How to Test Heat Exchangers

Clean the exterior of the heat exchanger to remove any debris on the unit.

Open the access panel and visually inspect the heat exchanger using a telescoping mirror and a powerful flashlight on both the exterior and interior of the heat exchanger. Look for signs of extreme deterioration, open cracks or split seams. If there are any larger cracks, they are usually identifiable using this visual inspection. However, the furnace needs to be off to perform this test, which gives the heat exchanger time to cool. Metal expands when heated, and the crack may not be fully visible while the furnace is turned off.

Have someone turn the furnace on and watch the flame for changes in its behavior. Sit in front of the furnace to observe the flame, but keep a safe distance to avoid burns. The call for heat from the thermostat will start the furnace but not the blower. The flame will be present for a short period before the blower turns on. If there is a crack in the heat exchanger, the air from the blower will make the flame look like it is dancing or changing color. There should be no change in the flame's behavior when the blower turns on.

Perform the methane (CH4) procedure, recommended by the American Gas Association (AGA). To do this, cut a 3-inch hole in the plenum, just above the heat exchanger. Allow the furnace to run for five minutes. With the heat exchanger still warm, inject the tracer gas into the bottom of the heat exchanger at a rate of 7 cubic feet per hour. Move a detector probe back and forth in the 3-inch hole for two minutes. If an unacceptable leak is detected, the probe detector will light up or signal the pressure of the tracer gas.

For a final test, turn on a combustion analyzer and select oxygen levels on the device. Place the test lead into the venting and turn on the furnace. The oxygen levels should remain the same when the flame first begins and when the blower motor turns on. A sudden drop in oxygen level could suggest a crack that is allowing the blown air to enter the combustion chamber.