Thursday, August 27, 2009


Quite often, when you encounter a problem with a piece of HVACR equipment, the problem turns out to be electrical in nature. In fact about 80% of the prob;ems are electrical in nature. So you need to have a clear understanding of how to read wiring diagrams before you frequently can find the source of the trouble although a wiring diagram may appear very complex when viewed as a whole, it can be simplified by breaking it down into smaller parts. If you look at the individual circuits, one at a time, the overall diagram and machine functions become much easier to understand.

Control systems, for example, usually are designed so that an individual circuit controls only one function of a piece of equipment—starting and stopping a compressor by means of a control relay for instance, or controlling a solenoid valve by means of a pressure switch. Naturally, many variations are possible, but the main point to remember is that an individual circuit usually is associated with a single, basic component function.

The basic type of wiring diagrams used in the HVACR industry today is the ladder diagram, so called because it looks like the symbols that are used to represent the components in the system have been placed on the rungs of a ladder.

(PSR) potential start relay:

Can you say what type of motors they all are? What type of capacitors?

The illustration above is a simple air conditioning schematic without heat. It is a simplified ladder diagram of the wiring you could expect to find in a residential split or package system. The thermostat is typical of the older thermostats, and also shows how a digital thermostat controls the unit, without any heating, programming, time delays, and solid state circuitry drawn in.

A dotted line was drawn around the thermostat to make it easier to see that these are the switches in the thermostat, and that while the thermostat is electrically a part of the circuit, it's actually located on the wall usually about five feet in a room inside a building In this air conditioning schematic there is a dotted line around the potential start relay to make it easier to explain what it does, and how the switch opens and drops the start capacitor out of the circuit when counter electromotive force (emf) energizes its coil.

To read this schematic, start at the top, and go down line by line. The top line is the primary of the control transformer. If the breaker and disconnect are on and L1 and L2 are live, 208 volts will be applied to the primary. At the second on line down, 24 volts will be induced in the secondary of the transformer, which is the source of the control voltage.

On line three of this air conditioning schematic, let’s say the room is warm, so the thermal switch (T STAT) contacts are closed. When we move the manual switch up to the 'Cool' position, 24 volts will flow through the 'Cool' switch, thermal switch, low pressure switch, and high pressure switch, and energize relay C (Contactor).At the same time, 24 volts has flowed through the fan-on contacts and energized relay FR (Fan Relay).

When relay “C” is energized, the “C” contacts close and energize the condenser fan motor and compressor, and when relay “FR” is energized the “FR” contacts closed and energized the evaporator fan motor. In this air conditioning schematic, the compressor and condenser fan will continue to run until the manual switch is turned off, or the room cools down and the thermal switch contacts open, or either the low pressure or high pressure switch opens.

The evaporator fan will run continuously if the fan switch is left in the 'On' position. If the evaporator fan switch is set in the 'Auto' position, the fan will run when the compressor runs, and turn off when the compressor turns off. If you are unsure in the operation of how a potential relay operates look on this site for an explanation of operation for a potential relay and a current relay the two most popular relays used on single phase compressors.

PS Motors are permanent split capacitor with run capacitors.

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.