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Table of Contents-
IndexHVAC Control Variations
Control variations include one, two or three speed blower/fan motor switches to
vary air velocity. Air or electrical toggle switches and rocker switches may be
used in system control to replace cable control. Air cylinders open or close duct
doors and radiator shutters. Air pressure hoses and fittings have a long service
life and are easy to repair. The following controls are discussed in this section:
1.
Air Controls
2.
Electrical Devices and Schematics
3.
Combination and Supplemental Controls
4.
Fan Clutches, Radiator Shutters, and Override Controls
1. Air Controls
The development of air controls has improved the operation and function
of HVAC systems. A basic form of air control may use a single switch and
cylinder to operate a duct door. In more complex systems air cylinders are
used to control fresh and recirculating air, heat/defrost and air condi-
tioner vents. A pressure switch is often used to control the compressor
clutch.
The most common air control system is a modular design. Modular
units are simple and compact as illustrated in Figure 5-7. An air block
uses a pintle and programmed control lever to control air cylinders.
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SPRING LOADED
PINTLES
O-RINGS
AIR FLOW
AIR FLOW
AIR FLOW
AIR BLOCK CLOSED
(CYLINDER VENTED)
AIR BLOCK OPEN
CONTROL LEVER
In air control systems a Legris push-on fitting has gained popularity with
manufacturers. The Legris design speeds air tube installation and pro-
vides a positive leak free air seal where tubing is attached.
Some Frigidaire AC systems have a
thermal limiter and superheat
switch
combination to protect the compressor when system pressure is
too low. A low system pressure can mean there is not enough refrigerant
inside the system and/or insufficient refrigerant oil circulation to properly
lubricate the compressor. Figure 5-8 illustrates the function of the ther-
mal limiter and superheat switch. When the fuse in the thermal limiter
melts, the clutch circuit is broken to stop the compressor. The thermal
limiter must be replaced when this happens.
HVAC Control Variations
Figure 5-7
This cutaway view of an air
block shows the components
and describes their function.
The air cylinders are not
shown.