Water Heater Replacement with a Plugged T&P Valve and a Closed System
A homeowner replaces a 40-gallon atmospheric gas water heater with a 50-gallon, 40,000 BTU/h unit in a 6-foot by 6-foot basement closet with an 8-foot ceiling. The closet has louvered bifold doors and no dedicated combustion-air ducts. The old T&P relief valve had been dripping, so the homeowner screwed a brass plug into the valve outlet and removed the discharge pipe. The new heater sits on the concrete floor; there is no drain pan and no floor drain. A 3/4-inch pressure-reducing valve was installed on the main three years ago. The flue is a 4-inch single-wall connector that runs 6 feet horizontally with no rise into an unlined exterior masonry chimney. The gas connector is a 1/2-inch appliance connector that passes through the closet wall.
1. Evaluate the T&P relief valve and its discharge. What must be corrected, and why is the plug dangerous?
Read the worked answer
The plug must be removed and the valve replaced, not merely unplugged and reused. A T&P valve is the final safety device for a closed tank; plugging it eliminates the only path for pressure relief if the thermostat fails or thermal expansion raises pressure above the valve setting. The result can be a tank rupture or an explosion. A dripping valve is a symptom to diagnose, not a leak to cap.
The replacement discharge pipe must be the full size of the valve outlet, typically 3/4 inch, and must not be reduced. It must be made of material rated for the temperature, must run by gravity with no traps, must have no valve or plug, and must terminate not more than 6 inches above the floor or at an approved outside location. The end must not be threaded, and the discharge must be visible and directed so it cannot scald anyone. If the valve drips after replacement, check for thermal expansion, a failed valve, or excessive pressure; do not cap it.
2. The PRV was installed three years ago and there is no expansion tank. How does that relate to the dripping T&P valve, and what is the correct fix?
Read the worked answer
A pressure-reducing valve creates a closed system: water cannot push back out through the PRV when it expands. As the heater raises the water temperature, thermal expansion increases pressure until the T&P valve opens and drips. The drip is doing its job, but repeated operation shortens valve life and wastes water. The missing expansion tank is the likely cause, not a defective T&P valve alone.
Correct the system by installing a potable-water expansion tank sized for the system volume, precharged to the static supply pressure, and located on the cold-water side downstream of the PRV and backflow protection. Check the system pressure with a gauge; a typical target is around 50-60 psi, not at the relief setting. If the T&P valve has been plugged or has operated repeatedly, replace it. Verify that the PRV is functioning and that there is no thermal trap or check valve trapping pressure between the heater and the fixtures.
3. Evaluate the combustion air and flue arrangement. Is the closet adequate as installed?
Read the worked answer
Probably not. The closet is 6 x 6 x 8 = 288 cubic feet. For an unconfined space using inside air, the usual rule is 50 cubic feet per 1,000 BTU/h, so a 40,000 BTU/h heater needs about 2,000 cubic feet. This closet is a confined space, and louvered bifold doors are not a reliable permanent combustion-air opening unless their free area is calculated and accepted. The safe correction is to provide two permanent openings sized per the code, or duct outside air to the appliance space.
The flue also has problems. A 4-inch single-wall connector running 6 feet horizontally with no rise is not a proper draft connector; it must slope upward toward the chimney at not less than 1/4 inch per foot, be as short and direct as practicable, and be sized to the draft hood outlet. An unlined exterior masonry chimney can cool the flue gases, cause condensation, and fail to develop draft; it may require a listed liner. Correct the vent connector and verify the chimney is suitable and lined before returning the heater to service. The 1/2-inch connector passing through the closet wall is also a defect: appliance connectors may not pass through walls, floors, or ceilings.