Water Heater Replacement in a Sealed Utility Closet
A property manager is replacing a failed 40-gallon atmospheric water heater in a 6-foot by 8-foot interior utility closet of a 1970s apartment building. The new unit is 50 gallons and 40,000 Btu/h. The closet has a solid door with only a 1-inch gap at the bottom and no combustion-air grilles. The existing T&P relief valve is rated 150 psi at 210°F, and its 3/4-inch copper discharge runs horizontally through the wall to the exterior, ending 8 feet above grade with a threaded cap. The heater sits on a plywood platform with no drain pan. The building recently installed a double-check backflow assembly on the domestic service and an expansion tank at the far end of the hot-water line. The gas supply is 1/2-inch black iron with a 25-foot equivalent length. Before turning on the gas, the inspector red-tags the installation and asks the contractor to correct the relief discharge, combustion air, and pan/expansion issues.
1. What is wrong with the T&P relief valve discharge, and how should it be corrected? Explain the reasoning, not just the list.
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The relief discharge is wrong in several ways. A T&P relief valve must discharge by gravity through a pipe the same size as the valve outlet, with no reduction, no trap, and no valve or cap. The threaded cap at the exterior end is a direct hazard: if the valve opens, the cap can block discharge and the tank can fail. The termination is also 8 feet above grade. Typical model codes require the discharge to terminate not less than 6 inches and not more than 24 inches above the floor, or at an approved exterior location where it will not cause injury or damage; a high exterior termination can freeze, is hard to observe, and can spray anyone below. The horizontal run through the wall must slope downward and be protected from freezing, and the discharge must remain visible so the owner can see weeping. The correction is to repipe full-size to within 6–24 inches of the floor and terminate with a plain end (no threads) over a floor sink or approved receptor, or terminate outside at a safe height with a downward bend and no cap. Do not plug the line to stop a drip; repair the cause.
2. Is the closet adequate for combustion air, and what would a compliant correction look like?
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The closet is a confined space. Its volume is about 6 × 8 × 8 = 384 cubic feet. At the usual 50 cubic feet per 1,000 Btu/h rule, that volume supports only about 7,680 Btu/h, far below 40,000 Btu/h. A 1-inch undercut is only one opening and is nowhere near the required net free area. The usual correction for a confined space is two permanent openings: one within 12 inches of the top and one within 12 inches of the bottom, each sized at 1 square inch per 1,000 Btu/h if communicating directly with a ventilated space, or sized per the duct table if using vertical or horizontal ducts. For 40,000 Btu/h that is at least 40 square inches net free area per opening under the simple rule, and more if louvers or grilles reduce free area. The door undercut does not count as a designed combustion-air opening. Also confirm the draft hood and vent are listed and that the closet is not depressurized by an exhaust fan.
3. What is required for the drain pan, and how does the closed system affect the installation?
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A drain pan is required where a leak would damage the structure or finish, and a plywood platform in a closet qualifies. The pan should be at least 1.5 inches deep, at least 2 inches larger than the heater, and drained by a 3/4-inch line to an indirect waste receptor with an air gap or air break, or to the exterior. The pan drain may not be trapped unless the trap is protected from freezing and evaporation, and it may not connect directly to the sanitary drain. The backflow assembly creates a closed system, so the expansion tank is necessary; check that it is on the cold-water side, precharged to the local static pressure, and sized for the 50-gallon heater. If the tank is undersized or waterlogged, the T&P valve will weep and the pan will catch it. The pan is not a substitute for a properly sized expansion tank, and the T&P discharge must not be piped into the pan.