The Relief Pipe the Basement Finish Buried

Why this matters

A basement finishing crew framed a closet around a water heater and, to get the drywall flat, cut the relief valve's discharge pipe off a few inches past the valve and let it end inside the new wall. For eleven months nothing happened. Then the valve started opening at night, and every time it did, hot water ran down the inside of a finished wall where nobody could see it. The complaint that came in was a musty smell. This case follows how the finishing shop worked back from the smell to the pipe, why the valve was opening at all, and the check the crew now does before any wall closes near a water heater.

Safety first, before anyone opens a wall

When water is coming from somewhere near a water heater and nobody knows where, two hazards come before diagnosis.

• Scald. A relief valve discharges water at or near tank temperature. Do not put a hand or face near the valve outlet, the end of its pipe, or a wet opening in a wall beside it. Do not lift the valve's test lever to see whether it is the source.
• Water and electricity. This heater was electric, and the wet wall carried two receptacles. Before any opening, the tech had the homeowner switch off the breaker feeding those receptacles and confirmed with a plug-in tester that both were dead. If water had been reaching the heater's own wiring or element covers, the heater's breaker would have gone off too, and an electrician, licensed where your state or locality licenses it, would have checked it before it was turned back on.

If a relief valve is discharging continuously and hot, turn off the heater's energy supply (the breaker for an electric unit, or the gas control to OFF on a gas unit), keep people clear of the discharge, and call a plumber, licensed where your state or locality licenses it, the same day. Do not cap, plug or bend the pipe to stop the water. That is how this house got here.

The complaint

Eleven months after the basement was finished, the homeowner called the finishing shop: a musty smell in the new rec room, and paint on the baseboard of the utility closet's outside wall had bubbled. The wall was an interior partition, the closet's side wall, shared with the rec room.

The finishing shop's lead, Tomas, went out. A pin-type moisture meter on the rec room side read high along the bottom of that wall and normal at chest height. The wet band ran about 4 feet along the wall and about 14 inches up from the floor. The carpet edge was damp to the touch.

The candidates, and what killed each one

A supply line leaking inside the wall. The closet wall had a cold-water line in it feeding a laundry box on the far side. A pressurized supply leak runs all the time, so the water meter would show it. Tomas had the homeowner shut every fixture and the ice maker, and he watched the meter's low-flow indicator for 30 minutes. It did not move. Dead, as far as it went: a meter test only sees flow during the minutes you watch it, and a source that runs a few times a night would be invisible to it.

Seepage through the foundation. The pattern argued against it. The wall was an interior partition, 9 feet from the nearest foundation wall, and the slab between them read dry. Seepage wets from the outside wall inward; this wall was wet in the middle of the room with dry slab around it. Dead.

The water heater tank or its fittings. Tomas looked from inside the closet. The tank's base, the drain valve and the floor under the heater were dry. The cold and hot connections on top were dry. A leaking tank wets the floor around itself first, and this floor was dry. Dead, for now.

Condensation. A cold-water line sweating in a cavity can wet a bottom plate, but not 14 inches up the drywall in a basement where the air read 41 percent relative humidity. Dead.

That left one thing on the heater that is supposed to discharge water: the temperature and pressure relief valve, or T&P valve, which opens to relieve the tank if pressure or temperature climbs past its setting. Residential T&P valves are commonly set to open at 150 psi or 210 F; the setting is on the valve's tag.

Following the pipe

From inside the closet, Tomas found the valve on the side of the tank with a short length of copper coming off it. The copper ran sideways about 5 inches and disappeared through a hole in the closet's side wall, the same wall that was wet. There was no pipe on the rec room side.

With the receptacle circuit still off and the heater's breaker off too, so a discharge could not start while his hands were in the bay, he cut a 6 inch by 6 inch inspection opening on the rec room side, level with the pipe's height, wearing eye protection and a filtering facepiece respirator under the shop's written respiratory program (29 CFR 1910.134, which 29 CFR 1926.103 applies to construction work such as this repair), because board wet that long can carry mold. The discharge pipe ended in the stud bay, cut square, about 30 inches above the floor, pointing at the back of the rec room drywall. The inside of the bay was wet down to the bottom plate, and the back of the drywall below the pipe end was stained in a fan shape. That was the source.

   CLOSET SIDE          STUD BAY           REC ROOM SIDE
   T&P valve            pipe cut short,
   [==]---- copper ---->| end 30 in up      | drywall back
                        |  water runs down  | stained in a
   water heater         |  inside the bay   | fan shape
                        |        |          |
   floor dry here       |  wet plate, wet   | baseboard
   ---------------------+--- bottom 14 in --+- bubbled ----

Reading the finishing crew's job photos afterward showed the sequence. Before framing, the T&P pipe had run down the side of the tank to about 4 inches above the floor, as it should. To get a straight wall, the crew had moved the framing tight to the heater, cut the downturn off, and passed the stub through the new drywall so the closet side looked tidy. The pipe that had been the valve's way out now ended in a closed cavity.

Why the valve was opening at all

A dry pipe for eleven months, then water. Something had changed, and a relief valve that opens is a symptom, not the fault.

Tomas asked the homeowner whether anything had been done to the plumbing recently. Two months earlier, a plumber had installed a pressure-reducing valve at the water service because street pressure was high. A pressure-reducing valve usually acts as a check against backflow toward the street, which turns the house into a closed system: when the heater warms a full tank, the water expands and has nowhere to go but up in pressure.

The finishing shop was not the plumber, so Tomas did not diagnose further than this. He did screw a pressure gauge with a maximum-reading needle onto the laundry hose bib and left it overnight. The next morning it read 62 psi static, and the maximum needle sat at 150 psi. Pressure in the house had climbed to the valve's setting during the night's heating cycle. That fits thermal expansion in a closed system: the T&P was doing its job, opening at its setting, and it was opening into a wall.

The fix, by the trades that own each piece

• The discharge pipe. A licensed plumber rerouted it as one continuous pipe of the valve's full outlet size, with no valve or cap in it, ending where it can be seen within 6 inches of the floor, which is what IRC P2804.6.1, in the 2021 edition as adopted, requires in substance. The pipe now runs down the closet side of the tank to a point 4 inches above the floor, over a pan with its own drain.
• The cause. The plumber installed a thermal expansion tank on the cold supply to the heater, charged to the house's static pressure. The overnight gauge test, repeated, read a maximum of 74 psi against a static of 62.
• The valve. Because it had cycled repeatedly, the plumber replaced it rather than trust it to reseat, and switched the heater's breaker back on only after the tank was full and purged of air at a hot tap, because an electric element energized in a part-empty tank burns out. The repeated overnight gauge test was the check that the valve stayed shut and the pipe end stayed dry through a full heating cycle.
• The wall. Under the same respirator and eye protection, with the area closed off from the rest of the basement, the finishing shop removed the wet drywall to 24 inches above the floor along the 4 foot band, dried the cavity until the studs and plate read at the same moisture level as dry framing elsewhere in the basement, then restored the receptacle circuit and confirmed both receptacles live with the tester, replaced the insulation and drywall, and replaced the carpet pad along that wall.

The finishing shop's account of what it owed was short: it had created the hazard, so it paid for its own repair and the plumber's reroute, and told the homeowner so in writing the same week.

What the crew does now

Before any wall, soffit or closet closes within reach of a water heater or boiler, the finishing lead traces the relief discharge from the valve to its end, with a photo of each, and writes one line on the work order: "T&P discharge traced, full size, uncut, ends visible within 6 in of floor or to drain." If the new framing would move or shorten the pipe, the framing changes or a licensed plumber reroutes the pipe before drywall, never after.

Two related checks were added to the same line because the same closet raises them:

• A gas water heater in a new closet needs its combustion air and vent clearances checked against the heater's manual and the adopted fuel gas code before the walls go up; enclosing it can starve it of air or depressurize it. See related: How a House Pulls Flue Gas Back Down a Vent.
• Access. The heater's drain, controls and relief valve stay reachable, so the closet door or an access panel is sized to the work, not just the tank.

The trace takes a few minutes. The thing it prevents took eleven months to show and a wall to fix.

References

• International Residential Code P2804.6.1, relief valve discharge piping, 2021 edition as adopted by your jurisdiction.
• Water heater manufacturer's installation manual for relief valve, clearances and combustion air.
• OSHA 29 CFR 1910.134, respiratory protection, applied to construction work by 29 CFR 1926.103.
• See related: T&P Valve and Discharge Piping Verification
• See related: The T&P Discharge Pipe and the Rules Other Trades Break
• See related: How to Trace a T&P Discharge Pipe to Its End