Most homeowners never look at the relief valve on a water heater until it drips. That valve is a safety device, and the pipe on its outlet is part of the same safety path. A capped, reduced, sagging, or misdirected discharge line can leave the valve doing its job on paper while the water has nowhere safe to go. The good time to check is now, with the house quiet and the tank cold.
What the Valve Is Doing
A temperature and pressure relief valve opens when the tank gets too hot or when too much pressure builds inside it. Water or steam leaves through the outlet and runs down the discharge line. It is a last line of defense, not a daily pressure control. If the valve weeps often, the cause is usually upstream: a failed pressure regulator, a closed system with no expansion control, or a water heater set hotter than intended.
Walk the Discharge Line
Start at the valve and follow the pipe to its end, using a flashlight and a tape. Check four things. The line should match the valve outlet size along its full run and should not step down in diameter. It should slope down and away from the tank, with no low loops that trap water, since trapped water can freeze on an exterior run. It should end in an open air gap above a floor drain or at an exterior location your local code allows, never capped, plugged, or submerged. The material should be rated for hot water discharge, because some plastic lines are not.
The most common mistakes are easy to spot once you know them: a reducer fitting at the valve, a cap or plug on the far end, a run tucked behind a finished wall with no visible termination, and a line that dumps onto a garage floor pitched back toward the tank. Any of these means the valve has a path that will not carry the water it is meant to carry.
Test the Valve Without Getting Scalded
Lift the test lever for a few seconds, stand to one side, and expect hot water and a rush of flow out the pipe. Watch the whole run and the end point while it discharges. Release the lever. The valve should reseat and stop. If it keeps dripping after reseating, the valve itself may be failing, and replacement belongs to a licensed plumber in most places. Do not plug, cap, or reroute the line to hide a drip. The discharge is hot, so keep children and pets well clear while you test.
Check the Expansion Tank Too
When the water supply has a check valve or backflow preventer, thermal expansion has nowhere to go. The relief valve then starts carrying a load it was never meant to carry day after day, and a leaking valve is often the first sign. An expansion tank handles that load. Tap the tank: the upper section should sound air-filled, and the lower section should sound solid with water. With the system isolated and the pressure off, check the air fitting with a tire gauge and compare the reading to the precharge printed on the tank label. A waterlogged or undercharged tank has lost its cushion, and the relief valve will keep doing the work until the tank is fixed.
Where the Parts Come From
ACHR News reported that F.W. Webb opened a new self-service store in Amherst, New York. The item describes the store as offering "a broad selection of plumbing, heating, HVACR, and industrial products" for contractors and other trades professionals. Check with the store before you go if you are a homeowner shopping alone. Bring the valve's rated pressure, rated temperature, and outlet size written down. A replacement is only the right part when those three numbers match the tank and the local code.
Keep the Roof Penetrations Dry
Some gas water heaters vent through the roof rather than the wall. Check that flue for a sound cap, a clean collar, and a flashing seal that sits tight to the shingles. If the penetration weeps or stains the ceiling below, the problem is in the roof, and a roofing contractor such as Keys Roofing can inspect the flashing and reseal it properly.
Run this check once a year, before the heating season and again after the first hard cold snap. Write the date on the tank. A discharge line you have walked is one you know will carry water when it has to.