5.2 Temperature & Pressure Relief Valves & Discharge Piping

Temperature & Pressure Relief Valves & Discharge Piping

Water heaters and pressurized hot water storage vessels represent one of the highest potential energy hazards in building plumbing design. Water under pressure does not boil at the atmospheric boiling point of 212°F (100°C). Under a typical municipal water distribution pressure of 60 psig (74.7 psia), the boiling point of water rises to approximately 307°F (153°C); at 125 psig, the boiling point reaches 353°F (178°C). If an unvented water heater experiences an unchecked thermal runaway due to a failed thermostat, the stored water becomes superheated liquid. A subsequent rupture of the vessel shell drops the pressure instantaneously to atmospheric, causing the superheated water to flash explosively into steam. Because steam expands to roughly 1,600 times the volume of liquid water, a standard 50-gallon domestic water heater undergoing a catastrophic failure releases energy equivalent to several sticks of dynamite—a phenomenon known as a Boiling Liquid Expanding Vapor Explosion (BLEVE). To eliminate this catastrophic hazard, IPC Section 504 establishes stringent mandates for Temperature and Pressure (T&P) relief valves and their associated discharge piping.

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T&P Relief Valve Operating Parameters & Listings (IPC 504.1–504.5)

Under IPC 504.4, storage water heaters operating above atmospheric pressure must have an approved, self-closing (levered) pressure relief valve and temperature relief valve, or a combination of the two. Combination T&P valves are the usual choice. Where a separate storage tank is used, relief valves are required on both the storage water heater and the storage tank (504.4.1). A relief valve may not be used to control thermal expansion (504.4); that is the job of the thermal expansion control in IPC 607.3.

Product Standards (IPC Sections 504.4 and 504.5)

• ANSI Z21.22 / CSA 4.4: IPC 504.4 requires relief valves to conform to ANSI Z21.22, Relief Valves for Hot Water Supply Systems.
• Labeling (504.5): T&P valves, and energy cutoff devices, must bear the label of an approved agency.
• Large vessels: Commercial heaters and tanks above certain input or volume thresholds are also built to the ASME Boiler and Pressure Vessel Code and carry ASME-rated relief valves. That requirement comes from the vessel standard and state boiler laws, not from IPC 504.

Mandatory Operating Thresholds

              T&P Relief Valve Critical Operating Thresholds

       ┌────────────────────────────────────────────────────────┐
       │           Maximum Pressure Relief Setting:             │
       │   150 psi (1034 kPa) or Vessel Working Pressure Rating │
       │                 (Whichever is lower)                   │
       └──────────────────────────┬─────────────────────────────┘
                                  │
                                  ▼
       ┌────────────────────────────────────────────────────────┐
       │          Maximum Temperature Relief Setting:           │
       │                    210°F (99°C)                        │
       │  (Thermostatic element immersed in upper 6" of vessel) │
       └──────────────────────────┬─────────────────────────────┘
                                  │
                                  ▼
       ┌────────────────────────────────────────────────────────┐
       │            Relieving Capacity (BTU/hr):                │
       │ Valve Steam Rating >= Appliance Hourly Input (BTU/hr)  │
       └────────────────────────────────────────────────────────┘

1. Pressure Limitation (504.5): The pressure setting may not exceed the tank or water heater manufacturer's rated working pressure or 150 psi (1035 kPa), whichever is less. For example, if a commercial tank is rated for a maximum allowable working pressure (MAWP) of 125 psi, the T&P valve pressure setpoint cannot exceed 125 psi.
2. Temperature Limitation (504.5): The temperature setting may not exceed 210°F (99°C). This parameter ensures the valve discharges before water reaches the atmospheric boiling point (212°F), preventing steam flash conditions inside the vessel.
3. Location (504.4.1): Relief valves are installed in the shell of the tank, and the temperature relief valve must be actuated by the water in the top 6 inches (152 mm) of the tank. No check valve or shutoff valve may be installed between a relief valve and the heater or tank it serves. Because hot water naturally rises due to thermal stratification, the hottest water always collects at the top of the vessel. Installing the valve on a remote tee or nipple where the probe is outside the top 6 inches delays thermal response and violates code.
4. Relieving Capacity (504.5): The relieving capacity of each pressure relief valve and each temperature relief valve must equal or exceed the heat input to the water heater or storage tank. For example, an instantaneous gas water heater with an input rating of 199,000 BTU/h requires a T&P relief valve with a certified ASME steam rating of at least 199,000 BTU/h.

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Relief Valve Discharge Piping (IPC Section 504.6)

The discharge pipe connected to a T&P relief valve conveys boiling water and steam during an emergency discharge. If the pipe is undersized, obstructed, improperly pitched, or fabricated from inadequate materials, it can burst, scald occupants, or trap pressure, rendering the relief valve useless.

The 14 Requirements of IPC Section 504.6

The discharge piping serving a pressure relief valve, temperature relief valve or combination valve shall:

          T&P Relief Valve Discharge Piping & Air Gap Geometry

            Cold Water Inlet      Hot Water Outlet
                   │                     │
                   ▼                     ▲
            ┌───────────────────────────────────┐
            │        Water Heater Tank          │
            │      T&P Relief Valve             │
            │       [<= rated pressure or 150 psi; <= 210°F]
            │      ┌───┐                        │
            │      │ O─┼──┐ Actuated by top 6"  │
            └──────┴───┴──┼─────────────────────┘
                          │
                          ▼  Full-size pipe from Table 605.4 or ASME A112.4.1
                          │  [Gravity flow, no traps, valves or tees]
                          ▼
                          │  [Unthreaded end]
                          ├── Air gap: min 2 x pipe diameter (1.5" for 3/4" pipe)
                          │            max 6" above floor or receptor rim
                          ▼
                ┌──────────────────┐
                │ ░░░░░░░░░░░░░░░░ │ Floor, pan or waste receptor in the same room
                └──────────────────┘

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Material Selection: Approved vs. Prohibited Piping (IPC Table 605.4)

One of the most frequent plan review corrections involves the specification of inappropriate plastic piping for T&P relief discharge lines.

Why Schedule 40 PVC Is Not Acceptable

Contractors sometimes propose Schedule 40 PVC (ASTM D1785) or PVC-DWV for relief discharge piping because it is cheap and on site. Under IPC 504.6 item 13, the discharge pipe must be a material listed in Table 605.4 (water distribution pipe), or a material tested, rated and approved for relief discharge under ASME A112.4.1. PVC is not listed in Table 605.4; it appears only in Table 605.3 for cold water service. Ordinary PVC is also rated for a maximum service temperature of about 140°F, while relief discharge can reach 210°F with flashing steam.

Code-Approved Materials for T&P Discharge Piping

Materials listed in IPC Table 605.4 include:

1. Copper or copper-alloy pipe and tubing (Types K, L and M): the most common choice, and easy to support rigidly.
2. CPVC (ASTM D2846, F441, F442).
3. Galvanized steel pipe (ASTM A53) and stainless steel pipe and tubing (Types 304/316).
4. Ductile iron pipe (AWWA C151, C115).
5. Polypropylene (PP) pipe or tubing (ASTM F2389).
6. PEX (ASTM F876), PE-RT (ASTM F2769) and the composite pipes listed in the table. Where tubing is installed with insert fittings, it must be one nominal size larger than the valve outlet and its outlet end must be fastened in place (504.6 item 14). Follow the manufacturer's instructions for support so the tubing does not move during a discharge.

Products tested, rated and approved under ASME A112.4.1 (water heater relief valve drain tubes) are also acceptable.

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Termination Scenarios: Indoor vs. Outdoor Geometry

The discharge terminal must safely dissipate boiling water without causing harm:

Scenario A: Interior Termination

• Approved Discharge Points (504.6 item 5): The floor, the water heater pan, a waste receptor (floor drain, floor sink, mop basin or hub drain) or the outdoors.
• Air Gap Location (item 2): The air gap must be in the same room as the water heater, where a discharge will be seen.
• Air Gap Dimensioning (item 10): The vertical distance between the end of the discharge pipe and the floor or the flood level rim of the receptor must be: $\text{Air Gap} \ge 2 \times D_{\text{pipe}} \quad \text{and} \quad \text{Air Gap} \le 6 \text{ inches (152 mm)}$ For a 3/4-inch pipe ($D = 0.75\text{ in}$), the minimum is $2 \times 0.75 = 1.5\text{ inches}$. The 6-inch maximum limits splashing of scalding water.

Scenario B: Discharge to the Outdoors

• Discharge to the outdoors is one of the permitted points (item 5), but the air gap must still be in the same room as the water heater (item 2). A common compliant arrangement discharges through an air gap into a receptor in the heater room whose drain is carried outdoors.
• The discharge must be readily observable, must not cause injury or structural damage, must flow by gravity and must not be trapped.
• Do not confuse this with the pan drain rule. The requirement to terminate 6 to 24 inches above the adjacent ground appears in IPC 504.7.2 for water heater pan drains extended outdoors. It is not a relief valve discharge requirement.
• Unthreaded Terminal (item 11): The end of the pipe has no threads, so nobody can cap it or attach a hose.

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Plans Examiner Verification Checklist: T&P Relief Systems

┌────────────────────────────────────────────────────────────────────────┐
│            Plumbing Plans Examiner Verification Checklist              │
│                         T&P Relief Systems                             │
├────────────────────────────────────────────────────────────────────────┤
│ [ ] Valve Standard: ANSI Z21.22, labeled by an approved agency       │
│ [ ] Pressure Setting: <= 150 psi AND <= tank rated working pressure  │
│ [ ] Temperature Setting: Maximum 210°F setpoint                        │
│ [ ] Sensor Location: Element immersed within top 6" of vessel          │
│ [ ] Capacity: Relieving capacity >= heater or tank heat input       │
│ [ ] Pipe Sizing: Full nominal diameter matching valve outlet (min 3/4")│
│ [ ] Pipe Material: Table 605.4 or ASME A112.4.1 (no PVC)            │
│ [ ] Slope & Drainage: Continuous pitch downward; zero uphill loops     │
│ [ ] Prohibited Fittings: No shutoff valves, check valves, or tees      │
│ [ ] Separation: Single device; not tied to pan drain or other relief │
│ [ ] Indoor Terminal: Air gap between 2x pipe diameter and 6" max       │
│ [ ] Air Gap Location: In the same room as the water heater          │
└────────────────────────────────────────────────────────────────────────┘