How It All Connects

System Review

One Building, One System

You've spent the last four series learning individual pieces. But on a real job, none of these exist in isolation.

A VAV box needs conditioned air. That air comes from an AHU. The AHU needs cold water to cool it and hot water to heat it. The cold water comes from a chiller. The hot water comes from a boiler. When the zone is cold and the VAV reheat coil fires, it's drawing from the same hot water loop as the AHU preheat coil across the building. All of it is connected. All of it is running at the same time.

The Full System

Chilled Water Plant: The chiller removes heat from water, producing a cold supply loop (typically 44-46 degrees F) that AHUs draw on to cool air. Two pumps maintain differential pressure. An OAT lockout prevents the chiller from running when outdoor air is already cool enough.

Hot Water Plant: The boiler heats water to a setpoint that resets with outdoor air temperature. Two pumps push the water through the building loop. AHUs use the hot water coil to preheat air in winter. VAV reheat coils draw from the same loop.

Air Handling Unit: The AHU mixes return air with outside air, filters it, then conditions it through the CHW coil or HW coil. A VFD-driven supply fan pushes the conditioned air into the duct network. The AHU doesn't know or care about individual zones.

VAV Box with Reheat: The VAV box throttles supply air to its zone using a modulating damper. When the zone needs cooling, the damper opens. When the zone needs heat, the damper drops to minimum and the reheat coil fires, fed from the same HW loop.

Conditioned Zone: The space being controlled. A zone sensor reads temperature and reports back to the VAV controller. Everything upstream exists to answer one question: is this zone comfortable?

# SUPERVISORY CONTROLS