August, 2019

Inspection News and Views from the American Society of Home Inspectors

Furnace Inspection Leads to Recommended Test


Tech Consultant’s note: The example described in this article exceeds the ASHI Standard of Practice (SoP); however, some inspectors may want to go beyond the SoP. Keep in mind that different inspectors may look at the same item and come to different conclusions. This article represents this author’s view. Yours may differ. ASHI encourages readers who hold different opinions to send a letter to the editor and the author will be given an opportunity to respond. All letters are subject to editorial review.

During an initial tour of a house on a 20˚F day back in December 2017, I found a high-efficiency condensing gas furnace in the basement with a plastic vent pipe and no pipe on its air intake. The plastic grid over the combustion air intake was discolored and cracked, consistent with heat damage. Also, I noticed a very slight gas odor at the intake, consistent with the odorant in natural gas.

Opening the front panel revealed corrosion, consistent with condensate leak below the inducer, but I did not find an active leak. A loose wiring fitting on the burner enclosure created an opening consistent with heat damage. According to the serial number, the furnace was manufactured in 1996, so it was about 21 years old at the time of inspection.

The thermostat fan switch was on, with room and set temperatures at 70˚F. I reset the fan to “auto” for testing. Using a combustible gas tester, I found no gas leaks at the gas piping. However, I detected a trace of gas inside the air intake while the furnace was off. This is consistent with residual gas left in the burners as explained in a 1994 article “The Truth About Gas Leakage Complaints and Gas Valves” by Honeywell ( Honeywell stated that the presence of trace amounts of gas at the burner in these situations is “safe and normal.”

I reset the thermostat higher and tested carbon monoxide (CO) at a hole in the vent. At ignition, CO increased, but rapidly fell to below 40 parts per million (ppm), then rose when the circulation blower came on. The CO test ended as CO rose over 600 ppm. I reset the thermostat lower, causing the burner and inducer to stop. I immediately felt a warm air flow at the air intake. When I repeated the test, I had similar results. (Note: Conducting instrumented tests exceeds the ASHI Standard of Practice, as well as many inspectors’ scope of work.) I found the ambient indoor air CO to be zero; however, the carbon monoxide alarms in the home were not operational.

Discharge of circulation air out the air intake when the inducer is off and while the circulation blower runs is consistent with heat exchanger damage. Air pressure around the heat exchanger is much greater than pressure inside the exchanger. Damaged heat exchangers leak from the air side to the combustion side.

Although CO may be produced when this air flow disturbs the burners, an inducer fan pulls CO and other products of combustion out through the vent. Our CO test in the vent follows methods taught by the Carbon Monoxide Safety Association ( and others. It looks for changes in CO in the vent when the circulation blower comes on. A significant change in CO in the vent, particularly rising CO, is consistent with heat exchanger damage (excess air entering the combustion chamber to change or cool the flame). 

These findings support the benefits of checking the heat exchangers of sealed combustion condensing forced air furnaces (Annual Fuel Utilization Efficiency [AFUE] of 90% or greater) for reversed air flow at the combustion air intake (interior or exterior, depending on the configuration of the air supply). No instruments are used in this method.

Regardless of other adverse conditions found, finding reversed air flow at the air intake warrants an inspection report recommendation to immediately contact a qualified heating contractor for a consultation.

Testing Method

  1. Operate the furnace by slightly raising the thermostat setting. (Set the fan switch to “auto.”)

  2. Prepare to check the air intake immediately after the burner and inducer stop. (If the intake is outdoors, you could ask your assistant or your customer to lower the thermostat, or you could wait for the burner to cycle off.)

  3. Place your finger or a tissue at the inlet opening while the heat exchanger is still warm and the circulation blower is running. Small heat exchanger defects may not be noticeable, but large ones may be felt as air discharge at the intake. Note any air flow discharge from the inlet opening.

  4. A discharge at the intake on a one-pipe system may be warm. A discharge at an outdoor intake on a two-pipe system may be cooler because the pipe may have cooled any reversed (discharge) air flow.

  5. If you find reversed air flow at the intake, retest it and try to watch the burners in the view port for flame changes when the circulation blower comes on. Also, check the burner enclosure for heat damage. These furnaces have a flame roll-out sensor, but the sensor can fail or the flames may not reach the sensor.

  6. If you find reversed air flow at the intake, do a follow-up test at the intake by simply switching on the thermostat fan. Check for air flow reversal at the intake. Flow reversal at the intake is consistent with a heat exchanger defect. However, seeing no reversal of flow in this case does not confirm that the heat exchanger is free of defects. This part of the test is done with the heat exchanger at room temperature. Defects may only open when the exchanger is hot and holes or cracks are expanded.

Important Note: If you do not find any functioning carbon monoxide alarms in the home, it is important to send a message to the occupants, typically through the real estate agent, to alert them to the lack of functioning alarms and an increased potential for carbon monoxide poisoning.

Author’s note: The author requested that this article be reviewed by the ASHI Technical Committee. The committee recommended inclusion of this opening note and the end note. 

The opinions expressed in this article are those of the author only and do not necessarily reflect the opinions or views of ASHI. The information contained in the article is general and readers should always independently verify for accuracy, completeness and reliability.

Roger Hankey is an ASHI Certified Inspector and he was the first ASHI® Member in Minnesota, ASHI #269. He has served Minnesota homeowners since 1975, and he is a Level II Certified Infrared Thermographer and a Licensed Minnesota Department of Health Radon Measurement provider. Contact Roger at or visit