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From the 2018 UPC Illustrated Training Manual

608.3 Expansion Tanks, and Combination Temperature and Pressure-Relief Valves.
A water system provided with a check valve, backflow preventer, or other normally closed device that prevents dissipation of building pressure back into the water main, independent of the type of water heater used, shall be provided with an approved, listed, and adequately sized expansion tank or other approved device having a similar function to control thermal expansion. Such expansion tank or other approved device shall be installed on the building side of the check valve, backflow preventer, or other device and shall be sized and installed in accordance with the manufacturer's installation instructions.

A water system containing storage water heating equipment shall be provided with an approved, listed, adequately sized combination temperature and pressure-relief valve, except for listed nonstorage instantaneous heaters having an inside diameter of not more than 3 inches (80 mm). Each such approved combination temperature and pressure-relief valve shall be installed on the water-heating device in an approved location based on its listing requirements and the manufacturer's installation instructions. Each such combination temperature and pressure-relief valve shall be provided with a drain in accordance with Section 608.5.

Water is, for all practical purposes, an incompressible liquid. Because it is incompressible, water placed in a closed container can build up high pressure when heated, even when heated from 40°F to room temperature. This is thermal expansion.

A building water distribution system having a check valve, pressure regulator, backflow preventer or other device that prevents pressure buildup in the building from being able to dissipate back into the source or water supply main regardless of the type of water heater used, must be equipped with an expansion tank or other means to control the thermal expansion pressures. The expansion tank will absorb excess pressure within the tank (see Figure 608.3a). The pressure relief valve relieves the excess pressure by discharging until the pressure drops below the set point (see Figure 608.3b).

When the water system is equipped with a storage water heater, pressures rise even more because of the increase in temperature and the high volume of water contained in the tank. All storage water heaters are required to be equipped with a combination temperature and pressure relief valve (T&P valve). Note in Figure 608.3b, the combination T&P valve has a temperature sensing tube. The pressure relief valve does not.

Storage water heaters must be manufactured to the ANSI Z21.10 series of standards. These water heaters are protected in three stages. The primary stage is the thermostat. Should the thermostat fail, the secondary stage or highlimit switch will turn off the source of energy to the heater. If the high-limit switch fails, the combination T&P valve opens to prevent a catastrophic failure of the water heater. Instantaneous water heaters having an inside dimensional width of three inches or less are exempt from having to meet the requirement for a T&P valve. An example of this exemption is the small single point of use heater for a hand sink. The typical dimension for these units are 11" x 5” x 2.8" and they have no storage capacity other than in the pipe itself to cause over temperature and pressure build up. When the need for hot water has been met the instantaneous water heater shuts off. If the water supply to this small heater has a backflow preventer, an adequately sized expansion tank shall be provided (see Figure 608.3d) even though a T&P valve is not required.

When installing a T&P valve it is critical that the installer check the rating plate on the combination T&P valve before installation to make sure that the Btu input rating of the water heater does not exceed the maximum Btu rating of the valve. The use of an undersized combination T&P valve could result in a catastrophic water heater failure should both the water heater thermostat and the water heater high-limit switch fail. In instances where there are two separate Btu ratings on the valve plate, the smaller of the two is used in making this determination. For more information, see Chapter 5.

Manufacturers of T&P valves require that the valve be installed with its temperature-sensing element immersed within the top 6 inches of the tank since this is where the hottest water in the tank is located (see Figure 608.3c).

Manufacturers recommend that they be manually opened at least once a year to ensure that they are functioning. In areas that have a high mineral content in the water, scale can form around the valve seat and render the valve inoperable. In such areas, it may be necessary to manually open the valve every three months or less. A water heater is a potential bomb that is prevented from exploding by three separate safety devices. The combination T&P valve is a lifesaver and is treated accordingly.

It must be emphasized that water heaters operating at pressures above 150 psi are classified as boilers. In fact, water heaters that have a storage volume greater than 120 gallons, have a Btu input rating greater than 200,000, or operate at temperatures higher than 210°F, are also classified as boilers and are manufactured to the ASME Pressure Vessel Standard.

The 2018 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.