Final answer:
Water heaters in garages must elevate burners and ignition devices unless they are electric water heaters, which don't have burners that can ignite flammable vapors. Natural gas water heaters are about 85% efficient, whereas electric heaters are 100% efficient in-transfer but may be less efficient overall due to fossil fuel-based electricity generation and transmission losses.
Step-by-step explanation:
In the question, we are discussing the safety regulations for the installation of water heaters, specifically the requirement that the burner and ignition devices must be elevated at least 18 inches above the floor to prevent ignition of flammable vapors.
Water heaters in residential garages must have burners and burner-ignition devices located at least 18 inches above the floor to prevent fire hazards from flammable vapors that may be present near the floor level. This rule can be exempted if: 1) they are electric water heaters. Electric water heaters do not have burners that might ignite flammable vapors. By contrast, natural gas or propane water heaters do have such burners, and as such, they need to be elevated if installed in a garage. The provision does not apply to water heaters located in basements where the risk associated with flammable vapors at floor level is not the same as in a garage.
Now let's assess the efficiency of different water heaters and their total fossil fuel energy use. A natural gas water heater has an efficiency of about 85% for transferring combustion heat to water, while an electric water heater can have an efficiency of 100% using a heating coil immersed in the water. However, when considering the electricity source, if it's from a natural gas power plant with a 40% efficiency in converting thermal energy to electricity, and it's transmitted at a 95% efficiency, we must calculate the overall energy usage.
To determine which method uses more total fossil fuel energy, and by what factor, we need to consider the entire energy conversion and transmission process for the electric heater. Even with perfect in-home efficiency for the electric heater, the inefficiencies in electricity production and transmission typically mean that using an electric heater powered by fossil fuels indirectly uses more fossil fuel energy compared to a natural gas heater.