Short definition
A TPR discharge tube is the pipe that runs from the side outlet of a water heater’s T&P relief valve down to within 6 inches of the floor or to an approved drain. It exists so that if the relief valve ever opens, scalding water and steam discharge harmlessly downward instead of spraying anyone nearby.
What it is
The discharge tube is a length of 3/4″ copper or CPVC (matching the T&P outlet thread) that screws or solvent-welds onto the T&P side outlet and runs downward, terminating near the floor or over a floor drain or pan. Code rules are unusually strict because the pipe has to do its job in a once-in-a-decade emergency:
- Full size. The tube cannot be reduced from the T&P outlet diameter. A 3/4″ T&P needs a 3/4″ tube the whole way.
- Downward only. No upward runs, no horizontal traps that hold water.
- Terminate within 6 inches of the floor. Or to an approved drain receptor with an air gap. Not above head height, not into a wall.
- No shut-off valve. Anywhere in the line.
- No threads at the end. Plain pipe end. Threads invite a future homeowner to “fix” the drip by capping it.
- Visible termination. An inspector or homeowner needs to be able to see the pipe end and notice if it’s dripping.
The tube has to handle 210°F water at 150 psi, which is why CPVC and copper are the common choices. PEX is sometimes accepted for the run with a metal terminal section; check local rule before using it.
Why it matters to a homeowner
A homeowner sees this term in two situations: a permit inspection failure, or a contractor’s invoice line item. Both come down to the same rules above.
The most common failure mode is a tube that terminates too high — a foot or more above the floor, or worse, run up and over to a sink. If the T&P ever opens, that geometry sprays scalding water sideways. An inspector will fail it; a buyer’s home inspector will flag it.
The second-most-common failure is a threaded coupling at the bottom, sometimes with a temporary cap “to stop the drip.” Capping a T&P discharge tube turns the relief valve into a non-functioning safety device. If the T&P is dripping, the answer is to fix the cause (usually thermal expansion) and replace the valve if needed — never plug the discharge.
When a plumber bids a water heater swap and the line item says “rework T&P discharge to code,” it’s because the existing tube doesn’t meet one of the rules above. Don’t waive it; the rework is usually under $50 in materials.
When you’ll encounter this term
- A permit inspector cites your T&P discharge as terminated too high or with a coupling at the end.
- You’re DIY’ing a water heater swap and the manual says “match existing discharge tube” — but the existing one is wrong.
- The TPR discharge tube freezes outdoors in a WA winter and back-pressure builds in the line.
- A home inspection report at sale time flags “T&P discharge does not meet code.”
Common variants and what a discharge tube is not
- TPR discharge tube vs. drain pan drain. Two separate pipes. The discharge tube drains the relief valve when it opens. The drain pan beneath the heater drains slow tank-bottom leaks. Some installs combine them at a floor drain; many keep them independent.
- Indoor vs. outdoor termination. Outdoor termination is acceptable in most WA jurisdictions if the line is frost-protected and visibly terminated. Indoors over a floor drain or pan is more common and easier to inspect.
- Hard pipe vs. flex. The discharge line must be rigid pipe, not a flex connector. Flex connectors aren’t rated for the pressure and temperature spike of a T&P discharge event.
Common failure modes
- Terminated too high. Sprays sideways during discharge. Cite-able on inspection.
- Capped or threaded end. Defeats the safety function. Replace the cap with an open pipe end.
- Reduced from 3/4″ to 1/2″. Restricts flow capacity; valve can’t relieve fast enough.
- Discharge into a closed pipe. Air gap to drain receptor required.
- Outdoor termination freezes in WA winter. Back-pressure builds. Insulate or relocate indoors.