – Applications Engineering Department
Compiled from: Trane Engineers Newsletter Live (2008), "ASHRAE Standards 62.1 and 90.1 and VAV Systems"
Why Combine 62.1 and 90.1?
In the modern construction world, ASHRAE standards play an increasingly crucial role in HVAC system design. Among them:
- ASHRAE 62.1 requires the supply of a minimum amount of outdoor air to ensure indoor air quality (IAQ).
- ASHRAE 90.1 sets limits on energy consumption, including restricting simultaneous reheating or cooling – phenomena common in VAV (Variable Air Volume) systems.
Engineers often face challenges when trying to meet both standards at the same time. However, the Trane technical document has shown that, with proper system design and control, both objectives can be achieved effectively.
📚 Source: Trane Engineers Newsletter Live – ASHRAE Standards 62.1 and 90.1 and VAV Systems, 2008
Common Conflicts in Design
1. Demand-Controlled Ventilation (DCV)
- 62.1 allows DCV to be used to reduce outdoor air when a space is not fully occupied.
- 90.1 mandates DCV in areas larger than 500 ft² with a density > 40 people/1000 ft² (e.g. auditoriums, dining halls, large lobbies).
➡️ The design should use CO₂ sensors, time-of-day scheduling or occupancy sensors to adjust the outdoor airflow (Voz) accurately.
2. Humidity Control
62.1 requires relative humidity to be limited to no more than 65%.
90.1 prohibits simultaneous cooling and reheating, unless:
- The airflow is reduced to 50%,
- Or a recovered heat source ≥ 75% is used,
- Or a special dehumidification (desiccant) system is used.
➡️ Solution: Use Fan-powered VAV Boxes, DOAS (Dedicated Outdoor Air Systems), or precise supply air temperature control.
3. Limits on Simultaneous Cooling – Heating
- 90.1 only allows reheat if the supply airflow does not exceed 0.4 cfm/ft² or the minimum ventilation requirement of 62.1.
- If not handled properly, the design may violate the energy limits of 90.1 or fail to provide the minimum ventilation of 62.1.
➡️ The minimum airflow setting in the VAV box must be set correctly, avoiding a setting too low, which causes a shortage of outdoor air, or too high, which wastes energy.
Solution: Smart VAV Design, Optimised Performance
✅ Combine DCV + VRC:
- DCV: Adjusts the outdoor airflow for each zone.
- VRC (Ventilation Reset Control): Calculates the ventilation efficiency of the whole system to optimise the outdoor air intake (Vot).
✅ Use the 62MZCalc spreadsheet:
- Trane provides a spreadsheet to help check whether the whole VAV system meets the requirements of 62.1 & 90.1.
- It fully simulates airflow, zone distribution and system efficiency.
Conclusion
Combining ASHRAE 62.1 and 90.1 in HVAC system design is not only a legal and technical-standard requirement, but also helps optimise the operating efficiency and long-term energy costs of the building.
👉 Design notes:
- Do not rigidly reuse old VAV configurations.
- Apply flexible control strategies: DCV, CO₂ sensors, reheat using recovered heat, or DOAS.
- Make use of simulation tools to confirm that the design satisfies both IAQ and the Energy Code.
📘 References:
- ASHRAE Standard 62.1 – Ventilation for Acceptable Indoor Air Quality
- ASHRAE Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings
- Trane Engineers Newsletter Live (2008), "ASHRAE Standards 62.1 and 90.1 and VAV Systems"
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