Air-Conditioning Control Systems

1. Air-Conditioning Control Systems Electricity for Refrigeration, Heating and Air Conditioning 7th Edition Chapter 15 Air-Conditioning Control Systems

2. Air-Conditioning Control Systems Upon 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 residential application • 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

3. 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

4. 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 condensing units.

5. 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 contactor that 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.

6. 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.

7. 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 the condensing unit correctly.

8. 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.

9. 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.

10. 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 available to 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.

11. 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 evaporator fan 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.

12. 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.

13. 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.

14. 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.

15. 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 some­what 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.

16. 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 manufacturer’s 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.

17. 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.

18. 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 manufacturer’s 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.

19. 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.

20. 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 method of wiring and the proper connection points.

21. Schematic Diagram of a Simple Air-Cooled Condensing Unit

22. Schematic Diagram of a Condensing Unit with Safety Controls

23. Schematic Diagram of a Small Air-Cooled Packaged Unit Used in a Residence

24. Schematic Diagram for Electric Air-Conditioning and Gas-Heating Unit

25. Control System of a Furnace