An air handling unit (AHU) control sequence coordinates the fan, the cooling and heating coils, and the outdoor-air dampers to deliver conditioned air at the right temperature and pressure. The core loops are supply-air-temperature control, duct-static-pressure control (with reset), and economizer/mixed-air control — wrapped in safeties that protect the equipment. In a VAV system, the AHU works in concert with the VAV boxes it feeds.
An AHU takes return air, mixes in outdoor air for ventilation, conditions the mix through coils, and pushes it into the ductwork with a fan. The control sequence has to hold the supply air at the right temperature, maintain enough duct pressure for the VAV boxes to draw what they need, bring in the right amount of outdoor air, and do it all efficiently and safely.
Several control loops run at once, coordinated so they cooperate rather than fight.
The supply-air-temperature loop modulates the cooling coil (and heating coil, if present) to hold the air leaving the unit at setpoint — commonly around 55°F for cooling. As the building’s zones demand more or less cooling, the coil valve modulates to keep that supply temperature steady.
Holding a low, steady supply temperature also does dehumidification work, which matters in Florida — the cold coil wrings moisture out of the air before it is distributed.
The fan must maintain enough static pressure in the duct for every VAV box to pull its required airflow. A static-pressure sensor in the duct drives the fan VFD: as boxes open and demand rises, the fan speeds up; as they close, it slows down.
The efficient refinement is static pressure reset — continuously lowering the pressure setpoint to the minimum that still satisfies the hungriest box. This keeps the fan as slow as possible, and because fan energy falls steeply with speed, the savings are real. It is a core reset strategy.
When outdoor conditions allow, the economizer opens the outdoor-air dampers to use cool outside air for free cooling instead of running the compressor or chiller. The mixed-air loop coordinates outdoor, return, and relief dampers to deliver the right mix.
In Florida, economizing is limited — outdoor air is often too warm and humid to use — so the sequence must be enthalpy-based, judging total heat content, not just temperature, before pulling outdoor air in. A naive economizer here imports moisture. See economizers in Florida.
The sequence wraps the control loops in protections: freeze protection on coils, high static-pressure cutout to protect ductwork, smoke-detector and fire-alarm interlocks that shut the fan down, and proof-of-flow checks that confirm the fan is actually moving air before coils run.
These safeties keep the equipment and building protected when something goes wrong, and they are a required part of any competent AHU sequence.
In a VAV system the AHU and its boxes are one coordinated system. Guideline 36’s trim-and-respond logic lets the boxes tell the AHU what they need: if boxes are starving, the AHU raises pressure or lowers supply temperature; if they are all satisfied at minimum, it relaxes both to save energy.
This demand-based coordination is what separates a modern, efficient air system from one running fixed setpoints all day. It is commissioned against the sequence at startup — see functional performance testing.
It coordinates the fan, cooling and heating coils, and outdoor-air dampers to deliver conditioned air at the right temperature and pressure. The core loops are supply-air-temperature control, duct-static-pressure control with reset, and economizer/mixed-air control, all wrapped in equipment safeties.
It continuously lowers the duct pressure setpoint to the minimum that still satisfies the most-demanding VAV box, keeping the supply fan as slow as possible. Because fan energy falls steeply with speed, static pressure reset is a significant energy-saving strategy.
Florida outdoor air is often too warm and humid to use for free cooling, and a naive economizer would import moisture the system must then remove. Florida economizer sequences must be enthalpy-based — judging total heat content, not just temperature — before bringing outdoor air in.
Typical protections include coil freeze protection, high static-pressure cutout, smoke-detector and fire-alarm interlocks that stop the fan, and proof-of-flow checks confirming the fan is moving air before coils operate. These protect the equipment and building when something goes wrong.
Suncoast Cold Systems installs, wires, and configures the HVAC controls integral to the mechanical systems we provide — and specifies open protocols (BACnet, Modbus, open supervisory platforms) so you own your building’s controls and data, with no proprietary dealer lock-in. Where a project calls for certified systems integration, we coordinate it within one accountable mechanical scope. Licensed Florida Class A Air Conditioning Contractor (FL #CAC1824642).