INVESTIGATION OF HEATING EQUIPMENT

INVESTIGATION OF HEATING EQUIPMENT

BY ARTHUR L. JACKSON

The heating system should be inspected in every investigation of a residential fire to determine if it could have caused the fire. If the investigation indicates that the system is the definite or a possible cause of the fire, it should be thoroughly examined.

The two basic fuels used in residential home heating appliances are fuel oil (usually number two grade) and natural gas. In areas without a natural gas utility, propane is used. Except for the gas-delivery mechanism, propane and natural systems operate essentially the same.

STEAM

Steam is the most common type of residential system used in the Northeastern United States. Steam boilers, whether fired with fuel oil or gas, function with the same operating controls. Usually, the controls for a fuel oil boiler operate on 120-volt current. Natural gas and propane heating-system controls operate on low voltage, usually 24 volts.

Operating controls. Among the operating controls on a typical oil-fired steam system is a primary safety control, which is connected to the oil burner. This control monitors the firing of the oil burner to ensure safe operation. If the flame is lost after initial ignition or if no initial ignition occurs, the burner shuts down and locks out on safety. The burner can be restarted only by manually resetting the lockout safety button on this control.

Older primary safety controllers were connected to the oil-fired appliance`s smoke pipe and had a safety delay of approximately 90 seconds. This stack controller monitored the heat produced by the fire; if no heat traveled through the smoke pipe, it would shut the burner down on safety.

Approximately 15 years ago, the primary safety controller was replaced with a cadmium sulfide flame-detection system mounted directly on the burner. A cad cell eye is located in the burner`s blast tube, and the light produced by the fire in the combustion chamber transmits a signal through the cad cell to the primary safety control. These newer types of primary safety controls have a safety shutdown of approximately 45 seconds–approximately half that of the older stack safety control relays.

In addition to the primary safety control, other operating controls are normally connected in series to the primary controller on a boiler. These controls include a pressure controller to operate the steam boiler on approximately two to five pounds of steam pressure. Additionally, a very important control to the fire investigator is the low-water cutoff control used to shut down the boiler if the water gets low.

The most common type of control on today`s market is a float control, which has a water chamber and a mechanical float assembly. The high incidence of failure for this type of control has led to the increased use of a new type of control that uses a probe instead of a float. The float controls usually become clogged with mud and muck from the steam boiler`s normal operation. The probe-type low-water cutoff control uses low voltage between two probes, which extend into the water side of a boiler. In the presence of water, the probe allows electric current to travel between the two internal probes. In the absence of water, this circuit is opened, and burner operation ceases.

* During the postfire investigation, you can easily test the float-type low-water cutoff assembly with an ohmmeter. If no water is present in the boiler, an ohmmeter between the two terminals on the back of this control should indicate an open circuit. If this circuit is in the closed position and there is no water in the boiler, the low-water cutoff control has failed.

This finding is important for the following reasons: If a boiler continues to operate with no water, the boiler will severely overheat. The investigator examining the internal flue surface of the boiler can easily detect a severe bluing and discoloration of the cast iron, clearly indicating that a dry firing has occurred.

Storage and fuel delivery. Fuel oil for the oil burner system is usually stored in outside underground storage tanks with a capacity of 550 gallons and higher or in inside or outside aboveground 275-gallon fuel oil storage tanks. Soft copper fuel lines with 38- or 12-inch outside diameters usually carry the fuel oil to the oil burner.

The fuel oil enters the burner through the fuel pump, which increases the pressure of the fuel oil to approximately 100 pounds pressure for delivery down through the nozzle draw assembly and out through the oil burner nozzle. The oil burner nozzle breaks down the fuel oil into millions of microscopic droplets to ensure virtually complete combustion with practically no residual smoke or soot. The fire in the combustion chamber then allows the heat produced to travel up through the upper portion of the boiler, the flue passages.

The flue passages allow the heat to transfer through the cast iron into the water side of the boiler. The heat generated in the combustion chamber then transfers to the water in the water jacket side of the boiler and turns the water into steam for distribution through the steam riser pipes and ultimately to the individual room radiators. The heat in the boiler combustion chamber reaches temperatures of between 1,5007F and 2,0007F.

Boiler Types

Two types of boilers are on the market today: dry base and wet base. The type of boiler is significant for the fire investigator for the following reasons.

Wet base boiler. In a full wet base boiler, the boiler water jacket assembly surrounds the entire combustion chamber on five sides–top, bottom, back, right, and left. The heat shield is in the lower portion of the boiler, in front of the combustion chamber to which the oil burner is connected.

Dry base boiler. On a dry base boiler, the entire bottom portion of the boiler surrounding the combustion chamber is a steel outer shell and has an insulating refractory material lining inside the combustion chamber.

Some fires result from a base burnout, usually the result of a lack of proper maintenance or an overfiring in the combustion chamber. When the base of a dry boiler burns out, the heat and products–heat, smoke, and soot–from the combustion chamber are expelled from the bottom side or back of the boiler instead of traveling up through the boiler`s vertical flue passages. If the boiler is in a room with a wood-combustible wall or if other combustibles are surrounding the boiler, these items can be ignited and cause a fire.

* During the postfire investigation, remove the boiler`s outer jacket assembly and examine the areas surrounding the combustion chamber. This type of base burnout cannot occur if the boiler has a full wet base.

Fuel Leak

Another potential cause of fire involving oil-fired heating equipment is a fuel leak at or near the oil burner itself. The most common cause of fuel leaks is loose fuel line fittings, a condition caused by the burner`s normal operating and firing or insufficient maintenance. (I have found that flare fittings are not susceptible to loosening from vibration and boiler movement.)

* During the postfire examination, examine the fittings. Inspect the integrity of compression-type fittings more closely. Usually when a fuel leak near the oil burner has caused the fire, the burner is heavily damaged. The fuel oil will travel along the floor into the vicinity of the combustion chamber and be ignited by the boiler`s normal operation. This occurs more readily when the boiler is a dry base boiler because the combustion chamber in this type of boiler is protected only by a steel outer shell and refractory insulation. The fuel oil on the floor migrates through this insulation and enters the combustion chamber.

The National Fire Protection Association Properties of Flammable Liquids, Gases and Volatile Solids lists the flash point of number two home heating fuel oil as 1267F to 2047F and its ignition temperature at 4947F.

As stated earlier, the combustion chamber temperatures in a normally firing oil burner combustion chamber are in the range of 1,5007F to 2,0007F, which could easily ignite the fuel oil migrating into it. The wet base boiler is usually set on cast iron legs and is two to three inches above floor level. Additionally, the area directly below the combustion chamber is usually filled with water, reducing the amount of heat transfer to the bottom of the boiler. Although it is possible for temperatures to reach 2007F and higher in this area–which could cause the fuel oil to vaporize, enter the combustion chamber, and burn back to the floor level–it is not as likely or as prevalent as in dry base boilers.

Additionally, if the fuel oil leak is at or near the fuel pump, fuel can be drawn in through the air intake, which is usually just behind the fuel unit, blown down the blast tube, and ignited from the boiler`s normal firing.

Other fuel leaks occur through deterioration or failure of the fuel oil pump seal, which is directly in the path of the combustion intake around the fuel pump`s shaft. Leakage in this area would also be drawn in down through the blast tube and ignited off the normal burner operation.

When the Boiler Is Involved

In some cases when the boiler is involved in a fire, the boiler`s exterior jacket may be already scorched. It may be difficult to tell whether the scorching is from external or internal overheating. Therefore, examine the inside of the boiler to determine if a dry firing has occurred. In addition to the discoloration of the boiler`s cast-iron flue passage, another good indicator that a dry fire has occurred is the absence of soot on the internal flue surfaces. Usually a small amount of soot builds up and, since soot is combustible, it will completely burn off when a dry firing occurs.

If the boiler was scorched from a fire unrelated to a boiler malfunction, the insulation provided by the boiler`s outer jacket assembly and the comparative airtightness of the combustion chamber and flue passages would prevent heat from entering the boiler and causing the conditions that would be observed when a dry firing occurs.

Fires have been mistakenly attributed to the steam pipes` contacting a structure`s wood framing during normal boiler operation. Subsequent investigation of these incidents revealed that overheating of a dry fired boiler actually ignited the wood-framing members. When a steam boiler dry fires, the cast iron or steel of which it is constructed absorbs tremendous heat and becomes cherry red. The heat is then transmitted up through the steel pipes through conduction. If the steam riser pipes are in contact with the building`s wood structural members, ignition could easily occur. This is not to suggest that the condition of pyrolysis does not occur. Pyrolysis, a well-known condition in the fire investigative field, refers to the continual heating and cooling of cellulose materials, such as wood, ultimately lowering the ignition temperature. I mention this to make certain that in your investigations you eliminate the possibility of a malfunction of a steam or hot water boiler when overfiring or dry firing occurs and the associated piping is heated beyond its normal temperatures.

NATURAL GAS-FIRED BOILERS

When investigating the burner assembly of a natural gas-fired boiler, closely examine every burner tube of the burner assembly to determine if the tubes ruptured or failed, thereby allowing natural gas to expel from the side or bottom of the burner tubes.

Normally, the gas is expelled up through the top slots, designed to limit the flow of natural gas and ensure a stable and clean flame above the burner assembly. The gas, introduced into these burner tubes through a small orifice, allows air to enter around the open end of the burner tube. The gas, either natural or propane, then mixes with the air and travels up through the slots on the top of the burner tube.

When a failure occurs in the burner tubes (resulting from a fracture, rot-out, or some other condition), the gas and air mixture travels to the path of least resistance, usually through these ruptures and holes. This makes the flame unstable and can direct the flame downward or sideways instead of up and through the boiler`s normal flue passages. In addition, the unstable flame has an imbalance of fuel and air and, in the presence of a rich mixture, causes a sooty and smoky fire, which ultimately will completely block the upper flue passages. When this occurs, the heat produced by the burner operation is redirected down and out the bottom of the boiler, causing a condition known as “flame rollout,” which would cause any combustible material in and around the boiler`s location to ignite.

Gas-fired heating boilers (propane or natural gas) operate in a slightly different fashion than those of oil-fired heating units. Although some power-flame units operate essentially the same as an oil burner–that is, gas is introduced through an orifice with a blower assembly providing combustion air–gas-fired units predominantly use a burner grid and natural drafting in their operation. These gas-fired boilers using the burner grid have the same operating controls as an oil-fired steam boiler in relation to the low-water cutoff and pressure control. As already noted, gas steam boilers usually use low voltage–24 volts–instead of 120 volts as oil-fired steam boilers. The important difference between oil-fired and gas-fired steam boilers is that the gas-fired boiler uses the natural drafting to carry the heat produced by the burners up through the boiler`s upper flue passages. As a result, the area directly below the gas burners is open to allow this natural drafting.

Consequently, the fire investigator must determine if the gas-fired boiler is on a combustible or noncombustible floor surface. Most manufacturers stipulate in their installation directions whether the boiler is suitable for combustible or noncombustible floors and supply a combustible floor kit that raises the boiler off the floor surface to prevent ignition of the floor surface below the boiler. If the floor around the base of a gas-fired boiler is the area of fire origin, you must determine whether the floor is combustible or noncombustible. I have seen instances where an installer placed ceramic tile over a plywood floor to make it noncombustible. This would not make the floor noncombustible. Heat would transfer through the tile and ignite the plywood subfloor.

Instead of a primary flame safety controller, gas-fired boilers use a thermocouple near the pilot light to ensure the safe operation of the gas burner assembly. Use the same procedures to examine the boiler`s interior as those used to determine if dry firing has occurred.

Another important point to remember is that all heating appliances–whether steam, hot water, gas, or oil-fired–have manufacturers` instructions detailing proper clearances of the heating equipment from combustible surfaces. These clearances are designed to ensure safe operation of the heating equipment during normal operation. Note that any boiler that malfunctions is capable of causing a fire even if the manufacturer`s clearance and installation requirements have been followed.

THE ROOM

During the postfire investigation involving heating equipment, carefully examine the area or room in which the equipment is installed, particularly the areas surrounding the smoke pipe and boiler top. In some cases, I have found metal hangers hanging from the boiler return piping in close proximity to the boiler smoke pipe. The smoke pipe on heating appliances generally heats to temperatures that could easily ignite clothing fabrics. Although the fire would burn away most of the clothing, the presence of metal hangers might indicate that combustibles were stored in close proximity to the heating equipment.

Additionally, examine the top of the boiler to determine if any combustible material, such as cardboard, was stored in that area. Many times, natural gas-fired hot water heaters are located near the heating boiler to provide hot water when the boiler is shut down during the summer months. Also, over the summer months, some items may have been placed on top of a boiler inadvertently while cleaning the basement and, if they are close to the smoke pipe, this combustible material can easily be ignited when the heating equipment operates for the first time in the fall.

THE WHITE GHOST

There has been some discussion concerning the phenomenon of a “white ghost” in relation to oil-burning equipment.1 The white ghost is merely atomized fuel oil and is much more dangerous in commercial and industrial applications, where large amounts of fuel oil are used for combustion. The potential for the white ghost has been greatly reduced as a result of technological advances in primary flame safety controls on large commercial and industrial boilers. Modern safety controls shut down burner operations in a millisecond should there be flame failure or no ignition. Additionally, the modern day flame safety controllers are computer-monitored and have sequential step ignitions that virtually eliminate the problem of the white ghost.

If you are confronted with circumstances beyond your expertise, call in others who can assist in the investigation. The team approach in a fire investigation helps to ensure that the conclusion will be competent and reliable. For help in evaluating heating appliances involved in fires, contact local dealers who work with the equipment on a daily basis. In addition to uncovering the exact cause of the fire, the knowledge gained can be applied to future investigations. n

Endnote

1. For additional information, see “The White Ghost” (Training Notebook) by Frank C. Montagna, Fire Engineering, May 1995, 16, and Letters to the Editor, Fire Engineering, August 1995, 43-46.

Examination of combustion chamber through the front lower door. Note the presence of soot. (Photos by author.)

(Top) Overall view of nozzle draw assembly. (Middle) This nozzle was disassembled in preparation for an examination of its interior. (Bottom) Nozzle draw assembly. Arrow indicates nozzle. Fuel oil spray is normally ignited by electrical spark from electrodes above nozzle.

(Top left) This dry base boiler (coal conversion) was the cause and origin of this fire. (Top right) Side view of oil burner showing flame pattern from below. (Bottom right) Test firing of oil burner. Flame is coming off burner head.

(Left) Oil stains are visible at the base of this boiler. (Right) An oil leak at the fuel pump seal.

Presence of soot in the upper flue passages indicates that this boiler did not dry fire.

(Top) “White ghost” produced by oil burner that is not firing. (Bottom) Front view of “white ghost” from oil spray.

n ARTHUR L. JACKSON is chief fire investigator for Peter Vallas Associates, Inc., a Hackensack, New Jersey-based company that provides fire and explosion analyses, investigation, and engineering services. He is also a fire official and a 29-year veteran of the Hasbrouck Heights (NJ) Fire Department.