Water Heaters and Hot Water Systems
Water Heaters and Hot Water Systems covers storage and instantaneous heaters, relief protection, expansion, venting, and distribution. The exam tests whether a heater is protected from over-temperature and over-pressure and whether its discharge is safe.
Core rules
- A temperature-and-pressure relief valve is required, set to open at 210°F or 150 psi, whichever occurs first.
- The T&P discharge pipe must be full size, not reduced, and must terminate safely with an air gap.
- A closed system with a backflow preventer or check valve needs a thermal expansion tank.
- Heaters installed in a pan must have a drain to an approved location.
- Gas heaters need proper venting and combustion air; electric heaters need disconnect and overcurrent protection.
Worked example
A 4,500-watt electric element heats water with an 80°F temperature rise. Input is 4.5 kW × 3,412 Btu/kWh = 15,354 Btu/hr. Recovery is 15,354 ÷ (8.33 lb/gal × 1 Btu/lb°F × 80°F) = 23.0 gallons per hour. Reasoning: the denominator is the energy needed to raise one gallon 80°F; dividing the available Btu/hr by that value gives recovery. Actual recovery is lower because of standby losses.
Traps that cost marks
- Capping or reducing the T&P discharge.
- Installing a relief valve but no expansion tank on a closed system.
- Terminating T&P discharge where it can scald someone.
- Sizing recovery without the temperature rise.