Determining the source of the problem in any HVACR equipment failure situation always begins with…

Step One:Consider the customer’s complaint and the symptoms.

Then, you can move on to…

Step Two:The specifics regarding the equipment, including wiring schematic if it’s an electrical problem, design characteristics, or other necessary factors if the problem area is refrigeration or airflow.

Which means you can then move on to…

Step Three:Follow a troubleshooting procedure that will systematically eliminate the different possibilities and zero in on the component or factor (or in some cases, combination of factors) responsible for the loss of equipment performance - the cause of the symptoms and the source of the complaint.

In the troubleshooting scenario we are about to present to you, you can follow the above steps to answer the question. Then, you’ll have the opportunity to compare your answer with ours.

Figure 1. (Click on the schematic for an enlarged view.)


In this particular troubleshooting situation, it’s the middle of July, and you’re dealing with a customer who has called to say that the unit that heats and cools their small office “isn’t cooling at all.” When you arrive, you find a 230-V, eight-year-old, rooftop package unit heat pump that employs a three-phase compressor. And you quickly confirm two things:

1.The thermostat is set in the COOL mode and the fan switch is set to the AUTO position.

2.The indoor blower motor is operating and blowing warm air.

With that evaluation accomplished indoors, you move to the rooftop and remove the access panel so you can check the schematic diagram (see Figure 1), and when you do, you find that:

• Checking with a voltmeter across the normally-closed CR contacts wired in series with the crankcase heater shows 0 volts.

• Checking with a voltmeter across the normally-open CR contacts wired in series with the contactor coil shows a reading of 230 volts.

• Checking with a voltmeter at the CR coil terminal connections shows a reading of 24 volts.

And here’s your three-part troubleshooting question:

1.What did you prove about the unit’s control transformer with your observation of the indoor fan motor when you arrived?

2.Which components did you eliminate as possibilities when you checked the contactor coil circuit?

3.Which component is the source of the no-run situation?

Compare your answers with ours byclicking here.

Publication date:09/08/2008