The California Mechanical Code (CMC) is one of thirteen parts of the California Building Standards Code that is adopted into law every three years by the California Legislature. The other 49 U.S. states adopt similar safety codes, which generally include the following titles (preceded by ): Building Code, Electrical Code, Fire Code, Mechanical Code, Plumbing Code, Residential Code, etc.

One of the primary objectives of the mechanical code is to help ensure that heating, ventilating, and air conditioning equipment installed in buildings are designed, operated, and maintained safely. Many HVAC systems utilize natural gas, which is highly flammable and can cause explosions.
This author investigated a natural gas explosion (also called deflagration, or subsonic combustion wave) that was caused by a series of maintenance errors, a heater malfunction, and a major code violation.

The maintenance errors caused natural gas to be released into a heater room, the code violation permitted the natural gas to accumulate in the room instead of being safely vented outdoors, and the malfunction permitted the heater to re-start automatically in an unsafe state, which ignited the explosion. Two workers received serious burn injuries from the incident, but the explosion wasn’t strong enough to damage the building.

It was difficult to rank the errors and defects according to their level of egregiousness, but the worst one was undoubtedly the combined design defect and construction defect associated with the building that housed the gas-fired heater. The heater was located at ground level inside a 23-foot tall enclosure construction from concrete masonry units (i.e., cinder blocks). The architect was responsible for the defective design, which contemplated heating equipment inside the room but didn’t incorporate the code-required ventilation area. The general contractor and appliance installer were responsible for allowing the heating equipment to be installed in the room without the proper ventilation.

The CMC requires ventilation at the top of any enclosure that houses gas-fueled appliances. The purpose is to vent natural gas (which is lighter than air) in the event of a substantial release of gas into the indoor space. Allowing flammable gas to accumulate in an enclosure is the first step in the process of forming an explosive device that lacks only an ignition source to turn into a horrific fireball or a destructive blast wave. The subject room was well sealed along the upper 75% of its height but was equipped with a louvered door at the bottom that effectively allowed combustion air into the room to supply oxygen for the heating appliance. Combustion products from the heater were vented directly to the outdoors by an electric blower, and the replacement air entered through the door louvers.

When the gas pipe developed a leak (the facts weren’t entirely clear about the size of the leak was or how it began), the gas rose to the ceiling and accumulated there, displacing the air below it to the outside environment through the door louvers.

This author performed a Large Eddy Simulation (LES) of the gas accumulation phase which showed the steady-state fuel gas concentration in the upper three-quarters of the room to be substantially greater than the Upper Flammable Limit for natural gas (approximately 15 percent by volume). This fact turned out to be the sole reason the building didn’t explode – a large fraction of the fuel gas present in the room had accumulated in zones that were too rich to burn (not enough oxygen present).
Nevertheless, when the employees were instructed to enter the room and shut off the gas to the heater, their motion created a flammable zone in some portion of the lower 25% of the room’s volume. When the defective heater ignited the flammable mixture as they were exiting the room, the fireball that was created pushed flames out through the open door and burned them badly as they tried to escape. Thankfully both survived.

Posted below are two videos showing the LES simulations for Case 1 – as installed without any venting at the ceiling, and Case 2 – as required by code, with a code-compliant opening of only 150 square inches of flow area at the top of the heater room. Case 1 shows high gas concentration (red) from ceiling down to the top louver of the entry door when gas is flowing and no significant dissipation after the gas source is shut off. Case 2 shows a temporary accumulation of moderate gas concentration (green) until the gas source is shut off, after which full dissipation occurs through the upper vent. The simulation runs approximately 24x faster than real time.

This gas accumulation simulation (along with testing of the defective heater, timeline analysis of witness testimony, and plumber standard of care analyses) helped the parties reach a resolution in this case.

Unintended water release in a residence is a frequent cause of damage (and insurance claims). Often, the water released is caused by a worn-out washing machine hose that ruptures, or an ice-maker supply tube that flexed one too many times and sprang a leak. These failures can result from a hose or tube defect, but often, they are caused by homeowner neglect or failure to replace an aged part.

When water damage begins suddenly in the attic space of a home, it may be tempting to blame it on a leaky roof. But if the weather has been clear for several days prior to the incident, an engineering investigation is in order.

Exactly this type of loss occurred in a 23-year-old Southern California townhouse.  The homeowners awoke in the middle of the night when they heard running water and found their bedroom floor covered with about an inch of water.  When the fire department responded to the homeowners’ emergency call, they found an air handling unit (AHU) in the attic was the source of the water streaming down the walls into the living space below. According to the homeowners, the air handler had been repaired in December, just two months before the incident, because it had not been providing any heat on cold nights during the early days of the winter season.

During our inspection, we discovered that the AHU contained 3 sets of coils: Two sets of coils in an older unit (blue and violet arrows in right photo below), and one additional coil that appeared brand new, but contained several cracked u-bends in the copper tubing (green arrows in left photo, coil removed in right photo). The first coil (blue arrow) was corroded and disconnected, and we determined that this was the hot water coil that had been in service until being replaced in December.  The second coil (violet arrow) was connected to refrigerant tubing that ran outside to a compressor/condenser unit. And the third coil (green double arrow in left photo indicating cracked tubing) was located downstream of the other two.

The cracks were obviously the source of the water leak, and we hypothesized that they were caused by freezing of the water inside the tubes, but the sequence of events wasn’t obvious until we examined the weather history from December to February. That winter, the weather was generally mild, but there was (a) a cold spell in December, about the time the AHU was repaired; (b) a spell of unusually warm temperatures in January, when air conditioning was needed in the townhouse; and (c) another cold spell right when the water release occurred, a time when heating was needed again.

It was obvious that when the HVAC technician installed the new heating coil, he violated a well-known mechanical code requirement – never install a water coil downstream of a refrigerant coil because the chilled air is cold enough to freeze the water, which causes ice expansion that leads to bursting of tubing and piping. Clearly this aspect of the installation was defective, but additionally, there was yet another defect that contributed to the damage.  He also failed to install a pan and drain underneath the new heating coil. When the homeowners called for heat during the February cold spell, and water began flowing out of the tubes that were ruptured during the January warm spell, it ran directly downward to the plywood flooring in the attic, rather than being directed outside via a drip pan and drain pipe.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)