The Water Heater Relief Valve
and Its Discharge Pipe: Missing, Capped, Uphill or Too Small
If a water heater’s controls fail, one brass valve is all that stands between a tank of superheated water and the house. The pipe below it has to do its job too.
Seventeen water heaters in Russ’s recent reports had a relief valve with no discharge pipe at all. Others had a pipe that was capped, too small, ran uphill, dumped into a bucket or stopped two feet off the floor. Across 269 houses, 30 had a relief valve or discharge pipe finding.
Water heaters are one of the few appliances in a house that can, in a rare worst case, explode. The relief valve is what makes that rare. The pipe is what keeps the valve from scalding someone when it works.
Scalding at the tap is a separate question. The post on hot water above 120 degrees covers the temperature Russ measures at every house.
What the relief valve does
Every tank water heater has a temperature and pressure relief valve, called a TPR or T&P valve. It’s a brass valve near the top of the tank with a small lever. If the thermostat or gas valve fails and the water keeps heating, or if pressure in the tank climbs too high, the valve opens and dumps water until things drop back down.
Most residential valves open at about 150 psi or about 210 degrees F. Water that hot flashes to steam when it hits open air. That’s why the pipe below the valve is as important as the valve itself.
What a correct discharge pipe looks like
Plumbing codes in Oregon and Washington set the basic rules, and they’re easy to check by eye:
- Same size as the valve outlet, usually 3/4 inch, the whole way
- An approved pipe material rated for hot water
- Runs downhill the whole way, so it drains by gravity
- No shutoff valve, cap, plug or threads on the end
- Ends close to the floor (the residential code says within 6 inches) or at a safe spot outside, where the water can be seen
The last point is there so someone notices. A relief valve that discharges into a hidden drain can run for weeks without anyone knowing the water heater has a problem.
Real Inspection Photos
Relief Valve Findings From Russ's Inspections
Discharge pipe problems on water heaters from Russ's recent reports.
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The relief valve discharge pipe on this heater was routed uphill instead of draining downward. -
Small copper tubing on the relief valve, much smaller than the valve outlet. -
This discharge pipe emptied into the utility sink, where a release would be hidden and hot. -
A discharge pipe ending in a bucket. The report recommended asking the sellers about leaks. -
The discharge pipe stopped too far above the floor, so a release could splash on someone nearby.
Missing, capped and too small
A valve with no pipe at all was the most common finding, in houses from Vancouver to Gresham. Russ’s report wording: the pipe is critical for safely directing hot water or steam away from people. When the valve opens with no pipe attached, it sprays sideways at about chest height.
An Oregon City water heater had a discharge pipe that was capped. Capping it disables the valve completely. A Brush Prairie heater had an undersized pipe, and a Portland heater had the valve connected to 3/8-inch copper tubing. A small pipe can’t carry the flow the valve is rated for, so the valve can’t relieve pressure as fast as it needs to.
Uphill, too high and other routing problems
In Washougal, the discharge pipes on two water heaters ran uphill to the outside. In Happy Valley, the same thing. A pipe that rises after the valve holds water in the low spot, and that water can corrode or block the valve, and freeze if the pipe runs outside.
A Beaverton pipe stopped too far above the floor, which means a discharge splashes at shin or knee height. A Portland pipe emptied into a laundry room utility sink, which hides the discharge and puts scalding water where someone might be washing their hands. In one Washougal house, the outdoor end of the discharge pipe appeared to have been used as an electrical ground, which is a job it isn’t meant for.
Buckets, stains and drips
A bucket under the end of the pipe is a clue. Russ found them in Sandy and Hillsboro and recommended asking the sellers about leaks, because people put a bucket there when the valve drips. In Damascus, a stain on the slab under the pipe pointed the same direction.
A relief valve that drips is doing its job, or failing at it. The common causes are high house water pressure, thermal expansion in a closed system that has no expansion tank, a thermostat set too high, or a worn valve seat. Russ measures house water pressure with a gauge on every inspection, so a high reading in the report and a dripping relief valve often go together. A licensed plumber should find the cause before anyone replaces the valve.
What Russ checks
He looks at the valve, the pipe size, material and route, where it ends, and for signs of past discharge like stains, mineral crust or a bucket. He checks the house water pressure. He doesn’t operate the valve, because an old valve that hasn’t moved in years may not reseal afterward. The post on water heater age labels covers how he estimates the tank’s approximate age from the data plate.
- Find the brass valve near the top of the tank and follow the pipe from it
- Confirm the pipe is the same size as the valve outlet, not a thinner tube
- Make sure it runs downhill and ends near the floor or outside
- Look for a cap, plug or valve on the pipe (there shouldn’t be one)
- Look for a bucket, a stain or white crust below the pipe end
The relief valve only protects the house if its discharge pipe is right. Missing pipes, capped pipes, undersized tubing and pipes that run uphill were the problems Russ found most. A dripping valve is a symptom, and a plumber should find the cause.
For gas water heaters, the post on backdrafting and combustion air covers the other end of the tank. The plumbing guide for before an offer covers the rest. To set up an inspection, you can book online.
Relief Valve FAQ
