Where a Water Heater Relief Valve Can Discharge (IRC P2804.6.1)
By Seamless Home Team, Residential project fulfillment · September 19, 2026
Quick answer
The discharge pipe from a water heater's temperature and pressure relief valve must discharge through an air gap located in the same room as the water heater, and it may terminate to the floor, to the pan serving the water heater, to a waste receptor or to the outdoors. It may not be directly connected to the drainage system. It must be at least the diameter of the valve outlet and discharge full size, serve only that one relief device, flow by gravity, and must not be trapped. The termination point must be readily observable by the building occupants and must sit not more than 6 inches, and not less than two times the discharge pipe diameter, above the floor or the flood level rim of the waste receptor. The end of the piping may not have a threaded connection, and the line may not contain valves or tee fittings. Those last two rules are the important ones: a threaded end invites a cap and a valve allows the line to be shut, and either one converts a safety device into a sealed pressure vessel.
A water heater has one device standing between normal operation and a rupture: the temperature and pressure relief valve. It opens when the tank exceeds a set temperature or pressure, and it dumps water until the condition clears.
What makes this an unusually interesting piece of code is that almost none of the rules govern the valve. They govern the pipe leaving it, and most of them exist for one reason — to make the line hard for a well-meaning person to defeat.
The requirements are in Section P2804.6.1 of the International Residential Code, and there are fourteen of them.
The fourteen requirements
The discharge piping serving a pressure relief valve, temperature relief valve or combination valve shall:
- Not be directly connected to the drainage system.
- 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 discharge full size to the air gap.
- Serve a single relief device and 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 in a manner that does not cause personal injury or structural damage.
- Discharge to a termination point that is readily observable by the building occupants.
- Not be trapped.
- Be installed to flow by gravity.
- Terminate not more than 6 inches and not less than two times the discharge pipe diameter above the floor or waste receptor flood level rim.
- Not have a threaded connection at the end of the piping.
- Not have valves or tee fittings.
- Be constructed of materials complying with Section P2906.5, or be tested, rated and approved for the application in accordance with ASME A112.4.1.
- Where insert fittings are used, be one nominal size larger than the size of the relief-valve outlet.
Fourteen items is a lot to hold, so it is worth grouping them by the job they do.
Three rules exist purely to stop the line being disabled
Items 11, 12 and 4 are the heart of the section, and they are not performance requirements at all.
No threaded connection at the end. A threaded end can accept a cap or a plug. A capped relief line converts the water heater into a sealed vessel whose only pressure relief is blocked, and that is the exact mechanism behind the tank failures this provision exists to prevent. The code does not warn against capping the line. It removes the fitting that would make capping easy.
No valves and no tee fittings. A valve is something that can be closed, deliberately or by someone who does not know what it is. A tee is an invitation to share the line with another device, which is also why item 4 requires the piping to serve a single relief device.
The termination is a band, not a minimum
Item 10 gives two bounds, and it is routinely remembered as one.
The end of the pipe must sit not more than 6 inches above the floor or the flood level rim of the waste receptor, and not less than two times the discharge pipe diameter above it. For a three-quarter-inch line that is a window between one and a half inches and six inches.
Each bound prevents a different failure. Terminate too high and a discharge of near-boiling water sprays outward instead of falling, in a room where somebody may be standing. Terminate too low — flush to the floor, or into standing water in a pan — and the outlet can be obstructed or submerged, which destroys the air gap and can allow siphoning back toward the tank.
The lower bound scaling with pipe diameter rather than being a fixed number is the tell that it is about the air gap: the gap has to be proportionate to the pipe producing the flow.
The air gap has to be in the same room
Item 2 is more specific than it first reads. It is not enough that an air gap exists somewhere; the air gap must be in the same room as the water heater. Combined with item 7 — the termination has to be readily observable by the building occupants — the intent becomes clear.
A relief valve that has started to weep is a symptom. It usually means thermal expansion on a closed system with no expansion tank, a failing valve, or a control problem. The code wants that water landing somewhere a person will see it, in the room where the appliance is, so that a slow fault gets noticed before it becomes a fast one.
This is the requirement most often lost on an otherwise careful installation. A line run neatly through a wall into a laundry drain in the next room has been improved out of compliance. So, in practice, has a line run outdoors to a corner of the house nobody visits: outdoors is a permitted termination, but the observability requirement follows the pipe out there, and in a freezing climate an exterior termination introduces a new way for the line to block.
Gravity, no traps, full size
Items 3, 8, 9 and 14 all protect flow.
The line must flow by gravity and must not be trapped. A rise or a dip holds water in the pipe after a discharge — water that corrodes the line from the inside, can freeze in an unconditioned space, and can obstruct the outlet. A blocked discharge line is functionally identical to a capped one, which is why this sits alongside the anti-defeat rules in seriousness even though it reads like a workmanship note.
The line must be not smaller than the valve outlet and must discharge full size to the air gap, so it may not be reduced anywhere. The valve is rated to relieve a given flow; a throttled line prevents it doing what it is rated to do. Item 14 closes the loophole: insert fittings reduce the internal bore, so where they are used the pipe has to go up one nominal size to compensate.
The pan is a separate requirement
Item 5 permits the discharge to terminate in the pan serving the water heater, and this causes a common confusion. Discharging into the pan is allowed. It does not make the pan optional or the pan drain optional.
Under Section P2801.6 the pan must be not less than 1-1/2 inches deep and drained by an indirect waste pipe of not less than 3/4 inch diameter. The pan drain and the relief discharge line are two different pipes doing two different jobs, and item 4 forbids connecting the relief line to piping serving anything else — so they do not get merged.
A relief line aimed into an undrained pan has relocated the discharge by about eighteen inches and solved nothing.
What actually gets found
The defects on this list cluster, and they cluster around good intentions:
- A cap or plug on a threaded end, usually because the valve was weeping and somebody stopped it.
- A shutoff valve in the line, for the same reason.
- The line run uphill to reach a drain that sits above the heater.
- Terminated in another room, or outside at the back of the house, in the name of tidiness.
- Reduced from 3/4 inch to 1/2 inch to fit an existing penetration.
- Terminated into a pan with no drain.
Not one of those is carelessness. Every one is somebody solving a visible problem — a drip, an awkward route, a hole that is the wrong size — without knowing what the pipe is for. That is the argument for settling the route in the scope rather than at the wall, which is the same argument that applies to where an HVAC condensate drain can discharge and to combustion air for the appliances in that closet. A water heater replacement frequently touches all three at once, and a quote that says "replace 50 gal water heater" addresses none of them.
The bottom line
The relief discharge line must air-gap in the same room as the heater, terminate between two pipe diameters and six inches above the floor or flood level rim, run full size and downhill without traps, serve that one device, land somewhere the occupants will see it, and end in a plain unthreaded pipe with no valves and no tees in its length.
Which edition and which local amendments govern is the authority having jurisdiction's answer, and plumbing provisions are amended at the state level more than most.
Seamless Home owns the design, permit and inspection layer on the work it runs, which is where the discharge route gets settled before the old tank comes out. Coverage is confirmed per service area rather than promised as blanket availability. Tell us what you are planning and we will tell you what the scope actually contains.
Frequently asked questions
Can a water heater relief valve discharge into a floor drain?
Only through an air gap, and the air gap has to be in the same room as the water heater. The discharge piping is not permitted to be directly connected to the drainage system, so a line run into a drain pipe or glued into a trap is a defect regardless of how tidy it looks. Discharging above a floor drain with a proper gap is acceptable, but the termination still has to satisfy the height rule and still has to be readily observable by the occupants. Running the line through a wall to a drain in another room fails on the same-room requirement even where an air gap exists at the far end, because part of the purpose is that somebody sees it discharge.
How high above the floor should a T&P discharge pipe end?
Between two bounds, not above a single minimum. The termination point must be not more than 6 inches and not less than two times the discharge pipe diameter above the floor or the waste receptor's flood level rim. For a 3/4-inch line that is a band between 1-1/2 inches and 6 inches. The two limits do different jobs. Too high and a discharge of near-boiling water sprays rather than falls, which is a scald hazard to anyone standing there. Too low and the outlet can be obstructed or submerged, which defeats the air gap and can siphon. An installer who remembers only 'about six inches' will usually land inside the band, but an installer who cuts the pipe to the floor has failed it.
Why can't the end of a relief valve discharge pipe be threaded?
Because a threaded end can be capped, and this is the rule with the clearest reasoning behind it. A capped or plugged discharge line turns the water heater into a closed vessel with its only pressure relief blocked, which is the precise failure mode that causes tanks to rupture. The code removes the opportunity rather than relying on nobody taking it. The same reasoning drives the prohibition on valves and tee fittings in the line: a valve is something that can be closed, and a tee is an invitation to share the line with another device. None of these three rules is about performance. All three are about making the line difficult to defeat.
Can the discharge pipe run uphill or have a trap in it?
No to both. The piping must be installed to flow by gravity and must not be trapped. A section of pipe that rises, or a dip that holds water, retains liquid in the line after a discharge. That standing water corrodes the pipe from inside, can freeze where the line runs through an unconditioned space, and can block the outlet. A blocked relief line is functionally the same as a capped one. This rules out routing that uses the discharge pipe as though it were an ordinary drain, and it is a common defect where the water heater sits below the point somebody wanted the water to go.
Can the relief valve discharge into the drain pan under the water heater?
Yes. The listed termination options are the floor, the pan serving the water heater or storage tank, a waste receptor, or the outdoors. Discharging into the pan is explicitly permitted. What it does not do is remove the need for the pan to be able to empty: under Section P2801.6 the pan must be not less than 1-1/2 inches deep and drained by an indirect waste pipe of not less than 3/4 inch diameter. A pan with no drain will overflow, and a relief discharge aimed into an undrained pan simply relocates the problem to the floor. The pan drain and the relief discharge line are two separate pipes doing two separate jobs, and the relief line may not be manifolded into anything else.
Can the discharge pipe be run outside?
Outdoors is one of the permitted termination points, but two other requirements follow the line outside with it. The termination must still be readily observable by the building occupants, and the discharge must not cause personal injury or structural damage. A line terminating at the back of a house where nobody goes satisfies the letter of 'outdoors' while defeating the purpose of the observability rule, because a relief valve that has begun to weep is telling you something is wrong and nobody will hear it. In freezing climates there is a second problem: an exterior termination can ice over and block, and a blocked line is the failure this whole section is written to prevent.
What size does the discharge pipe have to be?
Not smaller than the diameter of the outlet of the valve it serves, and it must discharge full size to the air gap, so it cannot be reduced anywhere along its length. Where insert fittings are used the pipe must be one nominal size larger than the relief valve outlet, because an insert fitting reduces the internal bore. Materials must comply with the water distribution piping requirements of Section P2906.5, or be tested, rated and approved for the application in accordance with ASME A112.4.1. The reason size matters is that the valve is rated to relieve a certain flow, and a line that throttles that flow prevents the valve doing the job it is rated for.