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Technical Letter


From Gas Forced Air Furnaces

Low Profile Problem

According to the National Center of Health Statistics, there were 594 deaths from unintentional carbon monoxide poisoning in 1991.  It is rare that news or information on this type of death reaches the public through the media.  The Vitas Gerulaitis death in 1994 raised attention to the problem but the public may not have been able to relate to this incident because of its unusual nature.  The tennis celebrity succumbed to carbon monoxide poisoning when a faulty swimming pool heater exhausted combustion gases into the cottage in which he was sleeping.

Without a detection device it is very difficult to determine that house air has significant levels of carbon monoxide(CO).  CO is colorless, odorless, and tasteless.   In the initial stages, CO poisoning will cause an individual to have a headache, to feel dizzy and nausious, and to feel fatigued.  These are all symptoms of the flu and can be misinterpreted.

Lack of understanding of the nature of CO has caused some homeowners to overreact with unnecessary purchases and in other cases to not react when appropriate.  Home occupants are better prepared to avoid problems with CO if they understand where CO comes from and how to prevent the intrusion of this poisonous gas into the home.


Burner Chemistry

The source of CO in the home is at any area of combustion and can include the furnace or boiler, stove burners, ovens, water heaters, and automobiles operating in a closed garage.  Of the home appliances, the furnace or boiler is generally the largest source of CO because of the amount of fuel consumed.

CO is usually a part of the by-products of combustion as described in the following.

Natural gas is a mixture of 80 to 95% methane (CH4 ), 5 to 15% ethane(C2H6) and a small amount of methyl mercaptan which is an odorant that allows for leak detection.   When the oxygen from air is mixed with natural gas in a burner fire, the products of combustion will be heat, non-toxic carbon dioxide (CO2), water vapor, and other by-products including CO.  The amount of CO in the by-product mix depends on the cleanliness and tuning of the burners, on the quality and quantity of combustion make-up air, and other factors.

When there is an inadequate amount of combustion make-up air, then insufficient amounts of oxygen will cause larger amounts of CO to form and less amounts of CO2.   When there are leakage problems with the furnace operation such that the products of combustion become part of the combustion make-up air, then CO2 will be burned to form more CO as a product of combustion.  The amount of CO buildup in a home will depend on the amount combustion gas leakage and amount of outside air changes in a home.  The more the combustion gas leakage and the less the outside air changes, the more serious the problem.


Furnace Operation

Forced air furnaces are designed so that the products and by-products of combustion are channelled to the flue pipe and exhausted to the atmosphere via the chimney after some of the heat is grabbed at the heat exchanger.  The blower pushes air around the heat exchanger and delivers heated air to the vent system. ( See sketch) The amount of heat delivered to the vent system as a percentage of the heat generated determines the the efficiency of the furnace.  The range of steady state efficiencies of furnaces in place today is at 60 to 90% or more.  For a 60% efficient furnace, 40% of the heat generated at the burners is lost mostly out the flue pipe.

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Damaged Flue System

CO, as part of the combustion gases, can escape into the living area of a home if the flue pipe or chimney is damaged or blocked or if the heat exchanger is damaged.

When the flue pipe or chimney is damaged or blocked so that the combustion gases including CO are not properly completely exhausted to the atmosphere, the conditions regarding furnace safety are most serious.  If the heat exchanger has small damage holes, normally only a small fraction of the combustion gases will infitrate the home.   A damaged or blocked flue pipe or chimney can mean that a large percentage or even the total amount of combustion gases including CO and CO2 will migrate into the home.  When the CO2 recirculates and becomes part of the combustion make-up air, increasing amounts of CO will be produced.   This cycle can repeat itself until dangerous levels of CO exist in the home.

The flue pipe or chimney can become blocked by falling masonry debris, the buildup of ash, fallen birds or animals, or in other ways.  Combustion gases can also escape into the living area at open joints or corrosion holes in the flue pipe.

One of the worst cases of deadly asphyxiation due to a malfunctioning flue pipe system occurred on the southwest side of Chicago several years ago.  A family of ten were killed by CO poisoning when the flue pipe became disengaged causing all of the combustion gases to escape into the living area.  The furnace was only one year old.  It had a flue pipe which was partially covered by a removable panel on the furnace housing where the disengagement occurred.  The family must have had some warning something was wrong because a furnace serviceman came to the house the day before the bodies were discovered.

While problems with the flue system are most serious, proper inspection can be accomplished with a combustible gas sniffing device and by a visual inspection.  If the flue system is substantially blocked, combustion gases will back-up at the furnace housing which can be detected by a sniffing device.  The sniffing device can also be used to detect other combustion gas leaks at flue pipe joints.

Damaged Heat Exchangers

Damage to heat exchangers that cause combustion gases to migrate into the heated air vent system should also be addressed during inspections.  Corrosion and stresses caused by repeated heating and cooling action can cause openings or fractures on the heat exchanger.  If it can be shown that a significant amount of gases including CO escape into the vent system, then the furnace should be replaced.  Repairing heat exchangers is generally not a practical alternative.

The process of determining which heat exchanger should be condemned and which has tolerable damage is probably the most argumentative area in inspections of gas forced air furnaces.  For instance, many furnace servicemen and home inspectors will condemn a furnace with small hairline cracks in the heat exchanger walls without further testing.   The American Gas Association recommends testing with the injection of trace gases into the heat exchanger and the use of sniffing devices capable of measuring gas leakage before passing final judgement.  Lennox Industries specifically tells servicemen working on their equipment in a procedure manual the " The presence of a crack in a heat exchanger does not mean the heat exchanger should be condemned."   Lennox recommends the procedure developed by the American Gas Association and specifically the use SENSIT/HETKIT heat exchanger test equipment.

A home inspector working within normal guidelines of non-invasive testing, cannot give complete assurance about the integrity and safety of a heat exchanger.  Visual inspections of the exchanger with a flashlight and mirror while the burners are in place have limited value.  Use of gas sniffing devices in the heating vents during normal operation is also of limited value.  Carbon monoxide detectors are only effective if the inspection is done during the heating season.  For best evaluation of a heat exchanger, homeowners should use servicemen that have test procedures recommended by the American Gas Association.

A heat exchanger in a furnace without jet type burners can have a hole in it, even a large hole of more than one-half inch in diameter, without any escape of combustion gases into the vent system while the fan blower is operating.  That is because the fan exerts a positive pressure over most of the heat exchanger surface.  Tests have shown that this positive pressure is effective in controlling gas leakages except in the upper reaches of the heat exchanger where due to the shape of air draft lines around the heat exchanger, this positive pressure will not exist.  For furnaces with jet type burners, holes in any location of the exchanger are susceptible to combustion gas leakage.   Tests have also shown that furnaces with induced flue draft fans, such as are found on high efficiency systems, will not leak gases into the heat vent system even if there are small holes in the heat exchanger.

This does not mean to suggest that heat exchangers with damage holes should continue in operation.  The AGA test method should be used as a pass or fail guideline along with common sense.


Safety Devices

Some furnaces have a heat sensitive device at the flue gas exhaust area to shut the furnace off if significant blockage occurs in the flue pipe.  Some high efficiency furnaces have induced draft flue gas fans which offer safety against leaking heat exchangers.

The best safety device for protection against CO poisoning is a CO detector.  The problems with CO detectors in Chicago this past winter were due to oversensitivity.   Newer models have corrected this oversensitivity.  Every home should have at least one detector with an adequate alarm.  It is recommended that each home should have 2 detectors with one in the bedroom to allow for a possible faulty detector.

Robert V. Gallo P.E.