3.4 Water Heaters: Installation, Relief Valves, Pans & Thermal Expansion
Key Takeaways
A combination temperature and pressure relief valve must conform to ANSI Z21.22, have a temperature setting of no more than 210°F and a pressure setting no higher than the tank's rating or 150 psi, and be sensed in the top 6 inches of the tank (IPC 504.4–504.5).
Relief valve discharge piping must be full size, unvalved, untrapped and unthreaded at the end. It must drain by gravity and end no more than 6 inches and no less than twice the pipe diameter above the floor or receptor (IPC 504.6).
Where leakage would cause damage, a tank water heater sits in a pan at least 1-1/2 inches deep with a drain at least 3/4 inch, made of 0.0236-inch galvanized steel or aluminum or 0.036-inch plastic, and plastic pans are not allowed under gas-fired heaters (IPC 504.7).
Where cold water reaches a storage heater through a check valve, pressure-reducing valve or backflow preventer, a thermal expansion device must be installed downstream of them, and the relief valve may not be used for this (IPC 607.3, 504.4).
An attic water heater needs a passageway at least 30 inches high and 22 inches wide, no more than 20 feet long, and a 30 × 30-inch level service space (IPC 502.3).
Why this section matters
Water Heaters has 6 questions of its own, and relief-valve and expansion questions also appear under System Installation. Most answers are in Chapter 5 of the 2021 IPC and Section 607. Gas-fired heaters must also follow the IFGC for venting, combustion air and gas piping (IPC 502.1). Electric heaters follow NFPA 70.
Installation and access (501–503)
| Rule | Requirement |
|---|---|
| 501.3 | Drain valve at the bottom of every tank heater, with an inlet at least 3/4 inch iron pipe size and a male hose-thread outlet |
| 501.4 | Locate for access for observation, maintenance, service and replacement |
| 501.5 | Water heaters must be third-party certified |
| 501.6 | Water from tankless heaters for domestic use must not exceed 140°F (shower valve rules still apply) |
| 501.7 | Tanks must be permanently marked with their maximum allowable working pressure |
| 501.2 | A combination water and space heating system that needs water above 140°F for heating must have an ASSE 1017 mixing valve limiting potable hot water to 140°F |
| 502.2 | Fuel-fired heaters may not be installed in a room containing air-handling equipment if that room is used as a plenum |
| 502.3 | Attic heaters need an opening and a passageway at least 30 inches high and 22 inches wide, no more than 20 feet long, with solid flooring at least 24 inches wide, a level service space of at least 30 × 30 inches, and a clear access opening of at least 20 × 30 inches (large enough to remove the heater) |
| 502.5 | A level working space of at least 30 × 30 inches in front of the control side |
| 503.1 | A valve on the cold water line to each heater, near the heater, serving only it, and accessible on the same floor as the heater |
In Alabama, Board rule 720-X-3-.01 allows water heaters to be exchanged by a certified plumbing contractor or a certified gas fitting contractor. Under rule 720-X-3-.02, gas piping up to and including the appliance shutoff valve is gas fitting. For gas-fired water heaters, the connection beyond that valve is also gas fitting (see section 7.4).
Safety devices (504)
- 504.1, antisiphon: A cold-water dip tube with a hole near the top, or a vacuum relief valve above the tank, must keep a storage heater from being siphoned empty.
- 504.2: Bottom-fed heaters and tanks need a vacuum relief valve conforming to ANSI Z21.22.
- 504.3: Electric heaters need a disconnecting means under NFPA 70. Other heaters need a separate valve to shut off the fuel.
- 504.4, relief valve: Storage heaters operating above atmospheric pressure need a self-closing (levered) pressure relief valve and temperature relief valve, or a combination T&P valve, conforming to ANSI Z21.22. It may not be used to control thermal expansion.
- 504.4.1, location: The valve is installed in the tank shell, and the temperature element must be actuated by water in the top 6 inches of the tank. A separate storage tank needs its own relief valve. No check valve or shutoff valve may be placed between a relief valve and the tank.
- 504.5, ratings: Temperature setting no higher than 210°F. Pressure setting no higher than the tank's rated working pressure or 150 psi, whichever is less. Relieving capacity at least equal to the heater's heat input.
Relief valve discharge piping (504.6)
| The discharge pipe must... | Why |
|---|---|
| Not connect directly to the drainage system | Prevents back-contamination |
| Discharge through an air gap in the same room as the heater | So discharge is visible |
| Be at least the size of the valve outlet, full size to the air gap | No restriction during a full-capacity release |
| Serve a single relief device | Discharges must not interact |
| Discharge to the floor, the heater's pan, a waste receptor or outdoors | An approved place |
| Discharge where it causes no injury or damage and is readily observable | So the problem gets noticed |
| Not be trapped, and flow by gravity | Prevents scale, freezing and blockage |
| End no more than 6 inches and at least two pipe diameters above the floor or receptor rim | 1-1/2 inches minimum for a 3/4-inch pipe |
| Have no threaded end, no valves and no tees | Nobody can cap or plug it |
| Be made of a Section 605.4 material or one rated under ASME A112.4.1 | Must withstand 210°F water |
| Be one nominal size larger than the valve outlet if insert fittings are used, with the outlet end fastened | PEX and similar tubing |
Drain pans (504.7)
- A pan is required wherever a tank-type heater or hot water storage tank could leak and cause damage, such as in attics, above finished ceilings or over wood floors.
- Pan material: galvanized steel or aluminum at least 0.0236 inch thick, plastic at least 0.036 inch thick, or another approved material. A plastic pan may not be installed under a gas-fired heater.
- Size and drain (504.7.1): The pan must be at least 1-1/2 inches deep and large enough to catch all dripping. Its drain must be at least 3/4 inch, made of a Table 605.4 material.
- Termination (504.7.2): Run the drain full size to an indirect waste receptor or floor drain, or outdoors 6 to 24 inches above grade.
- When a heater is replaced where no pan drain existed, no new pan drain is required.
Temperature and recirculation (607)
- 607.1: Hot water is required for bathing, washing, cooking and cleaning in residences. Nonresidential occupancies may use hot or tempered water for bathing and washing.
- 607.1.1: A heater's thermostat may serve as the temperature limit for fixture-delivery rules only if the heater meets ASSE 1082 or ASSE 1085.
- 607.1.2, tempered water: Controlled by an ASSE 1070 device set to 110°F maximum, an ASSE 1017 thermostatic mixing valve, or an ASSE 1082 or 1084 heater.
- 607.2: No more than 50 feet of developed length from the hot water source to a fixture. A circulating loop or heat trace counts as the source.
- 607.2.1.2: Demand recirculation pumps must start on a demand signal and limit water returned to the cold line to 104°F. Pumps between a heater and a storage tank may run no more than 5 minutes after the heating cycle ends.
- 607.4: Hot water is on the left side of a fitting, except ASSE 1016-type valves, which follow their own markings.
- 607.5 and 505.1: Insulate piping per the IECC. Unfired storage tanks are insulated to R-12.5.
Thermal expansion (607.3)
Heating water expands it about 1.7% between 40°F and 140°F. A check valve at the meter, a pressure-reducing valve or a backflow preventer traps that growth and makes the system closed. Section 607.3 requires a thermal expansion control device on the heater's cold supply, downstream of all check valves, PRVs and backflow preventers. It must be sized by the tank maker's instructions so system pressure stays within the 80 psi limit of 604.8.
Diaphragm expansion tanks have a factory air charge. Most manufacturers say to set the air charge to match the incoming static water pressure before connecting the tank. That is a manufacturer instruction, and you must still follow it under 303.2. Support the tank independently of the piping (308.10).
Exam traps
- Relief settings: 210°F, and 150 psi or the tank rating, whichever is less. Sensing in the top 6 inches.
- Discharge termination: at least two pipe diameters but no more than 6 inches above the floor or receptor. No threads and no valves.
- Pans: 1-1/2 inches deep with a 3/4-inch drain. No plastic pan under gas-fired heaters.
- The T&P valve is not an expansion device. Closed systems need a 607.3 expansion device.
What are the maximum temperature setting and the maximum pressure setting for a water heater's relief valve under IPC 504.5?
180°F and 160 psi, the standard water service rating
200°F and 125 psi, regardless of the tank rating
212°F, and the tank's rated pressure even if above 150 psi
210°F, and the lesser of 150 psi or the tank's rated pressure
A 3/4-inch relief valve discharge pipe ends above the floor next to the water heater. What termination height meets IPC 504.6?
Flush with the floor so it cannot be seen
At least 6 inches and no more than 24 inches above the floor
At least 1-1/2 inches and no more than 6 inches above the floor
Any height, as long as the end has a threaded cap
A water heater serves a home whose meter has a check valve. What does IPC 607.3 require?
Raising the relief valve setting to 175 psi to absorb the expansion
Removing the check valve so the system becomes open again
Relying on the T&P relief valve to discharge the expanded water
An expansion control device on the cold supply, downstream of the check
Sections you finish are checked off in the contents.