Water heaters, temperature control, and thermal expansion
Water heaters and safety
This domain tests selection, installation, temperature control, and thermal expansion. A storage water heater needs a listed temperature-and-pressure relief valve, with a discharge pipe that is not reduced, not trapped, and terminates safely. A closed potable system created by a check valve, PRV, or backflow preventer needs an expansion tank or other approved expansion control. Temperature control protects against scald: tempering or thermostatic mixing valves are used at fixtures and in health-care or public settings. Recirculation systems need insulation and balancing.
Installation details
Provide a drain pan where leakage could cause damage, seismic strapping where required, and combustion air and venting for gas units. The relief valve is a safety device, not a temperature-control valve. Do not cap, plug, or reduce its discharge. Plastic pipe must not be used for relief discharge unless listed. Label shutoffs and gas valves. Check the manufacturer's clearances and venting category.
Worked example
A heater is installed on a closed system after a PRV. The PRV prevents backflow to the main, so heated water has no place to expand. The installer adds a precharged expansion tank sized for the system volume and the pressure rise from cold to hot. The T&P valve is piped full size to within 6 in. of the floor and discharges to a safe receptor. A tempering valve at the lavatory limits delivery to the required maximum. Without the tank, pressure can lift the T&P valve and cause repeated discharge.
Traps
Capping the T&P; reducing its discharge; using the T&P as a mixer; omitting expansion control; installing a pan drain without a trap or air gap; wrong gas vent category; no combustion air.