Water Heaters, Temperature and Pressure Control
The safety devices
Every storage water heater needs a temperature and pressure relief (T&P) valve, rated for the tank, installed in the designated tapping. The discharge pipe must be the full size of the valve outlet, flow by gravity, have no valve or trap, be material rated for hot water, and terminate where discharge is visible and safe - typically 6 in. above the floor or outside. Never cap, plug or reduce a relief valve. Gas heaters need a vent connector, draft hood, combustion air, and clearance to combustibles.
Temperature control
Scald protection is a design responsibility: thermostatic or tempering mixing valves at the source or point of use, with maximum hot water at fixtures commonly limited to about 120°F and lower limits for tubs and showers. On a closed system - anything with a check valve, PRV, or backflow preventer upstream - install a thermal expansion tank sized to the system.
Installation details
Set the heater level, in a drain pan where leakage would cause damage, with the pan drain run to an approved receptor. Strap or anchor heaters where seismic rules apply. Provide a shutoff valve and union on the cold inlet and a full-port valve on the hot outlet, with dielectric protection at steel nipples. Keep unions and relief piping accessible.
Traps
- Reducing the T&P discharge pipe, or terminating it in a trap or drain where it could plug.
- Installing a check valve upstream without an expansion tank.
- Venting a gas heater with single-wall pipe through a wall or into a chimney serving another appliance without proper connections.
- Confusing a T&P valve with a pressure-only relief valve.
Rule of thumb
Relief discharge: full size, gravity, no valve, no trap, visible termination. If a question offers any other arrangement, it is the wrong answer.
Example: a 50-gallon heater with a 3/4 in. T&P outlet requires a 3/4 in. discharge terminating 6 in. above the floor drain.