At a time, within my memory span, the sidearm water heater was the method most commonly used to heat domestic water, with the exception of the tea kettle and the reservoir, on the cook stove. The heater was made up of a vertical coil of metal pipe, about two feet high, that was heated with either a gas or a kerosene burner. Another device that was used as a source of heat was a small, jacketed, coal or wood burner.
Regardless of the type of fuel used, the heater was connected, with piping, to the side of a thirty gallon water storage tank, hence the name “sidearm”. Those sidearm heaters were, indeed, a dangerous piece of equipment. I maintain that they were potentially lethal because there was no means of automatic control and, in most cases, there was no temperature or pressure relief valve on the tank.
At one time they were not especially dangerous when the water was supplied from a municipal water system. If the owner forgot to turn the burner off, the water only backed up into the city water system as it expanded from the excess heat. In a case such as this, the most serious thing that could result would be that your neighbors might get hot water from their cold water taps.
Later, cities adopted codes that required check valves in the supply lines immediately following their meters, because the hot water and steam was ruining their meters. After these checks had been installed, there was no place for the hot water and steam to go. Therefore, if left unattended, the pressure could rise to a point that the thirty gallon tank would rupture and the superheated water would flash into steam, resulting in a terrific explosion. These failures were not uncommon and because the heaters were, almost always, in the basement of the home, the resulting explosion would completely demolish the structure. These heaters have always been hazardous when used in connection with a water system that is supplied by a well and a pump, because the check valve in the well, performs the same function as the check in the city water line.