Abstract: A flammable vapor detector is used to detect flammable vapors in the event of a volatile organic liquid spill near a gas fired appliance, such as a hot water heater. The appliance is mounted at an elevation above the floor while the detector is mounted at or near the floor where the spill may occur. The detector is connected electrically to a safety gas cut-off solenoid valve in the appliance to urge the valve to cut-off the flow of gas to the appliance in the event that the detector detects flammable vapor. Because the detector is below elevation of the appliance, the detector reacts to higher concentrations of flammable vapors than the concentrations of flammable vapors at the higher elevation of the appliance to turn off the gas to the appliance before the combustion air to the appliance reaches a lower flammability limit. The appliance used to describe the invention is a gas fired hot water heater.