3.1 Water Heater Permits, Sizing (WA Table 501.1), Bedroom and Bathroom Locations, & Safety Devices
Key Takeaways
UPC 502.1 makes it unlawful to install, remove, or replace a water heater without first obtaining a permit from the AHJ.
Washington's Table 501.1 sets minimum first hour ratings from 42 gallons (1 to 1.5 baths, 1 bedroom) up to 80 gallons for larger homes.
A fuel-burning water heater in a Washington bedroom or bathroom must be direct-vent, or be in a dedicated closet with a listed gasketed self-closing door and outdoor combustion air (WA 504.1).
Unlisted water heaters require 12 inches of clearance on all sides and rear (UPC 504.3.2).
Recovery in gallons per hour equals input in Btu/h times efficiency, divided by temperature rise times 8.33.
Why Water Heaters Are a Separate Chapter
UPC Chapter 5 covers the water heater as a plumbing appliance. Washington removes the parts of Chapter 5 that deal with combustion air and venting; those go to the Mechanical Code (Section 3.3). What remains is testable: permits, sizing, location, safety devices, relief valves, seismic strapping, pans, and access. Section 608 in Chapter 6 adds the relief valve and expansion tank rules.
Permits and Inspection (UPC 502 and 503)
- 502.1: It is unlawful to install, remove, or replace a water heater without first obtaining a permit from the AHJ. A like-for-like replacement is not exempt.
- 503.2: The final water heater inspection is made after the work is complete.
- 501.1 (Washington amendment): Listed appliances are installed per the manufacturer's instructions. Unlisted water heaters are allowed only under 504.3.2. The amendment also sends combustion air and all vents and connectors to the Mechanical Code.
Sizing: Washington Table 501.1 (First Hour Rating)
The first hour rating (FHR) is the amount of hot water, in gallons, a heater can deliver in one hour starting with a full tank. It appears on the Energy Guide label. Washington's 2015-edition amendment sets these minimums for dwelling units:
| Bathrooms | Bedrooms | Minimum FHR (gallons) |
|---|---|---|
| 1 to 1.5 | 1 | 42 |
| 1 to 1.5 | 2 | 54 |
| 1 to 1.5 | 3 | 54 |
| 2 to 2.5 | 2 | 54 |
| 2 to 2.5 | 3 | 67 |
| 2 to 2.5 | 4 | 67 |
| 2 to 2.5 | 5 | 80 |
| 3 to 3.5 | 3 | 67 |
| 3 to 3.5 | 4 | 80 |
| 3 to 3.5 | 5 | 80 |
| 3 to 3.5 | 6 | 80 |
Nonstorage (tankless) and solar water heaters must also meet the applicable FHR. For replacement water heaters, the table note refers to the existing-building provisions in Chapter 1.
Recovery rate: the trade formula
Recovery (gph) = input (Btu/h) × efficiency ÷ (temperature rise × 8.33)
One Btu raises one pound of water 1°F, and a gallon of water weighs about 8.33 pounds. For electric heaters, 1 kW equals 3,412 Btu/h.
| Heater | Input | Efficiency | Rise | Recovery |
|---|---|---|---|---|
| Gas storage | 40,000 Btu/h | 0.80 | 90°F (40°F to 130°F) | 32,000 ÷ 749.7 ≈ 42.7 gph |
| Gas storage | 40,000 Btu/h | 0.80 | 60°F | 32,000 ÷ 499.8 ≈ 64.0 gph |
| Electric, one 4,500 W element | 15,355 Btu/h | 1.00 | 90°F | 15,355 ÷ 749.7 ≈ 20.5 gph |
| Gas tankless | 199,000 Btu/h | 0.95 | 70°F | 189,050 ÷ 583.1 ≈ 324 gph (5.4 gpm) |
Western Washington groundwater often enters in the 40s°F in winter, so design for the larger winter rise.
Location in Bedrooms and Bathrooms (Washington 504.1)
Washington replaces UPC 504.1. A water heater installed in a bedroom or bathroom must comply with one of the following:
- Closet option: A fuel-burning water heater may be in a closet located in the bedroom or bathroom only if all of these are met:
- The closet has a listed, gasketed door assembly and a listed self-closing device.
- The door has a threshold and bottom door seal.
- All combustion air comes from outdoors per the International Mechanical Code.
- The closet is for the exclusive use of the water heater.
- Direct-vent option: The water heater is a direct-vent type.
Under 504.1.1, self-closing doors must swing freely and close and latch every time. No hold-open feature is allowed. A bedroom clothes closet that also stores clothing does not qualify, because the closet must serve only the water heater. Electric water heaters have no combustion air or flue, so this rule targets fuel-burning units.
504.2: Water heaters other than direct-vent types are located as close as practical to the chimney or gas vent.
Clearances (UPC 504.3)
- Listed heaters (504.3.1): Follow the listing and the manufacturer's instructions. Clearances may not interfere with combustion air, draft hood relief, or service access.
- Unlisted heaters (504.3.2): 12 inches of clearance on all sides and rear. A combustible floor underneath must be protected in an approved manner.
Required Safety Devices
| Section | Requirement |
|---|---|
| 504.4 | Overpressure protection by an approved, listed device |
| 504.5 | Overtemperature protection by an approved, listed device, for both the heater and any hot water storage vessel |
| 504.6 | Relief devices installed per listing; no shutoff valve between the relief valve and heater or on the discharge pipe; the valve's Btu/h or steam relief rating must be at least the heater's input |
| 505.2 (Washington) | Storage heaters using energy other than gas (for example, electric or oil) need an over-temperature safety device in addition to the primary control, plus a combination T&P valve |
Most residential heaters combine these in a high-limit energy cutoff and a T&P valve (Section 3.2).
A Washington home has 2 bathrooms and 4 bedrooms. What minimum first hour rating does the Washington amendment to Table 501.1 require for the new storage water heater?
42 gallons
54 gallons
67 gallons
80 gallons
A homeowner wants a gas-fired, non-direct-vent storage water heater in a closet off the primary bedroom. Under Washington's 504.1, which set of conditions makes this acceptable?
A carbon monoxide alarm in the bedroom
Any solid-core door and a 12-inch clearance on all sides
A closet used only for the water heater, a listed gasketed door with a listed self-closing device, a threshold and bottom seal, and all combustion air from outdoors
A louvered door that supplies combustion air from the bedroom
A gas water heater has an input of 40,000 Btu/h and an efficiency of 80 percent. Incoming water is 40°F and the setpoint is 130°F. What is the approximate recovery rate?
64.0 gph
28.5 gph
42.7 gph
53.3 gph
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