Restaurant Water Heater Catches Fire Resulting in Fire and Water Damage
August 12, 2019
Regulated industry sector: Gas - Natural gas system
Qty injuries: 0
Injury description: NA
Injury rating: NA
Damage description: Minor fire damage, water damage due to fire suppression system
Damage rating: Moderate
Incident rating: Moderate
A commercial water heater located in a restaurant caught fire resulting in minor fire damage and significant water damage due to fire suppression efforts.
Site, system and components
A commercial water heater consists of a natural draft burner, heat exchanger, vent and combustion air supply. The natural gas burner is connected to a 2 PSI (high pressure) natural gas system via black iron pipe. A pressure regulator reduces the inlet pressure to the gas burner from 2 PSI to 7-14” WC (low pressure).
On a call for heat the gas pilot will light. Once the pilot flame has been proven the gas valve will open, supplying gas to the burner at a manifold pressure of 3.5” WC. A temperature control turns the tank burner on and off.
The burner relies on a supply of fresh air ducted into the room from outside in order to achieve complete combustion. Natural buoyancy entrains the products of combustion through the heat exchanger and out the top of where they are emitted to the outside via a galvanized metal vent.
Exhaust and make-up air system
A commercial kitchen consists of a make-up air and exhaust system. The exhaust system consists of a large hood, under which commercial cooking appliances are located, (i.e. Deep fat fryers, ovens, ranges). This exhaust hood incorporates a fan that draws grease laden vapours and products of combustion to the outside at a rate of 6000 CFM (cubic feet per minute).
A means of replacing the exhausted air is required in order to pressure balance the kitchen/restaurant. An indirect-fired make-up air unit supplies the commercial kitchen with supply air equivalent to the amount of exhausted air. The Make-Up Air unit consists of a supply fan, a gas-fired burner and a heat exchanger.
In a properly set-up kitchen the make-up air and exhaust systems are balanced to ensure that the amount of make-up and exhausted air are equal. This prevents the ambient pressure within the restaurant from being overly negative or positive. The make-up air and exhaust system are meant to be used simultaneously in order to ensure that the static pressure of the restaurant/kitchen remains close to neutral. This is of critical importance when natural draft appliances (i.e. a natural draft water heater) are installed within the same structure. Natural draft appliances are adversely affected by negative air pressure. Figure 1 below illustrates airflow in a balanced gas system contrasted with the actual configuration found.
A remote panel in the kitchen controls the make-up air and exhaust system. It is operated by kitchen personnel. The remote panel engages both the make- up air unit and the exhaust fan simultaneously.
A restaurant/kitchen was subject to a negative atmospheric pressure due to a faulty make-up air/exhaust system.
A gas contractor replaced the hot water tank located in a small mechanical room within the restaurant.
The mechanical room was subject to the atmospheric pressure in the restaurant due to a gap in the bottom of the mechanical room door.
The gas contractor performed tests on the water heater over the following days and found the water heater to be operating normally (these tests were likely performed without the exhaust fan running and under neutral atmospheric pressure conditions.)
Because the mechanical room contained some of the building’s few passive air openings (combustion air duct and water heater exhaust vent) the mechanical room was subject to a significant negative draft when the exhaust fan was running. Under these conditions air was pulled from the outside through the combustion air duct and vent. This resulted in the products of combustion spilling into the mechanical room. The significant air movement at the location of the water heater burner (in close proximity to the gap under the mechanical room door) in conjunction with the backdraft through the heat exchanger pulled the flame from the underside of the burner tray, melting the gas valve wiring and subsequently engaging the sprinkler system.
Facts and evidence
Make-up Air/Exhaust System
The wiring in the remote control panel that enables the make-up air and exhaust units was found to be modified in such a way as to bypass the make-up air unit. In this bypassed wiring arrangement the make-up air unit would not engage even when the remote panel switch was turned on. In this configuration only the exhaust fan would engage when the remote panel was turned on. This created a negative (pressure) atmosphere in the restaurant when the exhaust fan was running.
When the bypass was removed the make-up air unit was found to function simultaneously with the exhaust system.
The static air pressure inside the restaurant was measured at -.16” WC with the exhaust fan running and the make-up air off (this was the post incident as-found site condition). In a balanced system this pressure would be neutral.
An air balancing test performed under the as-found condition (i.e. Exhaust fan running, Make-up air disabled) showed a differential pressure of .0250” WC measured across the mechanical room door where the water heater was located. This measurement demonstrates that air was being drawn out from the mechanical room through the underside door gap.
At an undetermined time the make-up air/exhaust remote panel was modified (by-passed) so that the make-up air unit would not engage when the control was energized.
The combustion air inlet and the water heater vent are passive building openings susceptible to negative draft when the kitchen exhaust fan is running without make-up air. Backdraft occurs when the flow of flue exhaust gasses reverses in the vent, drawing the products of combustion back through the flue vent and heat exchanger exiting through the burner tray to the underside of the tank, causing the flame to roll out to the outside of the tank.
The aluminum pilot tubing on the underside of the burner tray was found to be melted indicating that the pilot tubing was subject to direct contact with the burner flame. The pilot tubing not be subject to excess temperatures under normal operating conditions. If the burner flame is drawn to the outside of the tank the pilot tubing would be in direct contact with the flame.
The wiring associated with the water heater gas valve was melted indicating that the burner flame was drawn to the outside of the water tank, at which point it made contact with the gas valve wiring.
Testing of the damaged tank at a remote facility indicated that the manifold pressure of the burner was close to the range specified on the rating plate.
The water heater had an appropriately sized vent and combustion air duct that would allow for correct operation under normal conditions.
The gap on the underside of the mechanical room is located near the water heater burner tray, resulting in significant air movement at that location when the exhaust system was running. An atmospheric burner is highly susceptible to air flow and the flame can be pulled off the burner and outside the tank when a negative pressure condition is present.
Restaurant staff stated that the temperature in the mechanical room was hotter than usual and they expressed their concerns to the installing gas contractor.
The gas contractor who installed the first tank stated that a draft test and gas manifold test were performed on the new water heater and found to be normal (kitchen exhaust fan may not have been running at this time and the ambient pressure may not have been negative).
The gas contractor who replaced the fire-damaged tank with a direct replacement stated that he observed a significant downdraft in the water heater vent and combustion air when the building openings were closed and the exhaust fan was running (No make-up air) . Upon initial start-up under these conditions he observed a yellowish flame coming up from the burner in close proximity to the gas valve. This demonstrates that the negative draft conditions in the restaurant were sufficient enough to pull the flame from out from the burner tray.
Causes and contributing factors
It is likely that the modifications made to the remote panel that by-passed the make-up air unit resulted in a negative building pressure that adversely affected the safe operation of the water heater burner by pulling the flames to the outside of the tank.
An interlock (i.e. air pressure switch or current sensing relay) between the exhaust fan and make-up air unit would have prevented the exhaust fan from running without proving that the make-up air unit was running in tandem. This interlock would help to ensure that the building atmosphere would not become negative.
The gas contractor did not recognize that the water tank was subject to negative pressure. Testing of the water tank with the exhaust fan running would have identified the negative pressure and its adverse effect on the water heater.
Figure 1: Airflow diagrams comparing the observed system configuration (failure scenario) with airflow in a balanced system
Photo 1: Water heater located inside the restaurant mechanical room
Photo 2: The melted wiring on the gas valve serving the water heater indicates that the flame was pulled outside of the burner tray.
Photo 3: The melted aluminum pilot tubing on the underside of the burner tray indicates that the burner flame was pulled to the underside of the burner tray
Photo 4: Remote panel that enables the make-up air/exhaust system. The “system” switch is meant to engage both the make-up air and exhaust systems.
Photo 5: Inside the cover of the remote panel showing the wiring modifications that disabled the make-up air unit
Photo 6: Make-up air unit located on the roof of the restaurant. This make-up air unit was disabled as a result of the remote panel modifications