Page 269 - HVAC Pump Handbook

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Configuring an HVAC Water System

266 The HVAC World

Cooling coils
such as fan-coil
or water source
heat pumps

DP1 DP2
C
Chiller

Chiller
Figure 9.11 Reverse return for low control valve differential
pressures.

9.6.2 Number of pipes in a loop system
There are three different numbers of pipes found in HVAC systems.
These are:

Two-pipe. Two-pipe systems have been used where there is only
heating or cooling; digital control has brought back the use of the
two-pipe system. Chapter 30 describes this in detail.
Three-pipe. This is an old configuration that is seldom used
today. There are two supply pipes and one return pipe; one supply
pipe is for heating, and the other is for cooling. There is a common
return pipe for both the heating and cooling systems. The possibili-
ties of energy waste are obvious with improper control.
Four-pipe: This has been the preferred piping arrangement for
buildings that have simultaneous heating and cooling loads. Most
buildings in the temperate zones are of this configuration and have
simultaneous heating and cooling loads, particularly in the spring

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