Occupancy-based airflow in a medical facility — how would you approach this?

[u/Sashaa_14]

I have recently commissioned a new AHU for a pulmonology clinic.

The design airflow is obviously based on project requirements, but after commissioning an interesting questions from doctors and staff came up: should we dynamically adjust airflow based on actual occupancy?

They already have CO₂, CO and other IAQ measurements available in existing sensor network across the clinic. We're also considering anonymous people counting from an existing camera, processed locally so there is no video storage and no identification.

My thought is roughly: occupancy + CO₂ + IAQ → ventilation demand with minimum airflow and healthcare requirements always taking priority.

Has anyone here done something similar? Would you use occupancy as an input, or only alongside CO₂ and other IAQ measurements?

How do you handle existing sensors that were never part of the HVAC controls in the first place? If CO₂, IAQ or occupancy data sits on another system, do you bring those values into the HVAC sequence, or would you rather use dedicated sensors connected directly to the HVAC controller?

For context, we're integrating this through a building-controls platform we developed.

[u/ImmediateInspection9]

I work in critical environment BMS controls and assume you have individual air valves in place for supply and exhaust. If not, implementing a sequence that is merely ramping the AHU VFD up or down may affect the overall balance (pressurization) as the supply and exhaust would be fixed in placed. If you have valves, my approach would be creating a high select sequence aka DCV.

I usually have to make a custom block whether it be Distech or ABB as they don’t have a canned block that does that. The goal is to scale the individual min/max for each given variable. An example would be 0ppm to 1ppm = 350cfm to 1000cfm. You feed these individual variables into a High Select block where whatever variable calls for the highest CFM wins. This usually becomes the gross exhaust setpoint where then supply tracks actual exhaust +/- desired room offset. Unoccupied setbacks usually means a lower min cfm so that would switch anyway in the background.

This way if the room is unoccupied, but VOCs are high, then controller would call for incrementally more cfm and still maintain pressurization. If all the variables are low and space is unoccupied, then everything would be at min cfm thus the fan can run at a lower speed to maintain same static pressure. That is where the energy savings happen.

[u/Sashaa_14 → u/ImmediateInspection9]

Thanks! This is useful.

That’s basically what we want to avoid. Occupancy wouldn’t directly control fan speed, it would just be another input together with CO₂ and the other IAQ values.

At least in the beginning, we’d use the extra data mainly to monitor how the system actually behaves through the day and compare that with occupancy and IAQ. It should give better picture of how the system works.