8.4 Water Heaters: Installation, Relief Valves, Discharge Piping & Drain Pans

General rules (IPC 501)

• 501.1: Chapter 5 governs the materials, design and installation of water heaters and their safety devices.
• 501.3: each tank-type water heater and hot water storage tank needs a drain valve at the bottom with an inlet of at least 3/4-inch nominal iron pipe size and a male garden hose thread outlet.
• 501.4: water heaters and tanks must be located and connected to give access for observation, maintenance, servicing and replacement.
• 501.5: water heaters must be third-party certified.
• 501.7: storage tanks and water heaters must show their maximum allowable working pressure, stamped or on a permanent plate.

Installation (IPC 502)

• 502.1: install per the manufacturer's instructions. Gas-fired heaters must also meet the International Fuel Gas Code; oil-fired heaters, the IMC; electric heaters, NFPA 70; and solar thermal systems, the IMC and ICC 900/SRCC 300.
• 502.1.1: elevation of ignition sources and protection from mechanical damage follow the IMC and IFGC. Under the IFGC, ignition sources of appliances in residential garages must be at least 18 inches above the floor, unless the appliance is listed as flammable vapor ignition resistant (FVIR). Appliances in garages must be protected from vehicle impact.
• 502.2: solid-, liquid- or gas-fuel water heaters may not be installed in a room containing air-handling machinery when that room is used as a plenum.
• 502.3 attics: an attic containing a water heater needs an opening and a clear passageway large enough to remove the heater. The passageway must be at least 30 inches high and 22 inches wide and no more than 20 feet long, measured along its centerline from the opening to the heater. It needs continuous solid flooring at least 24 inches wide. A level service space at least 30 by 30 inches must be at the front or service side. The clear access opening must be at least 20 by 30 inches, where that is large enough to remove the heater.
• 502.4: where earthquake loads apply, supports are designed for seismic forces per the IBC. In practice, residential tanks are often strapped in the upper and lower thirds.
• 502.5: appliances need access for inspection, service, repair and replacement without disabling fire-resistance-rated construction or removing permanent construction or other equipment. A level working space of at least 30 by 30 inches must be in front of the control side.

A full 50-gallon tank holds about 417 pounds of water (8.34 pounds per gallon), plus the weight of the tank. The supporting structure must carry this load (307.4 bars adding loads to trusses without verification).

Connections and safety devices (IPC 503 and 504)

• 503.1: the cold water branch to each water heater or storage tank needs a valve near the equipment, serving only that heater, on the same floor, without cutting off the rest of the cold water system.
• 504.1 antisiphon: an approved means must keep the tank from being siphoned, such as a cold-water dip tube with a hole at the top, or a vacuum relief valve in the cold supply above the top of the heater.
• 504.2: bottom-fed water heaters and tanks must have a vacuum relief valve meeting ANSI Z21.22.
• 504.3: electric water heaters need a disconnecting means per NFPA 70. Other types need a separate valve to shut off the energy or fuel supply.

Temperature and pressure relief (IPC 504.4 and 504.5)

• 504.4: storage water heaters operating above atmospheric pressure need an approved, self-closing (levered) pressure relief valve and temperature relief valve, or a combination valve, conforming to ANSI Z21.22. The relief valve shall not be used as a means of controlling thermal expansion.
• 504.4.1: relief valves are installed in the shell of the tank. The temperature relief valve must be actuated by water in the top 6 inches of the tank. With a separate storage tank, relief valves are required on both the water heater and the tank. No check valve or shutoff valve may be installed between a relief valve and the heater or tank.
• 504.5: relief valves and energy cutoffs must bear an approved agency's label. The temperature setting may not exceed 210°F. The pressure setting may not exceed the tank's rated working pressure or 150 psi, whichever is less. The relieving capacity of each pressure and temperature relief valve must equal or exceed the heat input to the heater or tank.

Why 210°F matters. Under pressure, water can be heated far above 212°F without boiling. At about 60 psi, it stays liquid up to roughly 307°F. If the tank fails, the superheated water flashes to steam, and its volume grows about 1,600 times. The temperature relief valve opens below 212°F so stored water never reaches that condition.

Capacity example: a 4,500-watt electric element is 4.5 kW × 3,412 Btu/h per kW ≈ 15,354 Btu/h, so the relief valve must be rated for at least that.

Relief valve discharge piping (IPC 504.6)

The discharge piping serving a pressure, temperature or combination relief valve shall:

1. not be directly connected to the drainage system;
2. discharge through an air gap located in the same room as the water heater;
3. be no smaller than the valve outlet, and discharge full size to the air gap;
4. serve a single relief device, and not connect to piping serving any other relief device or equipment;
5. discharge to the floor, the water heater pan, a waste receptor or the outdoors;
6. discharge in a manner that does not cause personal injury or structural damage;
7. discharge to a point readily observable by the building occupants;
8. not be trapped;
9. be installed to flow by gravity;
10. terminate not more than 6 inches above and not less than two times the discharge pipe diameter above the floor or the flood level rim of the waste receptor;
11. not have a threaded connection at the end;
12. not have valves or tee fittings;
13. be made of materials listed in Section 605.4, or materials tested, rated and approved for this use under ASME A112.4.1;
14. be one nominal size larger than the relief valve outlet where installed with insert fittings, with the outlet end fastened in place.

Example: a 3/4-inch discharge pipe must end between 1-1/2 inches (2 × 3/4 inch) and 6 inches above the floor.

The 6-to-24-inch dimension in Chapter 5 belongs to the pan drain that terminates outdoors (504.7.2), not to the relief valve discharge.

Drain pans (IPC 504.7)

• 504.7: where a storage tank-type water heater or hot water storage tank is in a location where leakage will cause damage, it must be installed in a pan of galvanized steel or aluminum at least 0.0236 inch thick, plastic at least 0.036 inch thick, or another approved material. A plastic pan shall not be installed beneath a gas-fired water heater.
• 504.7.1: the pan must be at least 1-1/2 inches deep and large enough to catch all drips and condensate from the tank. It is drained by an indirect waste pipe at least 3/4 inch in diameter, made of materials listed in Table 605.4.
• 504.7.2: the pan drain extends full size and terminates over a suitably located indirect waste receptor or floor drain, or extends outside the building and terminates at least 6 inches and no more than 24 inches above the adjacent ground. Replacement exception: where no pan drain was installed before, one is not required for a replacement water heater.

The relief valve may discharge into the pan (504.6), but its piping may not be tee'd into the pan drain or any other line.

Fuel-gas basics for water heaters

Gas venting and combustion air follow the International Fuel Gas Code (502.1). Under its standard method, an appliance room has adequate indoor combustion air if it has at least 50 cubic feet of volume per 1,000 Btu/h of total appliance input. Smaller rooms need two openings, one within 12 inches of the top and one within 12 inches of the bottom, connecting to more indoor space or to the outdoors. Category I (natural draft) water heaters use listed vents such as Type B, with clearances from the listing. Condensing heaters use the vent materials their manufacturer specifies.