How to Trace a T&P Discharge Pipe to Its End

Why this matters

The temperature and pressure relief valve on a water heater, the T&P valve, is the last thing between a heater whose controls have failed and a tank that ruptures. When it opens, it dumps water near boiling through its discharge pipe, and everything depends on that pipe being able to carry it: full size, falling all the way, open at the end, ending somewhere a person can see it. Basement finishes, laundry remodels and closet build-outs bury that pipe behind drywall, and somewhere out of sight it gets reduced, run uphill, or plumbed straight into a drain. Nobody sees the end, so nobody knows it no longer works until the day it has to. Any tech working in a mechanical closet can trace it in ten minutes without touching the valve.

The valve, its rating and the full discharge-piping rule set are owned by another card: see related, T&P Valve and Discharge Piping Verification. This card is how to follow the pipe you cannot see all of, and what to do with what you find.

Before you start: leave the valve alone

Do not lift the test lever. On an older valve, a lever pulled for the first time in years often does not reseat, and a valve left dripping or running on a heater in a finished basement is a flood, and the water coming out is hot enough to scald. Testing or replacing the valve is a plumber's job under the sibling card. Tracing the pipe needs nothing but your eyes, hands, a light and a tape.

Never cap, plug or valve the end, and never let anyone else. A capped discharge turns a working relief valve into a sealed tank.

If water is coming out of the discharge right now, you are no longer tracing a pipe. Keep everyone's hands and feet away from the outlet and any puddle, do not touch the valve, and get the customer's plumber or your shop's plumbing tech there. If the water is steaming or the tank is making rumbling or banging noises, the customer shuts off the heater's energy at the gas control or breaker if they can do it without reaching past the outlet, everyone stays clear of the heater, and the call goes to the plumber at once.

The steps

1. Start at the valve outlet and record its size. The discharge must be at least the size of the valve outlet for its whole length; most residential T&P valves have a 3/4-inch outlet. Note the pipe material and read any print line on it, because the adopted code limits what the discharge may be made of. The 2021 IRC accepts the materials on its water distribution pipe list (copper, CPVC and PEX among them) or pipe rated for the purpose under ASME A112.4.1; ordinary PVC drain pipe is not on that list, and some local codes and heater makers are stricter. Write down what you read rather than what you expect.

2. Follow the pipe by hand and eye, and stop at every joint. At each fitting, ask four questions. Does the size change (a reducing coupling, a bushing, a smaller tube pushed on)? Is there a valve, of any kind, which is never allowed? Is there a tee joining another line, which the pipe must not have, since a discharge serves one relief valve? Is the pipe still going down, or level to down? A section that rises, even a few inches, holds water at its low point, which is a trap, and the line no longer drains by gravity. Mark each fitting on a sketch with its distance from the valve.

3. Where the pipe enters a wall, ceiling or floor, find where it comes out. Measure from a fixed reference, such as an inside corner or a door frame, to the penetration, in both directions. Carry those measurements to the other side of the wall or to the room below and look there: behind appliances, in cabinet backs, at access panels, in an unfinished ceiling bay. A small inspection camera through an existing opening, such as an access panel or an open ceiling tile, is fine. Do not cut drywall to follow it without the customer's say-so; if the only way to see the rest is to open a wall, that is a finding in itself, and the decision is theirs. On a stepladder, keep both feet on it and the light in your pocket rather than your teeth.

4. Find the actual end, not where you assume it is. The end must terminate in the open, not connected directly to any drain line; where it discharges to a drain or a receptor such as a floor drain or a laundry tub, it must do so through an air gap, an open vertical space between the pipe end and the receptor, so a drain backing up cannot reach the valve and the discharge is visible. The end must not be threaded, so it cannot be capped. Under the 2021 IRC, as adopted by your jurisdiction, P2804.6.1 in substance also requires the discharge to flow by gravity, to have no trap, and for the air gap to be in the same room as the water heater, and sets a maximum height for the end above the floor or receptor; take that height from your adopted code, since the IRC, IPC and UPC state it in different sections and editions have moved it. Measure the end's height above the floor or receptor and record it.

5. Decide what must be reported. Any of these is a reportable fault: a reduction in size, a valve, a tee, an uphill run or trap, a direct connection to a drain without an air gap, an air gap in a different room from the heater, a threaded end, an end you could not find, or pipe of a material your adopted code or the heater maker does not accept. You do not need to decide which is worst. One is enough for the relief path to be unreliable.

6. Route the fix to a plumber, in writing. Correcting a T&P discharge is plumbing work, licensed where your state or locality licenses it. If you are the plumber, you correct it under the sibling card. If you are not, you report it; you do not reroute it with what is on the truck.

Worked example: the pipe that disappeared into the laundry wall

An HVAC tech replacing a furnace filter in a finished basement's mechanical closet notices the water heater's discharge pipe drops a short way and then vanishes into the closet's drywall side wall. The customer says the basement was finished eight years ago. The tech traces it.

MECHANICAL CLOSET            |  LAUNDRY ROOM
                             |
 [WATER HEATER]              |
  T&P valve o--+             |    1/2 PEX
  (52 in up)   | 3/4 copper  |   +----------+ rises
               | drops 14 in |   |          | 10 in
               +-------------+-[R]          |
               level run, 30 in  reducer    v
               through drywall   3/4 x 1/2  [T] tee into
                             |              side of the
                             |              washer
                             |              standpipe,
 floor ----------------------+--------------no air gap

Step 1. The valve outlet is 3/4 inch, 52 inches above the floor. The pipe leaving it is 3/4-inch copper, and copper's rating is not in question.

Step 2. The copper drops 14 inches, from 52 to 38 inches above the floor, then turns to a level run and enters the drywall. No fittings in the closet other than the elbow. So far, it passes.

Step 3. The tech measures 9 inches from the closet's back corner to the penetration, at 38 inches above the floor. The laundry room shares that wall, and the matching spot is behind the washer. With the washer pulled forward (customer's permission, water left connected, the tech watching the hoses), the pipe is visible: it comes through the wall after a level run of 30 inches, measured along the copper.

Step 4. On the laundry side, the copper ends at a reducing coupling, 3/4 by 1/2, and becomes 1/2-inch PEX. The PEX rises 10 inches, from 38 to 48 inches above the floor, then turns down and enters a tee cut into the side of the washer standpipe, sealed at the joint. The print line on the PEX reads a rating of 200 F at 80 psi. PEX is on the 2021 IRC's accepted list, so the material is not the fault here; the way it was run is. There is no open end anywhere; the line is piped into the drain.

Four faults, in the order the pipe meets them:

• Reduced size. 3/4 inch to 1/2 inch at the coupling. The discharge must be full size its whole length.
• Uphill run. The PEX rises 10 inches before falling. The low point at the coupling holds water, so the line is trapped and does not drain by gravity.
• Direct connection, no air gap. The tee joins the discharge to the drain system. A backup in the standpipe can reach the valve, and no one can see whether the valve has been weeping.
• Termination in the wrong room. Even with an air gap, an end in the laundry room is not in the same room as the heater.

The tech's note to the customer:

• Water heater relief valve discharge: starts correctly in 3/4 in copper, then on the laundry side of the wall is reduced to 1/2 in plastic tubing, runs uphill 10 in, and is plumbed directly into the washer drain with no open end.
• If the valve ever opens, this line may not carry the water away, and nobody would see it happening. Have a licensed plumber reroute it to end, full size, with an open air gap, in the mechanical closet.
• I did not touch or test the valve and did not alter the pipe. Washer pushed back as found.

The tech pushes the washer back, confirms both hoses are still tight and dry at their connections, and attaches the note and the sketch to the ticket.

What would change the call. If the copper had come through the wall and dropped, full size, to end over the laundry floor drain with a visible gap, the only remaining fault would be the room: the air gap is outside the heater's room, and that alone goes on the note. If the closet had a floor drain and the end of the copper had been found 6 inches over it, open, the tech would have nothing to report and the note would say the discharge was traced and found correct. If the pipe had vanished into the laundry wall and not reappeared anywhere the tech could reach without opening drywall, the finding is "termination not found," and that is reportable on its own: the code wants the end where people can see it.

How you know you traced it right

• Your sketch shows the valve, every fitting with its distance, every change of direction with its height, and the end.
• You saw the end yourself, or wrote that you could not.
• Every fault is named by what it is, not as "pipe looks wrong."
• The valve lever was never touched and the end was never capped, plugged or extended by you.
• Anything you moved to see the pipe is back where it was, and anything connected to water is checked dry.

References

• International Residential Code, 2021 edition, P2804.6.1 (relief valve discharge piping), as adopted by your jurisdiction
• Water heater and relief valve manufacturer instructions
• See related: T&P Valve and Discharge Piping Verification
• See related: The Relief Pipe the Basement Finish Buried