2021 IRC Chapters 28-29
A residential storage water heater is a tank of pressurized water at high temperature next to a fuel source or an electric element, in a closet, garage, attic, or utility room. The code's job in this chapter is to make sure that when something goes wrong, a stuck thermostat, a failed control, a closed system with no room to expand, the tank fails safely instead of catastrophically. This lesson covers where a heater can be installed, the drain valve and shutoff every installation needs, the pan that protects the building when a tank leaks, the relief valve and discharge pipe that are the last line of defense against overpressure, and the thermal expansion control that keeps a closed system from turning every heating cycle into a pressure spike. Venting and combustion air belong to the mechanical chapters and are covered in GCHS's mechanical course, not here.
P2801.4 requires a water heater to be located and connected so there is access for observation, maintenance, servicing, and replacement; a heater wedged behind shelving cannot be inspected for a slow leak or swapped out at end of life. P2801.5 sends location rules to Chapter 20, where M2005.2 says a fuel-fired water heater may not be in a room used as a storage closet, and a water heater in a bedroom or bathroom has to be in a sealed enclosure so combustion air is not taken from the living space (a direct-vent heater needs no enclosure). The fuel-gas chapter, G2406.2, adds that a gas appliance in such a room must be direct-vent, or sit in a space that opens only into that bedroom or bathroom, is used for nothing else, and has a solid weather-stripped self-closing door with combustion air taken from outdoors. In a garage, P2801.7 requires a water heater with an ignition source to have that source at least 18 inches above the garage floor, since fuel vapors settle near the floor, unless the heater is listed as flammable-vapor-ignition-resistant. A garage heater a vehicle could reach also needs protection: M1307.3.1 says appliances may not be installed where vehicles can damage them unless protected by approved barriers.
A heater tucked into an attic or crawl space is one of the most common access violations GCHS inspectors find in older houses. In an attic, M1305.1.2 requires an opening and a clear passageway large enough to remove the largest appliance, at least 30 inches high and 22 inches wide and not more than 20 feet long measured along its centerline (up to 50 feet if the passageway is at least 6 feet high and 22 inches wide for its entire length), with continuous solid flooring at least 24 inches wide, leading to a level service space at least 30 by 30 inches at the appliance. A luminaire controlled by a switch at the passageway opening and a receptacle at or near the appliance are also required (M1305.1.2.1). Under a floor, M1305.1.3 asks for a passageway at least 30 inches high and 22 inches wide and not more than 20 feet long, with a 30 by 30 inch level service space at the front or service side of the appliance and a rough-framed opening of at least 22 by 30 inches. A hard-to-reach heater is exactly where these provisions matter most, not where they get skipped.
Every storage water heater needs a drain valve at the bottom of the tank for emptying and sediment flushing, with an inlet of at least 3/4-inch nominal iron pipe size and a male hose-thread outlet (P2801.2); that valve is a maintenance fitting only and never substitutes for the relief valve covered below. Separately, P2903.9.2 requires a readily accessible full-open valve in the cold-water supply at or near the heater, so the tank can be isolated without shutting down the whole house. An inspector who finds a throttling gate valve seized with corrosion there has a real finding: it does not meet the full-open, accessible standard even though water still technically flows through it.
Every water-heating appliance and hot-water storage tank must be protected by either a separate pressure-relief valve and a separate temperature-relief valve, or a combination T&P valve (P2804.1). The temperature element has to sense the water in the top 6 inches of the tank, because heat stratifies and the hottest water sits at the top, and the valve opens at not more than 210 degrees Fahrenheit (P2804.4). The pressure valve opens at not more than 150 psi and at least 25 psi above the system pressure, and its setting can never exceed the tank's rated working pressure (P2804.3). The temperature number is not arbitrary: water under pressure stays liquid well above its normal boiling point, and released to atmospheric pressure above roughly that temperature it can flash instantly to steam, turning a tank failure into an explosion rather than a leak. A check or shutoff valve is never permitted between the relief valve and the tank, between it and the heating appliance, or between it and the end of its discharge pipe, because that valve is the tank's only protection (P2804.6). Each relief valve also needs its own discharge line; a line that serves two devices means a discharging valve can push hot water back through the other's outlet and makes it impossible to tell which tank failed. Bottom-fed tank-type heaters also need a vacuum relief valve (P2804.7).
P2804.6.1 controls how the T&P valve's discharge is piped away, and nearly every rule exists to keep the valve able to do its job when it matters. The pipe may not connect directly to the drainage system; it discharges through an air gap in the same room as the heater, to the floor, the heater's pan, a waste receptor, or the outdoors, at a termination point the occupants can readily see, because a weeping valve is a warning sign that a hidden termination would lose. It may not be smaller than the valve outlet and stays full size to the air gap, since reducing it anywhere builds back-pressure at the valve. It must be installed to flow by gravity with no traps or sags that hold water, with no valves or tee fittings, and it may not end in a threaded connection, since a threaded end invites someone annoyed by a drip to cap it, converting the tank into an unprotected pressure vessel. It must terminate not more than 6 inches, and not less than two pipe diameters, above the floor or the receptor's flood level rim, low enough that near-boiling discharge cannot spray a bystander yet high enough to keep the air gap. The pipe serves a single relief device and must be made of a material listed in Table P2906.5 or tested and rated for the service.
In the field
A common finding in GCHS rehab work is a water heater swap where the old T&P discharge was never reworked: reduced from 3/4 to 1/2 inch behind the tank, running up and over a joist, creating a sag that holds water. Both defects have to be corrected, not just the obvious one. A second common finding, in a converted attic, is a pan drain left ending inside the insulation instead of reaching daylight or an approved receptor; that drain has relocated the leak, not solved it.
Where a storage tank-type water heater is installed in a location where water leakage from the tank will cause damage, such as an attic, a finished space, or any location above occupied space, it has to sit in a pan (P2801.6). The pan is galvanized steel or aluminum at least 0.0236 inch thick, plastic at least 0.036 inch thick, or another approved material, and it is at least 1-1/2 inches deep (P2801.6.1). It drains by an indirect waste pipe of at least 3/4 inch, full size, that ends over a suitably located indirect waste receptor or extends to the exterior, landing 6 to 24 inches above the adjacent ground (P2801.6.2). A pan whose drain dead-ends in an attic or wall cavity defeats the purpose: the pan is there so a leak is both contained and noticed, and a hidden termination accomplishes neither. One allowance matters in a swap-out: where a pan drain was not previously installed, a pan drain is not required for a replacement water heater (P2801.6.1).
P2903.4 requires a means of controlling the pressure increase caused by thermal expansion, in practice an expansion tank, in two situations. The first (P2903.4.1) is downstream of a pressure-reducing valve where expansion pushes the pressure above the valve's setting. The second (P2903.4.2) is where a backflow prevention device, check valve, or similar device on a system with storage water heating lets expansion raise pressure. Both describe a closed system: one where a PRV, a backflow preventer, or a check valve stops water from flowing back toward the main. In an open system, water expanding as it heats simply pushes back into the main; once closed, that volume has nowhere to go, and pressure climbs with every heating cycle. Water expands only about 1 to 2 percent in volume from a cold supply temperature up to a typical 140-degree setting, but a closed system with no expansion tank cannot absorb even that small volume, so pressure spikes instead. The classic symptom is a T&P valve dripping only for an hour after a shower, exactly the window when the tank is reheating: that is thermal expansion, not a defective valve, and the fix is an expansion tank, not a higher-rated relief valve. Three devices work together and are easy to confuse: the PRV controls what the main sends in, the expansion tank absorbs the volume each heating cycle creates, and the T&P valve is the last-resort device that should never be doing routine work.
Lesson 2 covered the 120-degree maximum setting required at shower and tub/shower valves under P2708.4. That limit lives at the fixture valve, not the thermostat, because a thermostat drifts and disappears at the next replacement; turning a heater down to 120 degrees is never a substitute for a compensating shower valve. Where a combination water heater and space-heating system needs water above 140 degrees for space heating, P2803.2 requires an ASSE 1017 temperature-actuated mixing valve that tempers the water to not more than 140 degrees for domestic uses, and P2803.1 separately requires the connected piping and components to be suitable for potable water.
A worked example: a 40-gallon heater is fed through a PRV with no bypass, a closed system. Heating that water from cold to 140 degrees increases its volume by roughly 1.5 percent, or 40 times 0.015, about 0.6 gallon released into the piping every cycle. On an open system that volume pushes back toward the main; on this closed system it has nowhere to go, so pressure climbs until an expansion tank absorbs it or the T&P valve starts dripping to relieve it, which is why the code ties the expansion tank requirement to the presence of a check valve, backflow preventer, or PRV rather than to tank size or fuel type.
1. A water heater's temperature and pressure relief protection is set to open at no more than:
2. Under P2804.6.1, a T&P discharge pipe must terminate:
3. Why is a threaded end prohibited on a T&P relief valve discharge pipe?
4. An inspector finds a T&P discharge pipe reduced from 3/4 inch to 1/2 inch partway down, terminating 24 inches above the floor. What is present?
5. A pan is required under a storage water heater when:
6. A water heater pan must be at least how deep, with what minimum drain size?
7. An inspector finds a water heater pan drain in an attic that ends in the loose insulation instead of reaching a receptor or the exterior. This is:
8. Under P2903.9.2, the required shutoff valve at a water heater must be:
9. An inspector finds the cold-side shutoff at a water heater partly closed and seized with corrosion. The code-relevant finding is:
10. A thermal expansion control device is required under P2903.4 when:
11. A homeowner reports that the T&P valve drips only for about an hour after every shower. The most likely cause is:
12. Comparing three devices in the same water heater installation: a pressure-reducing valve, a thermal expansion tank, and a T&P relief valve. Which description is correct?
13. In the general case, a water heater installed in an attic is reached by a passageway that, under M1305.1.2, must be:
14. A water heater thermostat set to 120°F may NOT be used as: