Troubleshooting Gas Furnaces

In this specific troubleshooting case we have been asked to check on a gas furnace that, according to the customer, "won't heat".

When we arrive at the site, we find that the customer's general description of the situation gets more specific when they tell us that they "can hear something happening" but then the furnace goes quiet without providing any heat. When we start evaluating the operation of the system ourselves by making sure that the thermostat is calling, we are able to pin down the specific reason behind the equipment failure....although the burners ignite on a call for heat, the air handling system never starts, and the limit switch does it's job of shutting everything down.

Now that we have a specific failure in the sequence of operation of the equipment to investigate, we remove the access panel and check the schematic diagram shown below.


Figure One

Figure One


When we review the diagram in order to identify the specific test points on the printed circuit board that will allow us to proceed with isolating the problem component, we proceed:

With a voltmeter attached to PR2 and TP3 (Test Point 3) on the board, we initiate a call for heat and note that:

1. The Induced Draft Motor operates OK.

2. The glow coil operates.

3. The gas valve is energized and the main burner ignites.

4. After a delay, during which we never read 115 VAC, the system shuts down.

And, we've determined that the printed circuit board is the failed component.

The reason we know this is the specific failure that is keeping the equipment from operating is that when the furnace goes through its sequence successfully, it will deliver 115-volts at the two test points we monitored. However, we never read the voltage there, which indicates that the board has failed. 

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Troubleshooting HVAC Pages

Technical Training Associates
PO Box 2259
Green Valley, AZ 85622-2259
Phone: 520-625-6847

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