Purpose of ASHRAE 62.1
ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality, sets the minimum ventilation rates and air-cleaning requirements that protect health in commercial and institutional buildings. It is the document that decides how much outdoor air a system must bring in — which in turn drives the size of the heating/cooling load and the value of heat recovery.
Two Calculation Paths
| Procedure | Use |
|---|---|
| Ventilation Rate Procedure (VRP) | Sum of per-person + per-area outdoor air |
| Indoor Air Quality Procedure (IAQP) | Source control + contaminant modelling |
The VRP is the common route: required outdoor air = (people × Rp) + (area × Ra), then adjusted by a breathing-zone mixing factor (Ez) to size the system supply.
Why 62.1 Feeds Heat Recovery
- Higher occupancy or denser programming → more outdoor air → larger ventilation load.
- That load is exactly what an energy recovery wheel or plate exchanger offsets.
- In cold climates, 62.1's minimum OA can make preheating uneconomical without recovery.
Outdoor Air Fraction and the 90.1 Trigger
The outdoor-air fraction computed under 62.1 is the same fraction ASHRAE 90.1 uses to decide whether recovery is mandatory. So 62.1 sets the quantity, and 90.1 uses it to set the recovery duty. The two standards are linked by that one number.
Demand-Controlled Ventilation
62.1 permits CO₂-based demand control in many spaces. As occupancy falls, outdoor air modulates down — but the recovery device keeps saving energy across the whole turn-down range, which favours devices with stable effectiveness at part load (plate exchangers, well-purged wheels).
Practical Design Sequence
- Determine space types and occupancies.
- Apply the VRP to get required outdoor air.
- Compute the outdoor-air fraction.
- If 90.1 triggers recovery, select a device with certified effectiveness.
- Verify the system still meets 62.1 after accounting for recovery pressure drop.
Bottom Line
ASHRAE 62.1 is the "how much fresh air" rule. Pair it with 90.1 (the "recover it" rule) and you have the complete ventilation-energy case for specifying heat recovery in virtually any commercial building.
Worked Example: Outdoor-Air Fraction Drives Recovery
A lab has a design supply of 8,000 cfm with 6,000 cfm of outdoor air (OA fraction = 75%). Under ASHRAE 62.1 that is the required ventilation. Because 8,000 cfm > 7,500 cfm and the OA fraction (75%) is between 50% and 70%? No — it exceeds 70%, so it also meets the stricter >5,000 cfm / >70% trigger. Either way, 90.1 mandates recovery, and the 62.1-calculated outdoor air is the exact input that proves it.
Frequently Asked Questions
What is the difference between 62.1 and 90.1? 62.1 sets how much outdoor air is needed for health; 90.1 decides when you must recover energy from it.
Can I use demand control? Yes, CO₂-based control is permitted and recovery still helps at part load.
Does 62.1 require heat recovery? No — that is 90.1's job; 62.1 only sets the ventilation quantity.
What is Rp and Ra? Rp is the per-person outdoor-air rate and Ra the per-area rate; their sum sizes the system outdoor air.
In practice: treat ASHRAE 62.1 as the ventilation-quantity rule and ASHRAE 90.1 as the recovery-duty rule; together they define when, and how much, energy a commercial HVAC system must capture from its own exhaust air.
Coordinating 62.1 with the Energy Model
ASHRAE 62.1 belongs in the same workflow as the energy model, not in a separate silo. Once the ventilation rate procedure gives the required outdoor air and the outdoor-air fraction, feed both into the building energy simulation. The fraction is what triggers recovery under 90.1, and the quantity is what the recovery device must condition. Where the model shows recovery barely paying back, check whether demand-controlled ventilation can lower the average outdoor air without breaking 62.1; where it pays back strongly, document the saving for the code or incentive submission. Also confirm that adding recovery does not push supply velocities or pressures past what 62.1's mixing factors assume. Keeping 62.1, 90.1, and the energy model in one loop is the difference between a compliant design and a design that actually saves the energy it claims.