Built-Up Air Handling Units
Lesson 3.3
What Is a Built-Up AHU
A built-up AHU is not a packaged unit. It has no self-contained refrigerant circuit. Cooling comes from chilled water piped in from a central chiller plant. Heating comes from hot water from a boiler. The AHU is just a cabinet: a fan, coils, filters, and dampers assembled in a mechanical room.
The key difference from an RTU: the RTU stages compressors and gas heat, all or nothing in steps. A built-up AHU modulates. The chilled water valve opens 0 to 100 percent. The hot water valve opens 0 to 100 percent. The OA damper modulates. The fan runs at variable speed. Everything is continuous, not staged.
The Points
OAT (AI): Outside Air Temperature. Used for economizer enable logic and SAT reset strategies.
OA-DMR (AO): Outside Air Damper. Modulates 0 to 100 percent. Works in opposition to the RA damper.
EA-DMR (AO): Exhaust Air Damper. Tracks the OA damper position to relieve building pressure.
RA-DMR (AO): Return Air Damper. Modulates opposite to the OA damper.
RAT (AI): Return Air Temperature. The average temperature of air coming back from the building.
MAT (AI): Mixed Air Temperature. Measured after OA and RA combine in the mix box.
FLT-DP (AI): Filter Differential Pressure. As filters load, the pressure drop rises.
CHW-VLV (AO): Chilled Water Valve. Modulates 0 to 100 percent to maintain the SAT setpoint during mechanical cooling.
HW-VLV (AO): Hot Water Valve. Modulates 0 to 100 percent to maintain the SAT setpoint during heating mode.
FAN (DO): Supply Fan Enable. Starts and stops the fan based on schedule and occupancy.
VFD (AO): VFD Speed Command. The duct static pressure loop drives this output.
SAT (AI): Supply Air Temperature. The primary control variable for both the heating and cooling loops.
DSP (AI): Duct Static Pressure. Measured in the supply duct, typically about two-thirds of the way to the most remote VAV box.
The Sequence
Morning Warmup: Pre-occupancy, space below heating setpoint. OA damper closes. 100% return air through the hot water coil. OA-DMR: 0%, RA-DMR: 100%, CHW-VLV: 0%, HW-VLV: 72%, VFD: 100%.
Occupied Heating: OA damper holds at minimum for ventilation. HW valve modulates to maintain SAT setpoint. OA-DMR: 20%, RA-DMR: 80%, CHW-VLV: 0%, HW-VLV: 48%, VFD: 62%.
Economizer: OAT in usable range, space calling for cooling. OA damper opens wide. Free outdoor air cooling. OA-DMR: 78%, RA-DMR: 22%, CHW-VLV: 0%, HW-VLV: 0%, VFD: 70%.
Mechanical Cooling: OAT too warm for economizer. OA damper drops to minimum. CHW valve modulates. OA-DMR: 20%, RA-DMR: 80%, CHW-VLV: 58%, HW-VLV: 0%, VFD: 75%.
Unoccupied: Fan off. All dampers close. Setpoints widen significantly. All at 0%.
The DSP loop runs independently whenever the fan is on.
What to Watch For
- MAT sensor calibration is everything for the economizer: If MAT reads 5 degrees high, the economizer thinks the mixed air is warmer than it is. Verify MAT calibration first.
- OA and RA dampers must work in opposition: If they are not, you can either starve the fan of air or send the building into positive pressure.
- CHW and HW interlocks prevent simultaneous heating and cooling: The exception is dehumidification sequences.
- DSP sensor location changes everything: Should be about two-thirds of the way to the most remote VAV box, per ASHRAE guidelines.
- Minimum VFD speed is not optional: Always set a minimum speed, typically 15 to 20 percent. Without it, the fan can stall or run backwards.