Water Heaters and Hot Water Systems
Water Heaters and Hot Water Systems tests sizing, temperature and pressure protection, venting, and the code details that keep a storage tank from becoming a hazard.
Safety devices
- Every storage water heater needs a temperature-and-pressure relief valve set no higher than 210 F and 150 psi, and no higher than the tank's rated working pressure. The valve's Btu rating must equal or exceed the heater's input rating.
- The T&P discharge pipe must be full size, drain by gravity, terminate 6 in or less above the floor or outside, and must never be threaded, trapped, capped, or reduced.
- A drain pan is required where a leak would cause damage, drained to an approved indirect waste.
Closed systems and expansion
- Any check valve, PRV, or backflow preventer upstream closes the system. Add a thermal expansion tank sized to the system volume and temperature rise.
Venting and combustion air
- Category I atmospheric heaters use a draft hood and listed metal vent; Category III and IV units need their listed vent material, often AL29-4C, and condensing units need condensate disposal.
- Provide combustion air by the code's volume or opening method, confirm the vent rises, check clearances, and verify no backdrafting with a smoke or draft test.
Worked example
A 75,000 Btu/h heater with a 150 psi tank rating has a T&P valve rated 100,000 Btu/h and 125 psi. That is acceptable: the Btu rating exceeds the input, and 125 psi is below the tank rating. Substituting a 150 psi valve rated 60,000 Btu/h fails, because the Btu rating is below the heater input.
Traps
Capping the T&P, reducing its discharge, running it to a trapped drain, omitting the expansion tank, venting a Category I heater with PVC, and undersizing the gas piping.