The relief valve discharge pipe must run full size to a visible air gap, with no trap, valve or threaded end, and end 6 inches or less above the floor or receptor. A storage heater where a leak would cause damage needs a drained pan, but a replacement heater does not need a pan drain added.
Where must the relief valve discharge pipe terminate?
IPC Section 504.6 spells out the discharge piping serving a pressure relief valve, temperature relief valve or a combination valve. The pipe shall:
- "Not be directly connected to the drainage system," and instead "Discharge through an air gap located in the same room as the water heater."
- "Not be smaller than the diameter of the outlet of the valve served and shall discharge full size to the air gap."
- "Serve a single relief device and shall not connect to piping serving any other relief device or equipment."
- "Discharge to the floor, to the pan serving the water heater or storage tank, to a waste receptor or to the outdoors."
- "Discharge to a termination point that is readily observable by the building occupants."
- "Not be trapped," and "be installed so as to flow by gravity."
- "Terminate not more than 6 inches (152 mm) above and not less than two times the discharge pipe diameter above the floor or flood level rim of the waste receptor."
- "Not have a threaded connection at the end of such piping," and "Not have valves or tee fittings."
What does the water heater maker say?
The same ideas show up in manufacturer manuals. A. O. Smith's instruction manual for its commercial STH(X)-120 gas water heater lists the discharge pipe requirements as "Shall not be smaller in size than the outlet pipe size of the valve," "Must terminate a maximum of six inches above a floor drain or external to the building," and "Shall not have any valve or other obstruction between the relief valve and the drain." The manual is for that model, so use the manual that ships with yours, and follow both it and the code.
IPC Section 504.4.1 adds a rule for the valve itself: "There shall not be a check valve or shutoff valve between a relief valve and the heater or tank served." Section 504.4 says the relief valve "shall not be used as a means of controlling thermal expansion." If a relief valve keeps dripping because pressure rises when the heater fires, the answer is an expansion control device, not a bigger valve; see why a water heater relief valve leaks.
Does a commercial water heater need a pan?
Where a leak would do damage, yes. IPC Section 504.7 says: "Where a storage tank-type water heater or a hot water storage tank is installed in a location where water leakage from the tank will cause damage, the tank shall be installed in a pan." The next section sets the pan size and drain. The pan must be at least 1 1/2 inches deep and "of sufficient size and shape to receive all dripping or condensate from the tank or water heater," and it "shall be drained by an indirect waste pipe having a diameter of not less than 3/4 inch."
The pan drain "shall extend full size and terminate over a suitably located indirect waste receptor or floor drain or extend to the exterior of the building." There is one useful exception for swaps: "Where a pan drain was not previously installed, a pan drain shall not be required for a replacement water heater installation."
Does Phoenix change these rules?
Phoenix's published amendments to the 2024 IPC do not list Sections 504.4 through 504.7, so the base text applies to IPC-based plans. If the drawings cite the Uniform Plumbing Code instead, its own water heater chapter governs. A replacement in a commercial building still follows the permit rules of the city; see our water heater permit page for how that works in Phoenix.
What should a building owner check?
- 1Find the relief valve and follow its pipe. It should end in plain sight over a floor drain, receptor or outdoors, not capped, threaded or plumbed into a drain.
- 2Look for valves or tees anywhere on the pipe. There should be none.
- 3Check for a pan under any storage heater above finished spaces, and where the pan drains.
- 4If water keeps coming from the relief pipe, find the cause instead of plugging it. The A. O. Smith manual says the discharge pipe "Shall not be plugged or blocked."
Types of commercial water heaters and how often a commercial water heater should be flushed and serviced cover the rest. HQ is licensed and insured and replaces water heaters, with upfront pricing and free estimates, and the discharge piping and pan are part of what gets checked on a swap.