Makeup Air Units
Lesson 3.5
What Is a Makeup Air Unit
A Makeup Air Unit (MAU) brings in 100% outdoor air. There is no return air. No mixing. Whatever temperature and humidity exists outside is what you start with.
That distinction matters. A standard AHU blends outdoor air with return air, so the mixed air temperature stays relatively moderate. A MAU has no such help.
Where do you find MAUs? Commercial kitchens (hood exhaust), hospital operating rooms and lab spaces, parking garages, large industrial spaces.
The Points
| Type | Point | Description |
|---|---|---|
| AI | OAT | Outdoor Air Temperature |
| AI | DAT | Discharge Air Temperature |
| AO | HW_Vlv | Preheat hot water valve |
| AO | CHW-VLV | Cooling coil valve |
| AO | VFD-SPD | Supply fan speed |
| BO | FanEnable | Fan run command |
| BI | Fan_Status | Fan proof via current switch |
How It Conditions the Air
Heating mode: The hot water preheat valve modulates open based on a PID loop maintaining the DAT setpoint.
Cooling mode: The chilled water valve opens to cool incoming hot, humid outdoor air.
Freeze protection: When OAT drops below a freeze threshold (typically 32-38 degrees F), keep the preheat valve at a minimum open position any time the fan is off. A frozen MAU coil is an expensive failure.
Fan interlock: Coils do not open or energize until fan proof is confirmed.
Energy Recovery and Field Notes
Energy Recovery Ventilators: Many modern MAUs include an energy recovery wheel. It spins slowly between the exhaust air stream and the incoming outdoor air stream, transferring heat and sometimes moisture between them.
Stratification and coil face velocity: Large MAU coils sometimes have uneven air distribution across the face. Cold air entering at the bottom can freeze that section even when the average discharge temperature looks fine.
Condensation in winter: When warm moist exhaust air meets cold outdoor air in the energy recovery section, condensation forms. On cold enough days it can freeze and block the wheel.
# PLANT CONTROLS