Water Heaters and Hot Water Systems
Water heater questions test safety devices, seismic restraint, venting and sizing. California adds energy-code rules that push heat pump water heaters into new construction and require planning for them on alterations.
Safety devices
- Temperature and pressure relief valve rated 210 degrees F and 150 psi, mounted within the top 6 inches of the tank.
- Discharge pipe full valve outlet size (minimum 3/4 inch), no reducing fittings, no valves, no threads at the outlet, terminating not more than 6 inches above the floor or outside 6 to 24 inches above grade, and draining to a safe place.
- Expansion tank on any closed system - check valve, PRV or backflow device on the cold inlet.
- Drain pan, 1-1/2 inches deep, drained to a safe location wherever a leak would cause damage.
Seismic and installation
California requires two straps: upper in the top third, lower in the bottom third. In a garage the tank must be protected from vehicle impact. Gas units need a sediment trap, a listed connector, a shutoff valve, and correct venting - Category I appliances use Type B vent with the required clearances and combustion air.
Temperature and distribution
Hot water delivered to fixtures is capped at 120 degrees F in dwellings, with lower limits or thermostatic mixing valves for childcare, medical and assisted-living occupancies. Recirculation, insulation and pipe runs are controlled by the Energy Code; long dead legs waste water and fail inspection.
Worked example
A 40,000 BTU/h gas heater with 80 percent recovery efficiency, a 50 gallon tank, and a 90 degree F temperature rise. Recovery = (40,000 x 0.80) / (8.33 x 90) = 32,000 / 750 = about 43 gallons per hour. First-hour rating is roughly 0.7 x 50 + 43 = 78 gallons. A household with a 70 gallon peak-hour demand fits; a 90 gallon demand needs a larger burner or more storage.
Traps
- Capping, plugging or reducing the T&P discharge.
- Omitting the expansion tank after adding a PRV or backflow device.
- Using single-wall vent pipe in a chase or at the wrong clearance.
- No sediment trap, no strapping, no drain pan above a finished ceiling.