Replacement Water Heater in a Mechanical Closet
A 75,000 Btu/h, 75-gallon gas-fired storage water heater is being replaced in a 6 ft by 8 ft mechanical closet of a 40-unit apartment building with 8-ft ceilings. The cold inlet is 1-inch with a reduced-pressure principle assembly and a check valve immediately upstream. The temperature-and-pressure relief valve has a ¾-inch outlet, but the installed discharge is ½-inch copper that rises 3 ft, runs 12 ft across the ceiling, and ends in a threaded adapter pushed inside the rim of a floor drain that is trapped and connected to the sanitary drain. There is no drain pan and no expansion tank. The closet has a louvered door. The maintenance technician says the arrangement is fine because the pipe 'just drains into the floor drain anyway' and has never discharged in 15 years.
1. List every defect in the relief valve discharge and explain the failure each one creates.
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The discharge is wrong in at least six ways, and each one matters at the moment the valve actually opens.
- Size reduced from ¾ in. to ½ in. The discharge must be the full size of the valve outlet. A ½-inch line cannot pass the full relieving capacity, so the valve cannot relieve the tank, and the reduced line can be blown apart by the pressure it was supposed to remove.
- Rises 3 ft. Discharge must flow by gravity, downhill only. A rise creates a trap that holds water, scales up, and can freeze; the standing water also corrodes the valve seat.
- Twelve feet of horizontal run with a trap-like configuration. Any arrangement that can hold water or create backpressure on the valve is prohibited.
- Terminates inside the rim of a floor drain connected to the sanitary drain. The discharge must terminate through an air gap so there is no physical connection to the drainage system - this is a cross-connection between potable-side equipment and the sanitary sewer.
- Threaded adapter on the end. A threaded end invites a cap or hose; capping a relief discharge turns the tank into a pressure vessel. The end must be unthreaded and must not be valved.
- Termination height and visibility. It must terminate not more than 6 in. above the floor (or to the exterior) where discharge is visible and cannot cause injury or damage - inside a floor drain, no one sees it weep until the tank has failed.
Also verify the valve itself is rated for the heater's input and storage capacity and set no higher than 150 psi / 210°F, and that it is a new device, not the one that came off the old tank.
2. Why does this installation need an expansion tank, and why is the relief valve not the answer?
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The domestic system is closed: a reduced-pressure principle assembly plus a check valve upstream of the heater means expanded water cannot push back toward the main.
When the burner fires and the tank recovers, water expands - on the order of three-quarters of a gallon for 75 gallons through a normal temperature swing. In a closed system that volume has nowhere to go until pressure rises. Typical symptoms: relief valve weeping, dripping faucets and shower valves, noisy pipes, and eventually a failed tank or fitting.
The relief valve is a safety device of last resort, set near 150 psi. It is not designed to cycle on every recovery, and repeated weeping damages the seat so that it leaks continuously - after which it may not reseat at all when it is genuinely needed. Turning the thermostat down is not a fix either, because the tank still reaches the setpoint.
Required: a listed thermal expansion tank on the cold side of the heater, sized for the system volume, the heater capacity, and the differential between the PRV setting and the relief setting, pre-charged to the system static pressure. Alternative approved means of expansion control are acceptable; the relief valve is not one of them. Note the RP assembly also has to be tested annually, so this building already has a maintenance burden attached to the closed system.
3. The closet is 6 ft by 8 ft with an 8-ft ceiling and a louvered door. Address the drain pan and combustion air requirements.
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Drain pan. A pan is required where leakage from the heater or its connections would cause damage to the building or its contents - a mechanical closet in a 40-unit apartment building with no other drainage qualifies. Provide a pan at least 1½ in. deep with a ¾-inch minimum drain to an approved location (indirect waste to a receptor or to the exterior), and pipe the relief discharge separately so pan drainage and relief discharge are not combined into one undersized line.
Combustion air. The closet volume is 6 × 8 × 8 = 384 cubic feet, but 75,000 Btu/h at the standard 50 ft³ per 1,000 Btu/h indoor-air allowance requires about 3,750 cubic feet. This is a confined space by calculation, so the louvered door cannot be assumed to be adequate; its free area and the volume behind it must be evaluated, and if the adjacent space does not supply enough air, two permanent openings to outdoors are required.
Lay out the numbers on the drawing: opening free area per the code method chosen (the two-opening method has a minimum area requirement as well as a per-Btu requirement), the louver free-area derating, and the elevation of the openings (one within the top 12 in. and one within the bottom 12 in. of the enclosure). Also confirm the appliance is listed for the installation and protected against ignition of flammable vapors, and that clearances to combustibles are maintained.
Finally, put the correction list in writing: relief discharge, expansion tank, pan, combustion air - every item on it is a re-inspection point.