Water Heater Replacement and Thermal Expansion
A 40-gallon residential gas water heater fails and is replaced with a like-sized unit. The home has a pressure-reducing valve at the meter set to 55 psi, a closed system with a check valve at the meter, and no expansion tank. The new heater is installed with a temperature-and-pressure relief valve piped in 1/2-inch copper to a floor drain 12 feet away with two 90-degree elbows and a horizontal run. The homeowner reports the T&P valve weeping after the burner cycles. The plumber proposes to raise the T&P setting, and separately to cap the relief line so it stops dripping.
1. Diagnose the weeping T&P valve. What is the actual cause and what is the correct fix?
Read the worked answer
The symptom - weeping after each heating cycle - is the classic signature of thermal expansion in a closed system with no expansion control. The PRV and check valve make the system closed; when the burner fires, water expands and has nowhere to go, so pressure rises until the T&P relief opens. It is not a faulty T&P valve and not a pressure problem at the meter.
The correct fix is to install a listed thermal expansion tank sized for the system volume and the pressure differential between the PRV setting and the relief setting, or an equivalent listed expansion-control device. The T&P valve setting must not be raised; it is a safety device set at the manufacturer's rating and is not adjustable in the field. Capping the relief line is a life-safety violation and must be corrected immediately.
Other checks the answer should include:
- Confirm the PRV is actually functioning and not passing high pressure intermittently.
- Verify the expansion tank pre-charge matches the supply pressure and that it is installed on the cold side between the backflow preventer/PRV and the heater.
- Confirm the T&P valve is the correct rating for the heater and is not obstructed.
- Confirm the heater's thermostat is not stuck, which would cause continuous over-temperature rather than cyclic weeping.
2. Evaluate the T&P relief discharge piping as installed.
Read the worked answer
The discharge line has several problems to check against code:
- Material and size. The relief line must be the same size as the valve outlet or larger, and of a material rated for the temperature. Reducing to 1/2 inch from a larger outlet is not permitted if the valve outlet is larger; the discharge must not be smaller than the valve outlet.
- Length and fittings. A 12-foot run with two elbows is long for a gravity discharge; every elbow and every foot adds resistance and can prevent full-flow discharge. The line must be as short and straight as practicable.
- Slope and termination. The line must pitch downward to the termination and must terminate not more than 6 inches above a floor drain or receptor, with a visible air gap, and must not be trapped, threaded, or connected directly to the drain. A direct connection to a floor drain is a cross-connection.
- No valves or obstructions. No shutoff valve, no cap, no plug, and no reduction in size anywhere in the line.
- Termination point. It must discharge to a safe place where scalding water will not injure anyone and where the discharge is visible.
The correct answer is that the installation must be re-piped to full size, shortened and straightened where possible, and terminated with a proper air gap above a receptor - and the cap must be removed before the heater is returned to service.