(A) Thermal expansion is the build-up of water pressure in a water system due to the addition of heat. In extreme cases, thermal expansion has caused water heater explosions that have resulted in great property damage and the loss of lives.
(B) Thermal expansion is an important factor to consider whenever a backflow assembly is installed on a plumbing system that supplies a hot-water system. The backflow assemblies do not allow built up pressure in a water system to be back pressured and dissipated back into the water supply mains, contributing to the danger of a water heater explosion due to thermal expansion.
(C) (1) The chances of thermal expansion increase dramatically whenever a water heater is old or not well maintained. Usually, the first few times that a properly working temperature and pressure (T & P) valve is activated on a water heater, it can be relied upon to guard against thermal expansion. However, a T & P valve is not designed to be used repeatedly in this manner. In addition, T & P valves can be removed or altered and made inoperable by an uninformed property owner, and can also be affected by scale build up.
(2) There are three general types of products used to protect against thermal expansion. An expansion tank installed on the hot water system is one option. A second, less expensive option is a toilet tank ball cock product designed to vent water in the event of a line pressure build-up. This may require the installation of a pressure-reducing valve if line pressures regularly exceed 80 PSID. A third type of product would be a pressure relief valve on the cold water feed line to the water heater. A drain line would be needed here if the valve is installed indoors. Replacing old T & P valves is also a good idea, but not as effective as one of the above products.
(Prior Code, § 14-11-2)
(D) Ideally, the T & P valve should be considered as a back up to one of these other products. In actuality, however, many people rely strictly on their water heater’s T & P valve to handle any occasional excess pressure build up. In all cases, but especially where the T & P valve is the only safety feature in use, it is recommended that the T & P valve should be exercised at least once a year and replaced at least every five years.
(E) The greatest potential for thermal expansion problems would be from a poorly maintained water heater on a small water line with a backflow assembly installed just upstream of it. All maintenance personnel need to be encouraged to make sure their water heaters and T & P valves are maintained in good condition, checked regularly, and replaced every five years.
(Prior Code, § 14-11-3)