30 November 1998
(7) The designer may choose to use either four 2-way valves or one 3-way mixing valve, and one
3-way bypass valve for 2-position flow control applications as dual-temperature system changeover valves.
(8) Control valves are classified according to the action of the valve spring in moving the disk or
plug relative to the seat when the control signal or the power is removed. A 2-way valve that opens its flow
port under this condition is called a normally open (NO) valve, and one that closes its flow port under this
condition is called a normally closed (NC) valve. A 3-way mixing valve has both NC and NO inlet flow ports
connected to a common (C) outlet flow port. A 3-way bypass valve has both NC and NO outlet flow ports
connected to a C inlet flow port.
(9) The flow regulating characteristic of a valve is generally determined by the shape of a disk or
plug that passes through the flow port. The flow regulating characteristics used for control systems
covered by this manual are:
(a) Linear flow, in which the percent of valve travel equals the percent of maximum flow rate
through the valve.
(b) Equal-percentage flow in which equal increments in the percentage of valve travel produce
an equal-percentage change in flow rate from the previous flow rate, when a constant pressure drop is
(10) The applications of 3-way mixing valves covered by this manual require the use of valves with
linear flow characteristics. The applications of 2-way valves covered by this manual require the use of
valves with equal-percentage flow characteristics. This requirement results from the application of the
valves as modulating fluid control devices. The equal-percentage flow characteristic matches the
non-linear heat exchange characteristics of the HVAC equipment coils with a change in fluid flow that tends
to linearize the heat exchange output of the coil with a linear signal to the control valve. The linear-flow
characteristic is more suitable for mixing applications and for humidification.
(11) The purchase price of a control valve increases with its size. The installation cost of a control
valve also increases with its size, because of:
(a) The change from screwed ends to flanged ends
(b) Because larger valves and their weight require more installation and handling labor.
(12) At a pipe size of 4 inches or larger, a type of rotary control valve (known as a butterfly valve)
becomes economically suitable for HVAC control applications because of the combination of the price of
the valve and the installation costs. The butterfly valve has a disk that rotates on a shaft and closes against
a seat. The seat is concentric with the connected pipe. The butterfly valve can have flow control
characteristics similar to equal percentage when used with an appropriate actuator and positioner. In three-
way applications of valves for 4-inch pipe size and larger, the designer will show two valves on a common
pipe tee, with separate actuators that will operate the two valves simultaneously. One of the valves will be
NC, and one will be NO. The C connection can be either an inlet or an outlet. This allows the combination
of two valves and a pipe tee to function as a 3-way mixing valve or a 3-way bypass valve. Figure 2-1 shows
butterfly valves used in 3-way mixing and 3-way bypass arrangements on a common pipe tee.
Figure 2-1. Two butterfly valves on a common pipe tee.
b. Control dampers.
(1) Dampers are used to regulate the flow of air in ductwork in both modulating and 2-position