chapter 15 control systems

7/27/2019 Chapter 15 Control Systems

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Air-Conditioning Control Systems

Electricity for Refrigeration, Heating and Air

Conditioning 7th Edition

Chapter 15 Air-Conditioning Control Systems


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Air-Conditioning Control SystemsUpon completion of this chapter the student will be able to:

Understand the electrical circuitry of a residential condensing unit

Make all electrical connections to install a condensing unit in a residentialapplication

Troubleshoot a residential condensing unit

Understand the basic control systems used in residential air-conditioning

control systems

Understand the control systems used in light commercial air-cooled and

water-cooled packaged units Make all electrical connections for a complete residential installation

Understand the control systems used In gas heat electric air-conditioning

packaged units

Troubleshoot residential air conditioning system


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Key Terms

Air-Cooled Package Unit

Condensing Unit

Factory-Installed Wiring Field Wiring

Gas Pack

Indoor Fan Relay Package

Internal Pressure Relief Valve

Packaged Air-Conditioning Unit

Short-cycling

Water-Cooled Packaged Unit


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Basic Condensing Units

A condensing unit is the portion of a split air-conditioning system that is

mounted outside and contains the compressor, the condenser, the

condenser fan motor, and the necessary devices to control these

components. A split system is one that is divided into two parts, usually a condensing unit

(outside) with a fan coil unit (inside).

In most cases, the condensing unit is used with some type of equipment

that will produce the air flow, and a coil must be mounted in the air flow.

Most condensing units used in the industry today are air-cooled condensingunits.


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Condensing Unit Components for a Simple

Control System Most modem condensing units pick up their 24-volt power supply to operate

the control system from the furnace transformer or from an indoor fan relay

package. The evaporator fan motor usually is controlled by a relay when the system

contains a furnace or fan coil unit and the condensing unit.

The compressor and condenser fan motor in a condensing unit are usually

controlled by a contactor.

The simplest control system used on condensing units today is a contactorthat controls the operation of the entire condensing unit with the exception

of the internal overloads in the compressor and condenser fan motor.

Most condensing units without a high-pressure switch will have an internal

pressure relief valve in the compressor that opens if the discharge pressure

exceeds an unsafe level.


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Condensing Unit Components for Complex

Control Systems

The more complex systems incorporate a high-pressure and a

low-pressure switch. If any of the safety controls open, the compressor and condenser fan

motor stop.

Several manufacturers also use a device that protects the system from

short-cycling.

This device maintains a certain period of time between the cycles of the

equipment so the system does not cut on and off in rapid succession

(short-cycling).

This control system adds a relay and a time clock mechanism for this

purpose.


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Wiring

All condensing units come from the manufacturers with a wiring diagram

and, in some cases, with an installation wiring diagram.

Usually it is not difficult to follow the installation instructions and wire thecondensing unit correctly.


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Troubleshooting

Any troubleshooting of the condensing unit can be done from the schematic because of the simplicity of most

control systems.

By using the schematic diagram, and having an understanding of the components, you should have no trouble

with diagnosing a residential control system.


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Packaged Units

A packaged air-conditioning unit is built with all the components housed in one unit.

In most cases, packaged units are complete except for the power connections and the control connections.

Air-cooled packaged units are the most widely used types of packaged units.

The units are used on applications ranging from small residential cooling units to large commercial and industrial

air-cooled units. The smaller residential systems may be installed when the structure is built or at a later date.

The air-cooled packaged unit contains the compressor, evaporator fan motor, condenser fan motor if not of

remote design, and all the necessary controls.

Water-cooled packaged units are usually used on commercial and industrial systems.

The water-cooled unit requires a cooling tower or some means of supplying water to the condenser.

If a water-cooled unit is used, there must be interlocks to ensure that the cooling tower pump is operating when

the compressor is operating.


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Control System for Packaged Air-Conditioning

Systems Water-cooled packaged units utilize a line voltage control system with some

type of interlock to start the cooling tower pump or other accessories that

are required. Air-cooled packaged units can incorporate some form of heating as well as

cooling.

The smaller packaged units usually have some easy means of installing

electric resistance heat into the equipment.

The larger air-cooled packaged units make electric and gas heat availableto the customer.

Water-cooled packaged units and air-cooled packaged units usually use a

hot-water coil, steam coil, or resistance heat for heating purposes.


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Small Air-Cooled Packaged Units

The small air-cooled packaged units usually have a simple and easy-to-

follow control system.

The control system uses a contactor to operate the compressor and the

condenser fan motor and an evaporator fan relay to control the evaporatorfan motor.

All necessary safety components are also included in the control system.

The installation of the air-cooled packaged unit is relatively simple because

the only necessary connections are from the power source and the control

source or thermostat.


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Gas-Electric Air Conditioners

The basic control system for the air conditioning is the same as the one for

most small residential units.

However, several components are added to take care of the heating

control system.

The installation instructions should be followed when installing this type

of system.


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Rooftop Units

The air-cooled packaged units used in rooftop applications are complex

because they usually have some type of heating to go along with the air

cooling and, in some cases, some type of zone control.

These units are usually installed on the roof of a structure.

But they could just as easily be installed on ground level when the

necessary changes are made in the ductwork connections.

Often the electric connections of these systems are hard to install because

of the number of wires required for the control circuit and the power wiring.


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Field Wiring

Wiring that must be installed by the installation technician and is called

field wiring.

The factory-installed wiring is the wiring installed at the factory.

The factory wiring has been sized, color-coded, and installed in the control

system to operate the equipment properly.

The remainder of the wiring, whether it be the power wiring or control wiring,

must be connected in the field.


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Power Wiring

There are two power connections that must be made on a split air-

conditioning and heating system: the connections to the condensing unit

and the connections to the evaporator fan motor or furnace.

The electric supply wiring of a commercial and industrial system is some-what more complex than the simple residential systems.

The use of three-phase current in commercial and industrial structures does

not add to the complexity of the system, only to the number of power wires

that must be supplied to the system.


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Sizing Wires and Fuses

The distance that the power wiring must be run is important because of the

voltage drop that can occur on long circuit runs.

The installation instructions usually give a wiring chart with the length that is

allowable for each size of wire.

It is also important to follow the manufacturers recommendations for the

fuse or breaker size.

Before any heating, cooling, or refrigeration system can be expected to

operate properly, it must first be supplied with the correct size wire and fuse

to deliver the proper voltage to the system.


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Control Wiring

The control wiring of heating and air-conditioning systems is just as

important as the supply power wiring.

Low-voltage control systems are used on residential and small commercial

applications.

On large commercial and industrial applications, line voltage control

systems are often used.

Many large commercial and industrial systems are controlled by special

control systems: electric, pneumatic, or electronic.


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Residential

All residential packaged air conditioners use a low-voltage control system

that connects the thermostat to the unit.

Residential split systems are usually easy to install.

They all incorporate a 24-volt control system, with the low-voltage power

being supplied from an indoor fan relay package, the furnace, or the

condensing units.

The control system of a residential split system can be connected several

different ways, and the manufacturers installation instructions should be

followed. The more modern condensing units are usually designed without the

low-voltage transformer but with two low-voltage connections instead.


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Industrial

Control systems for relatively small commercial units are similar in design to

the residential low-voltage system.

The only exception is that the system uses a control relay that supplies the

condensing unit with 240 volts while using a normal 24-volt control system.

Two-stage heating or two-stage cooling can also be used with a low-voltage

control system.


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Sizing Wire

Control wiring in most cases is a small wire (No. 18 to No. 20) that is usually

covered by a rubber jacket when using a low-voltage control system.

Thermostat wire can be purchased single-stranded or multi-stranded, with

or without a rubber jacket protecting the small thermostat wires.

The wiring of air-conditioning and heating systems is one of the most

important factors in the installation of equipment.

The life of the equipment can be drastically cut by undersized wire or by

having loose connections.

The installation instructions are the best place to look for the proper methodof wiring and the proper connection points.


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Schematic Diagram of a Simple Air-CooledCondensing Unit


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Schematic Diagram of a Condensing Unit withSafety Controls


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Schematic Diagram of a Small Air-CooledPackaged Unit Used in a Residence


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Schematic Diagram for Electric Air-Conditioning

and Gas-Heating Unit


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Control System of a Furnace

chapter 15 control systems

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