Water Heaters and Hot Water Systems: Sizing and Safety Devices
Size by recovery and peak hour
Storage-type heaters are sized on peak-hour demand and first-hour rating, not tank volume alone. Recovery rate is the number that matters: gallons per hour = (Btu input × efficiency) ÷ (8.33 × temperature rise). Total demand combines the recovery of the heater with usable storage. For a given building, also consider recirculation to reduce wait time, insulation of hot lines, and the temperature at the fixture.
Temperature and pressure protection
A temperature and pressure relief valve is required, sized to the heater's input and rated at least as high as the tank. Typical settings are 210°F or 150 psi, whichever governs. The discharge pipe must be the full valve outlet size, must not be reduced, trapped, capped, or threaded at the end, must run by gravity, and must terminate safely - commonly 6 in. above the floor or to the exterior. A T&P valve is not a substitute for an expansion tank. Closed systems created by a check valve, PRV, or backflow preventer require a thermal expansion tank.
Installation details
Install a drain pan with a drain where leakage would cause damage (attics, ceilings). Strap heaters seismically where required. Provide combustion air and venting per the appliance listing and the gas code. Protect against scald with tempering or mixing valves where required. Tankless units are rated on flow at a specified temperature rise - check the cold-water temperature and simultaneous fixture use.
Traps
Sizing on tank volume only, missing the expansion tank, threading or reducing the T&P discharge, and assuming a higher-Btu heater always yields more hot water without checking efficiency and temperature rise.
Worked example
A 40,000 Btu/hr input heater at 80% efficiency with a 40°F rise: recovery = (40,000 × 0.80) ÷ (8.33 × 40) = 32,000 ÷ 333 ≈ 96 gallons per hour.