Water Heater Expansion and Temperature Relief

The TPR valve is discharging because the closed system has no expansion tank. When water is heated, it expands. In a closed system with a backflow preventer and PRV, the expanding water has nowhere to go, causing pressure to rise. Once pressure exceeds the TPR valve setting (150 psi), the valve opens to relieve pressure. The popping sounds are likely due to scale or sediment buildup in the tank, causing localized boiling and steam bubbles collapsing. This is common in areas with hard water. The installation has several code violations: (1) Missing thermal expansion control device. The code requires an expansion tank or other means to control thermal expansion in a closed system. (2) The TPR discharge pipe termination: it terminates 6 inches above the ground outside, which may be acceptable if it is not more than 6 inches above the ground and not less than 6 inches above the ground? Actually, the code typically requires the discharge to terminate not more than 6 inches above the floor or outside, and not less than 6 inches above the ground. So 6 inches above ground is borderline; it should be not less than 6 inches and not more than 6 inches? That would be exactly 6 inches, which is acceptable. However, the pipe must not be trapped and must drain by gravity. Also, the TPR valve must be rated for the heater's capacity. The new 50-gallon heater may require a higher capacity TPR valve; the existing 150 psi/210°F may be adequate, but the installer should verify. (3) The heater is in the garage, and the top is 18 inches above the floor. For gas heaters, the code often requires the ignition source to be at least 18 inches above the floor to prevent ignition of gasoline vapors. That appears to be met. (4) The TPR discharge pipe must be the same size as the valve outlet, typically 3/4 inch, and must not be reduced. The installer should check. The popping sounds also indicate that the tank may have sediment, which reduces efficiency and can cause the TPR to discharge due to overheating. The solution is to install an expansion tank, flush the tank to remove sediment, and verify the TPR discharge piping.

The thermal expansion tank must be sized based on the system volume and pressure. A common rule is to use a tank with a volume of about 2-3% of the water heater tank capacity, but it depends on the pressure. For a 50-gallon heater, a 2-gallon expansion tank is often sufficient for a 50 psi system. The expansion tank should be installed on the cold water supply line between the backflow preventer and the water heater, and it must be rated for potable water. The tank's pre-charge pressure should be set to the system pressure (50 psi) before installation. The TPR discharge pipe should be corrected to terminate not more than 6 inches above the floor or outside, and not less than 6 inches above the ground, and it must be piped to a safe location where discharge will not cause injury or damage. It should be the same size as the valve outlet (typically 3/4 inch) and must not be trapped. The pipe should slope downward from the valve. Also, the TPR valve should be tested annually, and the expansion tank should be inspected for proper charge. The homeowner should also have the water heater flushed to remove sediment to stop the popping sounds. The installation should be inspected by the local authority.