Water Heaters and Temperature Control
Safety devices
Every storage water heater needs a temperature and pressure relief valve rated for the tank, typically set to open at 210°F or 150 psi. The T&P valve is mounted near the top of the tank, in the hot outlet or a dedicated tapping. Its discharge pipe must be the same size as the valve outlet or larger, run by gravity, with no valve, no trap, no reduction, and no threaded end; it terminates within 6 in of the floor (or to the exterior) in a place where discharge is visible and safe.
Installation items
- Drain pan under the heater where leakage would cause damage, piped to a safe location.
- Seismic strapping where required.
- Combustion air and flue venting for gas units; proper clearance to combustibles.
- Expansion tank or thermal expansion control on a closed system.
- Scald protection: tempering valves and maximum delivered temperatures at fixtures (commonly 120°F for public use).
Common traps
- Capping or plugging a T&P valve.
- Reducing the discharge pipe diameter.
- Running the discharge uphill or terminating it where it can scald someone.
- Forgetting that a check valve or backflow preventer creates a closed system that needs expansion control.
- Confusing thermostat setting with relief valve setting.
Rule of thumb
Relief device protects the tank (210°F/150 psi); the thermostat or tempering valve protects people (about 120°F at the fixture).
Worked example: A heater with a capped T&P valve and a backflow preventer on the cold inlet fails on two counts: no pressure relief path and no thermal expansion control.