Water heaters and hot water supply systems
Domain 4 - Water heaters and hot water supply systems
This domain covers installation of storage and tankless heaters, relief protection, thermal expansion, venting and combustion air, and potable hot water distribution. Relief valve and venting items are the highest yield.
Relief and expansion
- Every storage water heater needs a temperature-and-pressure relief valve listed to ANSI Z21.22, with a Btu/hr rating at least equal to the heater input and a pressure rating not exceeding the tank rating. Typical setting: 210°F / 150 psi.
- Discharge pipe: same nominal size as the valve outlet, no reduction, no valve, no trap, gravity flow, terminating not more than 6 in. above the floor or to the exterior, with an unthreaded end and material rated for the temperature.
- A closed system (check valve, pressure-reducing valve, or backflow device on the supply) needs thermal expansion control, normally a listed expansion tank.
- A drain pan at least 1½ in. deep with a ¾ in. drain is required where leakage would cause damage: attics, ceilings, and floors above occupied space.
Venting and combustion air
- Category I appliances use a draft hood and a vent connector sloping up toward the chimney at ¼ in./ft; vent termination is at least 5 ft above the draft hood and 2 ft above anything within 10 ft.
- Condensing appliances are Category IV and use listed plastic venting sized per the manufacturer's instructions. The two categories cannot be mixed.
- Combustion air: the indoor-volume method of 50 ft³ per 1,000 Btu/hr, or two permanent openings sized 1 sq in. per 1,000 Btu/hr, one within 12 in. of the top and one within 12 in. of the bottom of the enclosure.
- In garages, appliances with an ignition source are elevated so the ignition source is at least 18 in. above the floor unless listed for garage installation.
Worked example
A 40,000 Btu/hr gas water heater sits in a closet with a louvered door. The installer runs the T&P discharge in ½ in. copper through a wall and terminates it threaded outdoors. Reasoning: three failures. The pipe was reduced from the valve outlet size, it must terminate unthreaded so it cannot be capped, and the valve must remain accessible. Separately, check combustion air: the closet volume must be compared with 50 ft³ per 1,000 Btu/hr (2,000 ft³ here). A typical closet is far smaller, so two openings or a listed outdoor-air duct is required.
Where marks are lost
- Sizing the T&P valve by pipe size instead of Btu/hr capacity.
- Piping relief discharge into a trapped line or a floor drain without an air gap.
- Adding a PRV or check valve and forgetting expansion control.