30 November 1998
e. Two-position control. The amount of heat delivered to a space from certain types of HVAC
equipment is controlled by turning the equipment on and by shutting the equipment off. This type of control
is also called on-off control. Examples of 2-position control are the starting and stopping of the fans of unit
heaters and fan coil units by room thermostats to maintain space temperature, and the opening and closing
of shutoff dampers when fans are started and stopped.
3. CONTROL SYSTEM EQUIPMENT.
a. Control valves.
(1) Control valves are used to regulate the flow of fluids in piping systems by compressing and
releasing a valve spring to move a valve closure disk or plug toward or away from the closure seat of a flow
port. The valves are used both in modulating and in 2-position control applications.
(2) Examples of the use of modulating control valves are:
Heating and cooling coil control valves.
Converter steam control valves.
Humidifier control valves.
(3) Examples of the use of 2-position control valves are:
(a) Dual-temperature water system changeover valves.
(b) Shutoff valves used in fan coil unit coils.
(4) Control valves are classified according to their flow regulating body patterns. A 2-way valve
restricts fluid flow in one direction, because it has one inlet and one outlet; a 3-way valve restricts flow in two
directions. The designer will use 2-way control valves for controlling the following types of HVAC
Coils served by variable volume pumping systems.
(5) A 3-way mixing valve has two inlets and one outlet, and a 3-way bypass (diverting) valve has
one inlet and two outlets. For the systems shown in this manual, the 3-way mixing valve is used in both flow
mixing and flow diverting type applications, except that a 3-way bypass valve is used as a dual-temperature
changeover control valve.
(6) In the flow mixing application, the 3-way valve is used to mix heated primary flow, from a boiler
or a converter, with system return flow to produce system secondary supply, for the purpose of controlling
temperature. When used on the return line from a coil, one of the 3-way valve's inlets is from the coil, and
the other inlet is from the bypass around the coil. The designer may choose to use 3-way mixing valves in
lieu of the 2-way valves shown in this manual for controlling the following types of HVAC equipment to
prevent deadheading of pumps:
Coils served by constant volume pumping systems.
As means of pump pressure relief in variable volume pumping systems.
As diverting valves around boilers or cooling towers.