This page on our website gives you an opportunity to evaluate a particular HVACR troubleshooting situation and, through the process of the systematic elimination of the possibilities, arrive at a diagnosis.
A GAS FURNACE THAT’S OPERATING INTERMITTENTLY
By Jim Johnson
This troubleshooting situation centers around a standard natural gas, induced draft, hot surface ignition system furnace. This is a new installation that has been accomplished in anticipation of the upcoming heating season, and the problem is that during the initial start-up the unit did not perform properly.
The customer’s description of the problem is that in one test cycle, the glow coil will ignite the fuel properly, while in another test, the burner shuts down prematurely. They also note that in some cases when the unit is started, the induced draft motor operates, then shuts down before the ignition is accomplished, but after disconnecting the power supply, the draft motor operates long enough to allow ignition, which can lead to burner ignition and blower motor operation in some instances. However, even if a complete sequence of operation is accomplished, the furnace shuts down prematurely even though the thermostat is set above the room temperature.
Since your initial evaluation of the mechanical and fuel aspects of this installation show no issues, you begin troubleshooting the electrical system (shown in Figure One above), and you verify the power supply with the following tests:
….L1 and L2 connections to the unit: 115 VAC
….Checking at the wiring connection at the power-in connection of the door switch to ground: 0 VAC
….Checking the opposite side of the line to ground: 115 VAC
You troubleshooting question: What do you do next?
A HEAT PUMP THAT WON’T COOL
By Jim Johnson
This troubleshooting problem involves a 230-volt, single phase 10-year-old heat pump, and the customer’s complaint is that the temperature in the house is too warm. When you arrive, you confirm that the indoor fan motor is operating and the inside air temperature is above the thermostat set point. You also find that the outside fan motor is operating normally, but the compressor is attempting to start and kicking off on overload. Figure One shows the manufacturer’s representation of the compressor circuitry and the diagram legend is shown in Figure Two.
What you find relative to the compressor wiring is that while the diagram doesn’t show it, a start kit consisting of a potential relay and start capacitor was added to this unit on a previous service call. Your electrical checks of the capacitors and start relay show the following results:
…4G2: 35 MFD
…Potential Relay Terminals 2 and 5: 2,500 Ohms
…Potential Relay Terminals 1 and 2: Continuity
…Start Capacitor: 220 MFD
With the compressor cooled down, your ohmmeter checks show the following:
…C to S: 3.5 Ohms
…C to R: 2.9 Ohms
…Terminal to Ground: Infinity
Your troubleshooting question: What is the next step you need to take in repairing this equipment?
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