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Safety Concerns with Heat Exchangers
By:New Jersey Home Inspector Michael Del Greco
Safety Concerns with Heat Exchangers
Submitted by: Michael Del Greco, New Jersey Home Inspector Lic GI 0121, President of Accurate Inspections, Inc, a New Jersey home inspection company
How unsafe is a failed heat exchanger in your forced-air system? The main safety concern with warm-
air furnaces, sometimes called "hot air heat," is keeping the products of combustion from mixing with
the air in the home and negatively affecting the health of the occupants.
When fuel is burned, three products are produced: (1) heat, (2) carbon dioxide (C02), and water (H20).
This example assumes complete combustion. If there is incomplete combustion, other products will also
be present. These may include the compounds such as carbon monoxide (C0), formaldehyde (HCH0)
and numerous other aldahydes, nitrogen dioxide (N02), and sulfur dioxide (S02). The technicians who set up furnaces try to keep the C0 to less than 100 parts per million (ppm) in the exhaust.
Problems develop when there is a blocked or partially blocked chimney and/or a failed heat exchanger.
A blocked chimney can fill the area where the heater is located and the first floor with toxic C0 gases in
a few hours, depending on how much air flow there is in the house. In most situations, a blocked chimney is relatively easy to clear.
A failed heat exchanger is much more difficult to determine, but, in almost all cases, is much less
dangerous than a blocked chimney. In fact, when the furnace's fan is running, the heat exchanger is pressurized from the house air side. In almost all cases, this pressure will not allow dangerous gases to
accumulate in the house air. The path of least resistance for these exhaust gases is up the chimney.
This may not be the opinion of most gas utilities in the country, which is somewhat understandable based on the liability exposure.
The pressure on the heat exchanger has a significant effect on the tendency of flue gases to pass from
one side of the heat exchanger to the other. If the fan is off, the pressure from the burner will cause the burner side to be positive and the C0 or C02 gas can pass to the house side. The amount of gas passing
from one side to the other is based on the size and location of the failure in the heat exchanger.
However, it is rare this amount would exceed the amount of C0 or C02 gases emitted from a kitchen gas range flame.
When the fan comes on, the house air side of the heat exchanger, in almost every case, is positive. The positive pressure from the house air or fan side would cause the house air to be pushed into the exhaust
side, not vice versa. The only exception may be some power burners which would maintain positive pressure on the burner side while the fan was on or a heat exchanger failure which was large enough to
get your fist into.
The main thing to remember is that high pressure will always move to a low pressure. There are a few other factors which must be added to be totally accurate. These would include the location of the failure and the design of the heat exchanger.
I am not trying to say that failed heat exchangers are safe, but would like you to know it is rarely as
much of a concern as we hear from most information sources.
One last item: According to the American National Standards, it is almost impossible to construct a heat exchanger that is entirely air tight. Therefore, any test method developed to detect flue gas leakage
needs to have quantitative aspects. It would not be desirable to identify as unacceptable any heat exchanger leakage that meets the equirements/standards for heat exchanger joints. This standard says the leak should not be more than 2% of the flue gases with the internal pressure raised to .1 water
column (WC) static pressure.
Information provided by Michael Del Greco, New Jersey Home Inspector Lic. GI 0121, American Society of Home Inspectors Member 102273, Pesident of Accurate Inspections, Inc. A West Paterson New Jersey Home Inspection firm.
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