A Phoenix home's plumbing code calls for a temperature and pressure relief valve with a proper discharge pipe on the water heater, a pan under it where a leak would cause damage, a pressure reducing valve above 80 psi, an expansion device in closed systems, fixture shutoff valves, anti-scald shower valves and vacuum breakers on hose bibbs.
1. Temperature and pressure relief valve
Section 504.4 says "Storage water heaters operating above atmospheric pressure shall be provided with an approved, self-closing (levered) pressure relief valve and temperature relief valve or combination thereof." The valve goes in the tank shell, and 504.4.1 adds "There shall not be a check valve or shutoff valve between a relief valve and the heater or tank served." It must carry an approved agency label with a temperature setting of not more than 210°F and a pressure setting that does not exceed the heater's rated working pressure or 150 psi, whichever is less (504.5). If yours drips, see why a water heater relief valve leaks.
The discharge pipe is part of the device. Section 504.6 requires it to "Discharge through an air gap located in the same room as the water heater," to avoid traps, to have no threaded connection at the end, no valves or tees, and to end "not more than 6 inches" above the floor or receptor, and it may not be "directly connected to the drainage system."
2. A pan under the water heater
Where a tank-type water heater sits in a location "where water leakage from the tank will cause damage," Section 504.7 requires a pan at least 1 1/2 inches deep, drained by an indirect waste pipe of at least 3/4 inch. For a replacement, the code says "Where a pan drain was not previously installed, a pan drain shall not be required for a replacement water heater installation." More on that in water heater drain pan overflow.
3. Pressure reducing valve above 80 psi
Section 604.8 starts: "Where water pressure within a building exceeds 80 psi (552 kPa) static, an approved water pressure-reducing valve conforming to ASSE 1003 or CSA B356 with strainer shall be installed." The valve must bring the pressure in the building distribution piping down to not greater than 80 psi static. Read whether you need a pressure reducing valve.
4. Thermal expansion control
Section 607.3 applies "Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer." In that closed system, heated water has nowhere to go, so a thermal expansion control device (usually a small expansion tank) goes on the cold supply, and the pressure must not exceed what 604.8 allows. The code adds that the relief valve "shall not be used as a means of controlling thermal expansion." See do I need an expansion tank.
5. Shutoff valves
Section 606.2 calls for shutoff valves on the fixture supply to each plumbing fixture, other than bathtubs and showers in one- and two-family homes, on the supply to each sill cock, and on the supply to each appliance or mechanical equipment. Section 606.3 requires access to them. Know where yours are before something leaks; how to shut off water to a fixture shows how.
6. Anti-scald shower valves
Section 412.3: individual shower and tub-shower valves must be "balanced-pressure, thermostatic or combination balanced-pressure/thermostatic valves" and must have a means to limit the maximum setting to 120°F, field-adjusted per the manufacturer. Tub-only valves need a temperature-limiting device under Section 412.5. See pressure-balancing versus thermostatic shower valves.
7. Vacuum breakers on hose bibbs
Section 608.16.4.2: "Sill cocks, hose bibbs, wall hydrants and other openings with a hose connection shall be protected by an atmospheric-type or pressure-type vacuum breaker or a permanently attached hose connection vacuum breaker." The reason is simple: a hose left in a bucket or pool must not be able to pull that water back into the house supply. See hose bib vacuum breakers.
What isn't on this list
Smoke and carbon monoxide alarms, GFCI outlets and gas shutoffs fall under other codes (for outlets, see GFCI protection near sinks). Water heaters that burn gas also have their own installation rules.
What to do about it
Walk your house with this list. If a relief valve drips, a pan is missing, pressure is above 80 psi, or a hose bib has no breaker, those are straightforward repairs. HQ is licensed and insured and gives free estimates with upfront pricing for water heater work and residential plumbing. Existing homes aren't forced to be rebuilt: Section 102.2 lets lawfully existing plumbing stay in use if it creates no hazard, while 102.4 says additions, alterations and repairs must conform to what a new system requires.