Boilers & Furnaces
Heating plants in homes and commercial structures must be closely evaluated for involvement in the cause of a fire. Most furnaces and boilers are fueled by either compressed gas (natural gas or liquefied petroleum) or #2 fuel oil. Heating devices using solid fuel will be discussed separately. An understanding of the characteristics for the specific fuel being used is a must prior to beginning the investigation. Such factors as the specific gravity, vapor density, polar or non-polar solvent, ignition temperature, and flash point are just a few of the factors that are also extremely important in determining the origin and cause of a fire. Once the type of fuel and its characteristics are identified, the type of appliance needs to be evaluated.
Most home heating plants (whether forced air or heated water) work on the same basic principal. Some type of medium, either air or water, is heated and circulated throughout the structure. Most forced-air furnaces have a heat exchanger that is heated by the combustion of fuel as air is circulated around it. The key for proper operation of this type of system is the complete separation of the exhaust gases and the heated air. If a failure occurs in the heat exchanger, the result can be hot and toxic gasses being circulated into the conditioned air space. The potential for deadly carbon monoxide levels is a primary concern when a failure of a heat exchanger occurs.
In certain applications and installations, if a failure in the heat exchanger occurs, the cabinet of the furnace may become hot enough to ignite nearby combustibles that ordinarily would not be subject to elevated temperatures. In a boiler situation, the release of carbon monoxide from a leaking heat exchanger can be as deadly as that of a forced-air furnace. If the boiler is in a common air space with the living quarters, the danger is even greater.
Many times, when heating plants are involved in the cause of a fire, it is due to the placing of combustibles too close to the unit. Exhaust temperatures of heating plants that are less than 90% efficient can easily reach over 400 degrees Fahrenheit at the exit point from the unit. This is more than ample temperature to ignite combustible items that may be in contact with the exhaust. Materials such as cloth or clothing that are in contact with a normally operating heating plant can cause an insulating effect on the portion of the unit it contacts. This condition can cause temperatures to elevate above normal due to the restriction of cooling airflow.
Furnaces having an efficiency rating of over 90% and that are fitted with PVC (plastic) vent systems are much less likely to be involved in fire causation. These units’ exhaust inherently operates at a much lower temperature than the less efficient units. Even if a malfunction of the burner occurs in a high efficiency unit, many other sensors and safeguards are in place to prevent overheating of the plastic exhaust system.
At Donan Engineering we have fire investigators, mechanical engineers, and electrical engineers with many years of experience in evaluating failure modes of home and industrial heating plants.