Troubleshooting Challenge: A Gas Furnace That’s Operating Intermittently
Your troubleshooting situation involves 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 (see the schematic diagram here), 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
Your troubleshooting question: What do you do next?
Compare your answer with ours by clicking on the PDF link below.
Publication date: 9/8/2014
For information on Jim Johnson’s HVACR technician training DVDs, go to http://techtrainassoc.com/training-videos/hvacr-training-videos/.