This document discusses different types of HVAC control systems, including pneumatic, electric, electronic, and direct digital control (DDC) systems. It outlines the key components and operating principles of pneumatic systems, then describes advantages and disadvantages of electric/electronic, DDC, and traditional control systems. DDC systems allow for remote monitoring and precise control through a central computer, but integrating different component brands can require additional programming.

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CEM 350
Facility Systems Design & Construction

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Sound operation of system is directly
influenced by selection and
functioning of control system
Temperature control systems have
seen dramatic changes recently
Brought about by desire to conserve
energy and efficiency
Most dramatic change is advent of
DDC (direct digital control) and
Internet (LAN, WAN – local and wide
area networks)

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Traditional controls systems:
 Pneumatic
 Electric
 Electronic
 And now DDC
 DDC with Internet ties

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Traditional controls systems:
 Pneumatic
 Electric
 Electronic
 And now DDC
 DDC with Internet ties

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Pneumatic control systems utilize
compressed air to supply energy to
control valves, damper operators etc.
Pressurized air circuits are small
diameter copper or plastic tubes from
1/8” dia. to 5/16” dia. Rarely exceeds
½” diameter for main air
Construction tip: Protect tubing
installations such as column lay-in or
cable tray, conduit, etc.

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For effective operation, we need (1):
 Source of clean, dry compressed air
 Source is usually electric air compressor
with filtering removal of 99.9% of
particulates 0.01 micron or smaller
 Filter must also remove oil introduced into
air stream by compressor
 Smallest droplet of oil or water will render
pneumatic controller with 0.005” orifice
useless
 Remove water w/ refrigerated drier

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For effective operation, we need (2):
 Pressure reducing valve (PRV) (80 psi to
20 psi – if large sysytem with long runs,
consider 30-80 psi to reduce pipe sizes
 Air lines are copper or plastic tubing
 Sensing elements – measure what is to be
controlled – T-stats, relays, etc.
 Control devices such as valve and damper
operators
 Spring ranges – installed on pneumatic
operators to provide full movement from
open to closed (3 to 13 psi throttling
range) so spring relaxed at 4 psi, closed
at 11 psi

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For effective operation, we need (3):
 Normally open valves/dampers (NO) in
open position when no air pressure acts
on operator. NO used when “fail-safe”
condition requires full flow through valve
or damper. In cold climates, heating
valves are NO; warm climates, cooling
coil chiller valves are NO; return air
dampers typically in NO position
 Normally closed valves/dampers (NC) –
spring pressure keeps valve in closed
position if no air pressure on the operator

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Pneumatic advantages
 Extremely durable – 40 year life if keep
air system clean
 Simple operation so personnel understand
 Maintainability – controls readily checked
with an air gauge and calibrated
 Very powerful and can operate large
systems
 Generally lower costs on large projects

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Pneumatic disadvantages
 Subject to oil and water contamination –
oil-less air compressors not practical as
they are too expensive and maintenance
intensive
 More hardware required for initial system
and for modifications – with DDC just re-
program
 Greater deviation than DDC controls –
issue with low or light load conditions
 Springs require calibration whereas
electronic controls almost no calibration

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Electric and Electronic Control
Systems – electricity is the power
source and electric wiring transmits
power from controller to operator
Advantages
 Logic panels available for similar projects
or larger standard control systems (pre-
packaged)
 Inexpensive installation for smaller
projects – pneumatic systems cost-
effective above 50 tons HVAC capacity
 Remote control of space sensors easily
accomodated

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Electric and Electronic Control
Systems – Disadvantages
 Higher operator hardware cost
 Shorter service life (15-20 years) as
electronics burn out due to voltage
problems, short circuits, etc.
 Higher installed costs for larger
projects – some jurisdictions require
control wiring to be in conduit
 Operators not available for large
valves

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Direct Digital Control (DDC) – control
accomplished via digital computer;
employs microprocessors with
software; control sequences entered
into computer with software; set points
by keyboard entries or touch screens
Computer continuously monitors
controlled variable; transmits
difference in values as digital pulse
that is converted to modulating signal
by a transducer
Transducer sends electric or
pneumatic signal to controller

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Direct Digital Control (DDC)
Advantages
 Simplicity of changing control sequences
and set points – often made without
adding hardware to system
 Example: Increased space heat gain has
caused occupant complaints; from
computer VAV system can have box
airflow increased, t-stat dead band
reduced, cooling proportional band
narrowed, system shutdown enacted later,
etc.
 Central control point – monitor entire
facility; improvement in security as

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Direct Digital Control (DDC)
Advantages
 Simplicity of changing control sequences
and set points – often made without
adding hardware to system
 Example: Increased space heat gain has
caused occupant complaints; from
computer VAV system can have box
airflow increased, t-stat dead band
reduced, cooling proportional band
narrowed, system shutdown enacted later,
etc.
 Central control point – monitor entire
facility; improvement in security as

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Direct Digital Control (DDC)
Advantages - 2
 Remote monitoring – can allow
expert to “look-in” on operational
issues
 Energy efficiency – can modify
temperatures easily for occupied
and unoccupied schedules
 Very precise control

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DDC system components need to
“talk” to each other –
Disadvantages:
 Older components and systems
difficult to “talk” to each other such
as controlling automatic dampers
and valves – programming required
– proprietary issues between
competing vendors
 Newer systems – more commonality