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HVAC Engineer Says Codes Are Pushing Building Owners Toward Occupied Standby—but Comfort Risk and Recovery Time Still Set the Ceiling
ASHRAE and energy codes are making it easier for building owners to cut ventilation during the workday—but engineers say that doesn’t mean every building should.
At NexusCon, John Vilani, an HVAC engineer with Grumman/Butkus, laid out how ASHRAE 62.1, Guideline 36, and IECC C403 now explicitly allow outdoor air to be reduced to zero during occupied standby when no occupants are detected. The intent: dense urban buildings shouldn’t ventilate as if they’re fully occupied all day.
“That means between 8 a.m. and 5 p.m., you’re allowed to slack temperature setpoints and actually go to zero ventilation air,” Vilani said. “That’s a new thing.”
Using a Manhattan office tower as an example, Vilani walked through conference-room math showing how outdoor air can be cut by 74% under partial standby—or 100% when a space is truly empty. In theory, it’s a straightforward energy win.
In practice, it’s messier.
“When you deploy occupied standby, you need to think about recovery rate,” Vilani said. “If 30 people suddenly walk into that conference room, you may not be able to do the setback.”
The building’s mixed-use zones—private offices, open offices, and large conference rooms sharing common air handlers—forced engineers to tune zone-level sequences carefully. Over-aggressive setbacks risk slow thermal recovery and visible comfort failures, especially in high-profile spaces.
“There’s still some art to the science,” Vilani said. “You have to analyze the data to see where you can get away with it.”
The takeaway: codes may enable occupied standby, but engineers—not formulas—still set the ceiling.
Learn more:
- Watch the full presentation from NexusCon 2025
- Sign up for the Nexus Labs newsletter to get five similar stories for owners each Wednesday:
ASHRAE and energy codes are making it easier for building owners to cut ventilation during the workday—but engineers say that doesn’t mean every building should.
At NexusCon, John Vilani, an HVAC engineer with Grumman/Butkus, laid out how ASHRAE 62.1, Guideline 36, and IECC C403 now explicitly allow outdoor air to be reduced to zero during occupied standby when no occupants are detected. The intent: dense urban buildings shouldn’t ventilate as if they’re fully occupied all day.
“That means between 8 a.m. and 5 p.m., you’re allowed to slack temperature setpoints and actually go to zero ventilation air,” Vilani said. “That’s a new thing.”
Using a Manhattan office tower as an example, Vilani walked through conference-room math showing how outdoor air can be cut by 74% under partial standby—or 100% when a space is truly empty. In theory, it’s a straightforward energy win.
In practice, it’s messier.
“When you deploy occupied standby, you need to think about recovery rate,” Vilani said. “If 30 people suddenly walk into that conference room, you may not be able to do the setback.”
The building’s mixed-use zones—private offices, open offices, and large conference rooms sharing common air handlers—forced engineers to tune zone-level sequences carefully. Over-aggressive setbacks risk slow thermal recovery and visible comfort failures, especially in high-profile spaces.
“There’s still some art to the science,” Vilani said. “You have to analyze the data to see where you can get away with it.”
The takeaway: codes may enable occupied standby, but engineers—not formulas—still set the ceiling.
Learn more:
- Watch the full presentation from NexusCon 2025
- Sign up for the Nexus Labs newsletter to get five similar stories for owners each Wednesday:
This is a great piece!
I agree.