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Heat Exchanger Failure

Dangers of a leaking furnace heat exchanger

This web page has been written to provide information regarding cracked heat exchangers on warm air furnaces. This information does not apply to hot water boilers (baseboard, Hydro air, for radiant heat).

A warm air furnace is structurally designed to isolate the air warmed to heat your home, from the products of combustion. Combustion by-products (also referred to as flue-gases) are created from the flame that was used to warm the furnace. Whether a furnace burns wood, fuel oil, propane or natural gas, the flue gases (smoke) need to be vented to the outdoors.

Flue gas that is vented outdoors via chimney, direct venting or power venting needs to be removed from the furnace completely. Flue gas is comprised mostly of Carbon Dioxide (CO2) and Water Vapor, but may also contain some unsafe elements such as unburned hydrocarbons and carbon Monoxide (CO), which is a deadly tasteless and odorless gas. The combination of carbon monoxide and unburned hydrocarbons should never be introduced into the warm air that circulates from the furnace to warm the home.


The furnace heat exchanger is responsible for the separation of flue gasses and the filtered warm air. Heat exchangers are manufactured to be air tight and made to withstand the heat from the burner’s flame during operation. A furnace heat exchanger can crack or burn through due to age or being overheated for a variety of reasons. A furnace with a compromised heat exchanger must be taken out of service until the heat exchanger or the furnace is replaced.


Q&A

I am told my heat exchanger has failed, however my furnace seems to running fine, and my CO detectors have not gone off, is my system still safe to operate?

No. In many furnaces, ignition and combustion take place first followed by a several minute delay before the air blower system starts. During this initial start up phase, flue gases comprised of of CO2, Water vapor, CO (Carbon monoxide) and unburned hydrocarbons will enter the ductwork due to positive pressure from the combustion chamber. After the air blower system engages, the air pressure may reverse, causing air from the blower system to enter the combustion chamber. While this often prevents dangerous flue gases from entering the ductwork, it severely affects the combustion operation of the oil or gas burner by introducing excess air into the combustion chamber. To obtain clean and reliable combustion, there has to be a precisely measured balance of fuel and air, if this balance is disrupted, the burner will become unstable, unreliable and potentially dangerous.

Am I wasting fuel by operating a furnace with a faulty heat exchanger?

Yes. In addition to being unstable, the burner system will also become grossly inefficient, and can lose more than 50% of its energy efficiency.  The flue gas of a properly tuned oil burner will produce a benign mixture comprised mostly of CO2 and water vapor, with nearly undetectable levels of CO and unburned hydrocarbons, however an oil burner operating within a furnace that has a breach in the heat exchanger will often produce dangerous levels of carbon monoxide (CO), carbon particulate emissions, and varying degrees of unburned hydrocarbons.

If I buy a replacement furnace, is it likely the heat exchanger will fail again?

Low cost furnaces have a life expectancy of only 10-15 years.  Sippin Energy recommends furnaces with limited lifetime warranties, such as the ThermoPride Brand.


Testing Methods

A heat exchanger can be reliably tested by utilizing combustion testing equipment.  This type of test is necessary, because in most cases of heat exchanger failure, the breach cannot be seen with the naked eye.

  • Combustion Analyzer – CO2 or O2 method
    • Properties of combustion are measured during most combustion related service calls. These vital components of combustion include; Carbon Dioxide, Oxygen, flue pipe negative pressure, combustion chamber pressure, temperature, and degree of smoke are recorded, and are needed to calculate a combustion efficiency test.
    • To test a heat exchanger with CO2 or O2, a reading is taken with the burner running in a stable state, WITHOUT THE FURNACE BLOWER OPERATING. That reading is observed and recorded. When the furnace blower comes on, a technician will observe and record  ANY change in the O2 or CO2 original reading. If the blower has ANY influence in the reading, the heat exchanger is not air-tight.

Heat exchanger failures can also be characteristically different depending on the location of the breach. For example, in a lowboy style furnace (like the one illustrated above), a breach in the vent connector portion of the heat exchanger will likely introduce flue gases into the duct system continually because the breach is located immediately before the blower assembly. If this is the case, it can often be evidenced by flue gas contamination of the air filter, which will show evidence of soot contamination (see photo).