3.4 Seismic Strapping, Thermal Expansion Tanks, Pressure Regulators, & Temperature Controls (WA 507.2, 608.2, 608.3)

Seismic Strapping (Washington 507.2)

Unrestrained water heaters topple in earthquakes. That can break gas connections, start fires, and flood buildings. The unamended 2015 UPC requires strapping only in Seismic Design Categories C through F. Washington applies the rule to every water heater. Its amendment reads:

507.2 Seismic Provisions. Water heaters shall be anchored or strapped to resist horizontal displacement due to earthquake motion. Strappings shall be at points within the upper one-third and lower one-third of its vertical dimensions. At the lower point, a distance of not less than four (4) inches shall be maintained from the controls to the strapping.

What the rule means on the job:

1. Two strap zones: one strap in the upper third of the tank height and one in the lower third.
2. Control clearance: the lower strap must stay at least 4 inches away from the gas valve or thermostat, so the strap does not interfere with service or crush the controls.
3. Structure: anchor to framing or masonry that can resist the load. Listed strap kits give fastener sizes and must be installed per their instructions.
4. No height rule from the floor: Washington's text does not require the lower strap a specific distance above the floor. The measured requirement is the 4 inches from the controls.

Thermal Expansion in Closed Systems (Washington 608.3)

Heating water makes it expand, roughly 1 to 2 percent between typical cold supply and storage temperatures. For a 50-gallon tank, that is about half a gallon to nearly a gallon. In an open system, the extra volume pushes back through the meter into the main. In a closed system, the water cannot escape. Because water barely compresses, pressure climbs rapidly until the T&P valve weeps at 150 psi, and fill valves and connectors are stressed along the way.

Washington's 608.3 amendment:

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 … 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 … 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.

Exception: Instantaneous hot water systems installed in accordance with the manufacturer's installation instructions.

Devices that create a closed system include:

• A purveyor's dual check or other backflow preventer at the meter
• A pressure regulator (PRV); UPC 608.2 requires an expansion tank downstream of every regulator
• A containment assembly (DC or RP) on the building service
• Any check valve on the building supply

Installing the tank

• Put the tank on the cold water side, on the building side of the closing device. It is usually near the heater's cold inlet.
• Support the tank so its weight, full of water, does not hang on the piping.
• Set the air pre-charge per the manufacturer's instructions. Typically that means matching the incoming static pressure or the regulator setting. A tank left at a factory charge below line pressure fills with water and loses much of its capacity.
• Potable-water expansion tanks must comply with NSF 61 (608.2).

Pressure Regulators (UPC 608.1 and 608.2)

More 608.2 details that make good exam questions:

• Regulators 1-1/2 inches and larger do not require a strainer.
• The regulator and strainer must be accessible aboveground, or in a vault with a properly sized, sloped drain to daylight. They must be protected from freezing, and the strainer must be cleanable without removing the regulator or disconnecting the piping.
• When sizing with Table 610.4, base the pipe size on 80 percent of the reduced pressure. For example, with a regulator set at 70 psi, size using 56 psi, which falls in the 46–60 psi column.
• An expansion tank must be installed in the cold water distribution piping downstream of each regulator. It prevents thermal expansion pressure and maintains the regulator setting.

Temperature Controls

• 504.5: Every water heater or hot water storage vessel needs overtemperature protection by an approved, listed device.
• 505.2 (Washington): A storage heater using energy other than gas needs an overtemperature safety device in addition to its primary temperature control, plus a combination T&P valve.
• In practice, the primary thermostat maintains the setpoint. A high-limit energy cutoff (ECO) shuts off the burner or elements if the thermostat fails, before the T&P valve's temperature element (about 210°F) opens. Most ECOs require a manual reset, so a tripped ECO is a signal to find the failed control.
• 507.1: The AHJ may require a dielectric insulator on the heater's water connections.

Exam trap: The expansion tank goes on the building side of the check valve or regulator. A tank installed on the street side of the device cannot absorb expansion that is trapped inside the building.