Give me the rules for the T&P relief valve discharge pipe on a tank water heater.

IRC P2804 / UPC 608.5 Key Rules for T&P Discharge Piping:

1. Material: Copper, CPVC, PEX, galvanized steel, or polypropylene. No PVC (fails at 140°F; relief valves discharge at 210°F). PEX/CPVC must be rated for 180°F+ (check printing).
2. Size: No reduction. Pipe ID ≥ valve outlet (typically ¾"). No necking down.
3. Termination Point: Indirect waste receptor (floor drain/standpipe): Air gap ≥ 2× pipe diameter (min 1½" for ¾" pipe). End 6" max above receptor flood rim. IRC P2804.6.1 To grade/outside: End 6"–24" above grade. IRC P2804.6.2 No threads, caps, or valves* at terminus.
4. Routing: Gravity drain only. No traps, tees, or elbows that hold water. IRC P2804.6.1 Slope: Minimum ¼" per foot fall toward terminus. IRC P2804.6.1 Length: Max 30 ft (IRC) or 15 ft (UPC) total developed length unless manufacturer allows longer. Every 90° ell = ~5–10 ft equivalent. Freeze protection: Required in unconditioned spaces (insulate/heat trace). IRC P2804.6.1
5. Support: Secure per material code (e.g., CPVC/PEX every 32"–48") so pipe cannot move during discharge thrust.
6. Common Errors: Discharging into water heater pan (prohibited—pan drain too small), using PVC, uphill runs, capping end "to stop drips" (valve is defective—replace it), reducing to ½" PEX.

Pro Tip: Run ¾" copper or CPVC straight down to a floor drain with an air gap. Simplest code-compliant path. If no drain exists, pipe to exterior 6"–24" above grade with a screened elbow (bugs) and ensure slope.

Verify local amendments; some jurisdictions require hard pipe (copper/steel) only.

The temperature and pressure (T&P) relief valve discharge pipe on a residential tank water heater must comply with the International Residential Code (IRC) and International Plumbing Code (IPC), as adopted by most jurisdictions. Key rules:

1. Material: Must be rigid, heat-resistant piping — typically copper, CPVC, or galvanized steel. PVC, PEX, or flexible tubing is prohibited (IRC P2804.6.1, IPC 504.6).
2. Size: Must match the valve outlet — usually ¾-inch NPT. Do not reduce pipe size (IRC P2804.6.2).
3. Length & Configuration: - Maximum length: 30 feet (IRC P2804.6.3). - Maximum of four 90° fittings (equivalent length counts toward limit). - Must drain by gravity to a safe location — no traps (IRC P2804.6.4).
4. Termination: - Must discharge within 6 inches above the floor or waste receptor (IRC P2804.6.5). - Not permitted to discharge into a crawlspace, attic, or under a building (scald/flood hazard). - Must be visible and observable — not concealed (e.g., inside walls or ceilings).
5. No Valves or Threads: No shut-off valves, plugs, or threaded connections downstream of the T&P valve (IRC P2804.6.6).
6. Air Gap: If discharging into a sink or drain, must maintain an air gap ≥1 inch above the flood rim (to prevent backflow).

Critical: Improper installation can lead to explosive failure. If unsure, consult a licensed plumber — T&P valve safety is life-critical. Always verify local amendments to IRC/IPC.

(Sources: IRC 2021 Sections P2804.6.1–P2804.6.6; IPC 504.6)