Home Inspector Training & Inspection Report Review Service:
Useful Articles - We periodically post articles from our experience to share what we've learned. Often they describe information we learned the hard way. See below for a furnace inspection sample.
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Contact us by email for more information or to schedule any of these home inspection training opportunities.
Sample training article:
Furnace inspection leads to recommended testOn a 20˚F day in December 2017 an initial tour of the house found a sealed combustion condensing gas furnace in the basement with a plastic vent pipe and no pipe on its air intake. A plastic grid over the combustion air intake was discolored and cracked consistent with heat damage. Also a very slight gas odor was found at the intake consistent with the odorant in natural gas.
Opening the front panel revealed corrosion consistent with condensate leak below the inducer but no active leak was found. A loose wiring fitting on the burner enclosure created an opening consistent with heat damage. The furnace was manufactured in 1996 per the serial number (about 21 years old.)
The thermostat fan switch was ON with room and set temperatures at 70˚F. Inspector reset the fan to AUTO for testing. A combustible gas tester found no gas leaks at the gas piping. However, a trace of gas was detected inside the air intake while the furnace was OFF. This is consistent with residual gas left in the burners as explained in a 1994 Honeywell document found here. Honeywell states that the presence of trace amounts of gas at the burner in these situations is “safe and normal”.
The thermostat was reset higher and carbon monoxide (CO) tested 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 was ended as CO rose over 600 PPM. The thermostat was reset lower causing the burner and inducer to stop. A warm air flow was immediately felt at the air intake. The test was repeated with similar results. (NOTE: Instrumented tests exceed the ASHI® Standard of Practice and many inspectors’ scope of work.) Ambient indoor air CO was found 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 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. ) While 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 Assn. (COsafety.org ) 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/cool the flame).
These findings support the benefit of checking the heat exchangers of sealed combustion condensing forced air furnaces (AFUE 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.
1. Operate furnace by raising thermostat setting slightly. (Fan switch at AUTO.)
2. Prepare to check the air intake immediately after the burner and inducer stop. (If intake is outdoors, have a helper or customer lower the thermostat, or 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 since the pipe may have cooled any reversed (discharge) air flow.
5. If reversed air flow is found at the intake, retest 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 reversed air flow is found at the intake, a follow up test can be done by testing at the intake by simply switching the thermostat to Fan ON. Check for air flow reversal at the intake. Flow reversal at the intake is consistent with a heat exchanger defect. HOWEVER, no reversal of flow is this case is NOT confirmation that the heat exchanger is free of defects, since this part of the test is done with the heat exchanger at room temperature. Defects such as cracks may only open when the exchanger is hot and expanded.
Regardless of other adverse conditions found, finding reversed air flow at the air intake warrants an inspection report recommendation of immediate attention by a qualified heating contractor.
Footnote: If no functioning carbon monoxide alarms are found, a message to the occupants, typically through the real estate agent, is advised to alert them to the lack of functioning alarms and an increased potential for carbon monoxide poisoning.