Water Heaters and Temperature Control
Relief protection is the core topic
Every water heater needs a temperature and pressure (T&P) relief valve sized for the tank's input, installed in the designated tapping, with the sensing element in the tank's upper portion. The discharge pipe must be the full size of the valve outlet, run by gravity with no traps, no valves, no reducing fittings, and no threaded end; it terminates not more than about 6 inches above the floor or safely outside. Never plug, cap, or valve a relief valve - that is the single most common exam scenario.
Temperature control
Scalding prevention matters: fixtures used for bathing and washing generally should deliver water no hotter than about 120°F, achieved with a tempering or mixing valve where storage is hotter. Hot water storage above 140°F is used in some designs to limit Legionella risk and then tempered down at the point of use. Handwashing sinks, public lavatories, and accessible fixtures have specific temperature limits.
Installation details
Provide a drain pan with a drain to an approved location where leakage would cause damage, thermal expansion control in a closed system, proper combustion air and flue venting for gas units, and elevation of ignition-source appliances about 18 inches above a garage floor (FVIR designs excepted). Strap or anchor per local seismic requirements and keep the unit accessible for service.
Worked example
A 40,000 Btu/h gas water heater is fitted with a T&P valve rated 150 psi and 100,000 Btu/h. The valve capacity exceeds the input, which is acceptable. But if the installer runs 1/2-inch pipe from a 3/4-inch valve outlet, the installation fails regardless of valve capacity.