Service & Maintenance / Extra Edition

Troubleshooting With Jim Johnson: An Older Gas Furnace That's Blowing Cool Air

October 8, 2007
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Jim Johnson

With this article, The NEWS is introducing a new online column called “Troubleshooting With Jim Johnson.” In this column, Jim Johnson, director of training for Technical Training Associates, will present a specific HVACR equipment problem and invite readers to submit their diagnosis. From those who submit the correct answer by the stated deadline, there will be a prize drawing. The answer, and the prize winner, will be announced in the next column, along with another problem for you to solve. Get ready to troubleshoot with Jim.

In this troubleshooting situation, we have a customer who has called to say that their furnace “isn’t working.” In response to the dispatcher’s questions, the homeowner is able to further describe the problem by saying that they pulled the plug on the unit because it was blowing cool air and nothing they did with the thermostat would turn it off.

Upon your arrival at the home, you find the following conditions:

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

1. The temperature in the house is 50°F.

2. The furnace, a standard upflow model that uses a spark ignition system for the pilot flame upon a call for heat, is unplugged.

When you plug the power cord into the 120-VAC receptacle, you note that the fan motor starts immediately. When you remove the access door, the motor shuts off. Consulting the wiring diagram in Figure 1, you begin your troubleshooting by attaching the leads of a voltmeter to the PR-1 and PR-2 connections on the furnace control board. Pushing in on the door switch shows a reading of 120 VAC. As your next test, you move the meter leads to the SEC-1 and SEC-2 terminals, and your reading with the door switch pushed in is 0 VAC. 

Your two-part troubleshooting question is:

1. Which component has failed?

2. Why does the blower motor run constantly?

If you have the answer to this question, click on and submit your diagnosis via The NEWS link. All correct answers received by the deadline of Oct. 30, 2007 will be entered into a prize drawing for a Fieldpiece SC46 Clamp-on Digital Meter. The correct answer to this question, along with the results of the prize drawing, will be published in the Dec. 3 online issue.

Publication date: 10/08/2007

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