Water Heaters and Temperature Control
Installation Basics
Water heaters must be listed, installed per manufacturer instructions, and provided with a temperature and pressure relief (TPR) valve. The TPR discharge pipe must be full size, gravity-drained, terminate within the specified distance above the floor or outside, and never be trapped, capped, or reduced. Know the difference between a storage-type heater, tankless, and indirect heater, and where each may be installed.
Temperature and Safety
Domestic hot water temperature limits protect against scalding, especially at fixtures used by children, the elderly, or persons with disabilities. Tempering valves and mixing valves are used to limit delivery temperature. Where a water heater is installed in a garage, ignition sources must be elevated or protected. Combustion air and venting for gas water heaters are governed by the fuel gas code and are frequently tested.
Traps in Exam Questions
Watch for questions that assume a TPR valve can discharge into a drain pan without a proper air gap, that a discharge pipe may run uphill, or that a tankless heater needs no relief valve. Another common trap: confusing thermal expansion control with the TPR valve - an expansion tank controls pressure from thermal expansion in a closed system but does not replace the relief valve. Also know that seismic strapping or anchoring requirements may apply and that drain pans are required where leakage could cause damage.
Worked Example
If a heater is rated at 40,000 Btu/h and the relief valve is rated in Btu/h and pressure, the valve rating must equal or exceed the heater input. A 40,000 Btu/h heater requires a TPR valve rated at least 40,000 Btu/h and the system pressure rating; the discharge pipe must be the same nominal size as the valve outlet.