Holding VPD with Continuous DX Modulation

The Challenge

In controlled-environment agriculture, vapor pressure deficit (VPD), not temperature alone, governs transpiration, CO2 uptake, and ultimately yield. Standard staged DX equipment at part load controls neither side of the VPD equation well: it overshoots on temperature and stops removing moisture during every off-cycle. A ±10 to 14°F discharge-air swing on every cycle moves leaf-surface VPD outside the target window, and each restart spikes space humidity as moisture re-evaporates off the coil across the canopy. Light-commercial splits in flower rooms can restart 20 to 30 times per hour, a duty that commonly shortens compressor life to 18 to 24 months.

The Solution

The installation keeps the standard split-system DX equipment and adds three elements working together. The APR‑E Valve is a refrigerant-side, closed-loop discharge-air controller: when discharge-air temperature deviates from setpoint, the control system repositions the valve to restore it, and the compressor keeps running at matched capacity instead of cycling. A Reliable Controls building automation system issues the command and trendlogs every point continuously, and Stratus Designs provided the HVAC and controls design, valve application engineering with Rawal Devices, commissioning, and remote monitoring. No specialty precision-cooling units were used.

The Results

From the facility's BAS trendlogs (Flower Room G2, 2026), annotated by the commissioning integrator:

• Discharge-air temperature held within ±0.88°F maximum deviation from setpoint under continuous modulation, versus ±10 to 14°F per cycle in uncontrolled on/off operation on comparable equipment
• Achieved VPD range of 0.128 kPa, versus 0.40 kPa uncontrolled
• Space relative humidity stability of approximately ±1.5%, versus ±6 to 8% spikes accompanying each compressor restart
• Through a complete lights-on ramp with a moving temperature setpoint (76 to 82°F), zone temperature tracked within about ±0.8°F typical, with a single worst deviation of 1.8°F during the 6°F setpoint ramp itself, and no restart spikes because there were no restarts

Note: ±0.88°F is the discharge-air deviation the APR‑E Valve directly controls, not the zone temperature. Zone tracking through the ramp is reported separately above. Data source: Reliable Controls BAS trendlogs from one licensed facility (identity withheld at operator request; verified by Stratus Designs).