Water Heaters and Temperature Control
Installation essentials
A storage water heater needs a temperature and pressure relief valve - rated for the tank's pressure and temperature, installed in the tank's top or on the hot outlet per the manufacturer, and never plugged, capped, or replaced with a lower-rated valve. The discharge pipe must be the full size of the valve outlet, gravity-drained, without traps or valves, and must terminate so discharge cannot injure anyone: typically 6 inches above the floor or to the exterior. The end is not threaded (to prevent capping). A drain pan with a drain to a conspicuous point is required where leakage would cause damage.
Expansion, venting, and combustion air
A closed system created by a PRV, check valve, or backflow preventer requires a thermal expansion tank; the T&P valve is not an expansion control device. Gas water heaters need proper venting (category, draft hood, or power vent per the listing), adequate combustion air for a confined space, a sediment trap on the gas line, and a shutoff valve. In a garage, ignition sources must be elevated off the floor per code. In seismic areas, strap the tank at the upper and lower thirds.
Temperature control
Stored water temperature and delivered temperature are both regulated. Tub and shower valves must be pressure-balancing or thermostatic and limit the mixed outlet to about 120°F to prevent scalding; public and institutional installations often require additional limits at lavatories and sinks.
Traps the exam sets: capping a T&P discharge, reducing its size, running it through a wall with a trapped or uphill section, or treating a failed T&P valve as a simple part swap on a closed system.