Operation of HVAC systems

Operation of a typical HVAC system and a typical HVAC control loop are discussed in the following paragraphs.

1. HVAC system operation. In the HVAC schematic shown in figure 4-1, interior building air is transported in ducts. A relief air port in the system discharges return air to the outside. An outside air port allows fresh air to enter the HVAC system. Three modulating dampers adjust the amount of outside air entering the system. Some of the return air from HVAC zones is mixed with outside air, filtered and divided into two streams. One supply air stream passes over a heating coil and is delivered to rooms located within the zone. The other supply air stream is cooled with a chilled water-cooling coil, before being supplied to the same rooms within the same HVAC zone. Excess supply air is then mixed together before flowing to other zones. Separate dampers in the heated and cooled air steams adjust air flow to the zone before mixing. Once the mixed air is taken to another zone it will be split again and heated and cooled in a similar way before being supplied to building rooms within the next zone.
2. HVAC control loop operation. Supply air is heated or cooled by passing over a heating coil (H/C) or a cooling coil (C/C) respectively. The flow of water in the coil is controlled by measuring duct air temperature with a sensor TT downstream of the coil. Refer to figure 4-2, Typical HVAC control loop. The measured temperature signal is interpreted by the controller TC and compared against a set point temperature established by a thermostat or an energy management control system (EMCS). To adjust liquid flow in the variable liquid volume (VLV) control valve, the controller produces an instrument signal usually 4 to 20 milliamps. In a pneumatic loop a signal converter, IP, converts this signal to a 3 to15 psig (pounds per square inch gauge) pneumatic signal. Tubing supplies the pneumatic signal to a pneumatic positioner (PP) on the heating/cooling water control valve VLV. The positioner is mechanically connected to a diaphragm actuator  indicating valve stem position. The actuator incorporates a spring which  maintains the valve in an open or closed position [normally open (NO) or normally closed (NC)] depending on the control scheme, when no air is supplied to the top of the diaphragm. The pneumatically positioned actuator and VLV valve are often assembled as a unit. The positioner interprets the pneumatic control signal and using the shop air supplied to it, directs more or less air to the side of the diaphragm opposite the spring on the actuator. This air pressure moves the diaphragm which moves the valve stem, changing the amount of liquid flowing through the valve and coil, therefore changing the amount of heating or cooling imparted to the air stream flowing over the coil.
   1. Water flow through the boiler is maintained at a constant rate with a three- way control valve. Temperature sensors in the packaged boiler allow burner controls to adjust fuel flow and therefore the BTU content of the heating water.
   2. Water flow through the chiller is maintained at a constant rate with a three- way control valve also. Temperature sensors in the packaged chiller allow controls to adjust refrigerant flow and therefore the BTU content of the chilled water.