3.2 Temperature and Pressure Relief Valves & Discharge Piping (UPC 504.6, 608; WA 608.3.1 and 608.5)

What the T&P Valve Protects Against

A storage water heater is a closed pressure vessel. If the thermostat and high-limit control both fail, water keeps heating past 212°F without boiling, because it is under pressure. At 50 psi, water stays liquid to about 298°F. At 150 psi it stays liquid to about 366°F. If the tank then ruptures, the superheated water flashes to steam and expands about 1,600 times in volume. A properly sized, properly piped T&P relief valve prevents that event by discharging water before the tank reaches those conditions.

When a T&P Valve Is Required

Ratings and Settings

• Pressure (608.4): Each pressure relief valve is an approved automatic type with a drain, set at not more than 150 psi. No shutoff valve may be installed between the relief valve and the system.
• Capacity (504.6): The relief valve's hourly Btu discharge capacity or rated steam relief capacity must be not less than the input rating of the heater. A 199,000 Btu/h commercial heater needs a valve rated for at least 199,000 Btu/h.
• Temperature: Standard residential T&P valves (ANSI Z21.22/CSA 4.4) open on temperature at about 210°F. The temperature-sensing element must sit in the hottest water near the top of the tank, in the heater's designated tapping. Never extend the valve out on a nipple or tee that holds the probe away from the tank water.
• No shutoffs anywhere in the relief path (504.6): Do not install a valve between the relief valve and the heater, or on the discharge pipe between the valve and atmosphere.

Discharge Piping: Washington 608.5

Washington's amended 608.5 reads as a checklist. The discharge piping serving a temperature relief valve, pressure relief valve, or combination valve must have no valves, obstructions, or means of isolation, and must be:

1. Full size: equal to the size of the valve outlet, discharging full size to the flood level of the receiving area, pointing down.
2. Rated material: rated for at least the operating temperature of the system and approved for the use.
3. Gravity discharge: discharging independently by gravity through an air gap into the drainage system, or outside the building with the end of the pipe not more than 2 feet and not less than 6 inches above the ground, pointing downward.
4. Safe: discharging so as not to cause personal injury or structural damage.
5. Not trapped: no part may be trapped or subject to freezing.
6. Unthreaded: the terminal end may not be threaded, so the outlet cannot be capped.
7. Not into the pan: discharge from a relief valve into a water heater pan is prohibited.

Washington exception: Where no drainage was provided, replacement water heating equipment only needs a drain pointing down from the relief valve and ending between 2 feet and 6 inches from the floor. No additional floor drain is required.

Why 6 to 24 inches? Below 6 inches the outlet can be blocked by snow, water, or debris. Above 24 inches the discharge splashes and can scald someone standing nearby.

Related Pan Rule

A drain pan under a heater in an attic or over a floor-ceiling assembly (Section 3.3) catches leaks from the tank. It is not a receptor for relief valve discharge. A T&P line that ends over the pan violates item 7, even if the pan has a drain.

Exam trap: "Relief valves may be tested by installing a ball valve in the discharge so the line can be isolated." Any valve or means of isolation in the discharge piping is prohibited.