Field-Ready · Lesson 4.1
Boilers and Chillers: The Big Picture
Where the heat comes from and where the cold comes from. Everything downstream depends on these.
Why Central Plants Exist
Every AHU, fan coil, and VAV box you have worked on so far needed two things: heat and cold. It got them from somewhere. That somewhere is the central plant.
The alternative to a central plant is distributed generation: every air handler has its own heat source and every zone has its own cooling. In a small building this works fine. In a large building or campus it creates a maintenance nightmare: dozens of small boilers, each with its own flue, controls, and service needs. Efficiency suffers because small systems cannot match the part-load efficiency of a large, properly optimized central plant.
A central plant concentrates the generation of heat and cooling into a small number of high-capacity machines:
Hot Water Plant
One or more boilers heat water to 140 to 180°F. That water circulates through the building, giving up heat at coils in AHUs and terminal units, then returning to be reheated. The boiler plant is the source of all building heat.
Chilled Water Plant
One or more chillers cool water to 44 to 48°F. That water circulates through the building, absorbing heat at coils in AHUs and fan coils, then returning to be rechilled. The chiller plant is the source of all building cooling.
The pumps that circulate the water, the cooling towers that reject heat, and the BAS sequences that optimize staging and reset setpoints are all part of plant controls. That is what the next several lessons cover.