T&P Valve Discharge on a Closed Water System
A 50-gallon residential gas water heater in a garage was replaced in kind. The house has a pressure-reducing valve at the meter set to 65 psi and a double-check backflow preventer, and the new heater has a check valve on the cold inlet. The thermostat is set to 140 degrees F to stretch the hot water supply. The T&P valve is rated 150 psi / 210 degrees F. The relief discharge is piped in 3/4-inch copper reduced to 1/2-inch copper through a wall, terminating 4 inches above the driveway. The owner reports a steady drip at the T&P valve that started about two weeks after the install and stops if the water heater is shut off.
1. Diagnose the drip. Support your diagnosis with numbers, and state whether the T&P valve is defective.
Read the worked answer
This is thermal expansion in a closed system, not a bad relief valve.
The math: the heater is isolated from the main by the PRV, the double check, and the new inlet check valve, so there is nowhere for expanded water to go. Heating 50 gallons from about 60 degrees F to 140 degrees F expands water roughly 1.6 percent:
50 gal x 0.016 = about 0.8 gallon of extra volume.
Water is essentially incompressible; forcing 0.8 gallon into a fixed 50-gallon steel vessel drives pressure up fast - far past the 150 psi set point of the T&P. The valve opens, relieves a few ounces, reseats, and repeats. That is exactly the intermittent drip reported, and it is confirmed by the drip stopping when the heater is shut off (no heat, no expansion).
So the valve is doing its job. The defect is the missing expansion control. Note also that 65 psi static plus expansion spikes is a lot of stress on the whole system; the fix protects fixtures and the heater tank as well as stopping the drip.
2. Evaluate the relief discharge piping as installed and list every correction required.
Read the worked answer
The discharge is noncompliant and dangerous in several independent ways:
- Reduced from 3/4 inch to 1/2 inch. The relief discharge must be the full size of the valve outlet, with no reducing coupling anywhere in the line. A reduced discharge chokes capacity and can prevent the valve from relieving fast enough.
- Terminates 4 inches above a driveway. The end must discharge to a safe location - typically not less than 6 inches and not more than 24 inches above the floor or grade, into a drain or to the exterior where it will not cause injury, damage, or a slip hazard. A driveway is a discharge point that scalds feet and children.
- Subject to freezing. An exterior termination in a cold climate can ice over and block the discharge entirely, which defeats the valve.
- Verify the end is not threaded or capped and that no valve, plug, or trap is installed in the line, and that the pipe material is rated for 210 degrees F.
- Verify the discharge runs by gravity with no low spots that will hold water and corrode the line.
Correct scope: replace the 1/2-inch section with 3/4-inch rated pipe, re-route to a safe termination with adequate clearance, and confirm no valves or reductions were added downstream.
3. The installer proposes to fix the drip by removing the check valve on the cold inlet. Evaluate that fix and give the correct solution.
Read the worked answer
Reject it. Removing the check valve may restore a path for expansion back into the main, but:
- The backflow preventer and PRV at the meter may themselves act as effective closed-system boundaries anyway, so the drip may simply return.
- A purveyor or code requirement for backflow protection at the meter is not the plumber's to waive; creating a connection that allows expansion to push hot water back toward the public main is a cross-connection problem.
- It addresses the symptom at the cost of a code violation.
Correct solutions, in order:
- Install a listed thermal expansion tank on the cold side downstream of the check valve, pre-charged to the system pressure (65 psi in this case) and sized for the 50-gallon system volume and the temperature rise - roughly a 2-gallon nominal tank for this application; follow the manufacturer's sizing table.
- Where an expansion tank is impractical, a listed thermal expansion relief valve rated for the application may be used, piped to a safe drain.
Separate point on the 140 degrees F setting: storage at 140 degrees F is allowed, but delivery to fixtures must be limited to about 120 degrees F. If the owner wants 140 degrees F storage, a master tempering (thermostatic mixing) valve must be installed at the heater outlet, and the relief discharge correction above still applies.