Water Heaters
Protection devices
Every storage water heater needs a temperature and pressure (T&P) relief valve, sized to match or exceed the heater's maximum Btu/h input and pressure rating, installed within the top 6 in. of the tank, and never valved off. The discharge pipe must be the same size as the valve outlet, flow by gravity, have no traps or reductions, terminate within 6 in. of the floor or to the exterior 6–24 in. above grade, and end without threads. Where leakage would cause damage, a pan at least 1.5 in. deep with a drain is required.
Installation rules
Gas heaters need combustion air, a vent connector sloped up toward the chimney, and clearance to combustibles. Electric heaters need a disconnecting means. Heaters in garages must be elevated so the ignition source is at least 18 in. above the floor. Seismic strapping is required in high-seismic regions. Thermal expansion control (an expansion tank) is required whenever a check valve, pressure-reducing valve, or backflow device creates a closed system - a T&P valve does not serve that purpose.
Traps in questions
- The relief valve must be rated for the heater's input Btu/h, not just its tank volume.
- Discharge piping that is reduced, trapped, or threaded at the end is a classic wrong answer.
- A T&P valve does not replace the thermostat or energy cutoff; both functions must exist.
- Relief discharge may not terminate where it can scald or where the valve can be isolated.
Worked example
A 40,000 Btu/h gas water heater requires a T&P valve labeled for at least 40,000 Btu/h at 150 psi/210°F. A 30,000 Btu/h valve is not acceptable, even if the tank is small, because the valve could be overpowered.