Weeping T&P Valve on a Mezzanine Water Heater
A 100-gallon, 199,000 Btu/h atmospheric gas storage water heater is installed on a mezzanine platform. The closet is 6 ft by 8 ft by 8 ft with a single louvered door providing 200 in² of free area. The flue is single-wall pipe pushed into an existing unlined masonry chimney, and the pipe touches wood framing at the ceiling penetration. The temperature-and-pressure relief valve is rated 210°F/150 psi and is piped in 1/2-inch PEX to a floor drain 6 feet away, terminating 1 inch above the drain; the valve outlet is 3/4 inch. Since a pressure-reducing valve and a testable backflow preventer were installed at the service, the valve has wept about a cup of water after each heating cycle. There is no expansion tank, no drain pan and no pan drain; the mezzanine framing is exposed above the kitchen's suspended ceiling. The rating plate lists a maximum working pressure of 150 psi.
1. Diagnose the weeping relief valve. What is the most likely cause, what must be ruled out, and what corrective work do you require?
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The most likely cause is thermal expansion in a now-closed system. A pressure-reducing valve plus a testable backflow preventer creates a one-way boundary: as the burner heats 100 gallons, the expanded water has nowhere to go and the pressure climbs until the relief valve lifts, which is exactly why it weeps after each cycle rather than continuously.
Rule out the alternatives before accepting that diagnosis: a relief valve with a damaged seat that will not reseat, a thermostat or high-limit failure driving over-temperature, and a pressure-reducing valve passing or creeping, which would raise static pressure even when the heater is idle. Put a gauge on a test port and watch pressure across a full heating cycle from cold; measure static pressure when the building is quiet. A relief valve that dribbles continuously at idle is not an expansion problem.
The fix is a listed thermal expansion tank sized for the system volume and pre-charged to the static system pressure, installed on the cold water side with proper support, plus verification that the relief valve rating does not exceed the heater's 150 psi maximum working pressure. Never accept removal, plugging or capping of the relief valve as a solution.
2. Evaluate the relief discharge piping and the missing pan. Which installation details are unacceptable and what do you require?
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The discharge piping fails on several counts. A relief valve discharge must be full size, meaning the same nominal size as the valve outlet, so reducing a 3/4-inch outlet to 1/2-inch PEX is prohibited. The material must be rated for the temperature and pressure the valve can discharge; PEX is not an acceptable relief discharge material in most listings, while copper, CPVC or other listed materials may be.
The discharge must flow by gravity, must not be trapped, must not contain valves or obstructions, must be installed so it drains fully, and must terminate with an air gap to a receptor or within the prescribed distance above the floor, with no threaded end that could be capped. Running it to end 1 inch above a floor drain is closer to correct, but the drain is not a receptor designed to take the full discharge, and the reduction and material still fail. Require full-size, listed piping terminating with a visible air gap to a service sink or an approved receptor.
The missing pan is separate: a water heater on an elevated platform above a ceiling requires a pan with a drain to an approved location, and the pan must be sized to the heater and drain by gravity. The relief discharge should not be used as the pan drain.
3. Do the combustion air supply and the flue meet the code? Support your answer with the calculation and state what you require.
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Combustion air: the closet volume is 6 x 8 x 8 = 384 ft³. For a confined space taking all air from inside, the usual criterion is 50 ft³ of space per 1,000 Btu/h of input, so this appliance needs roughly 199 x 50 = 9,950 ft³. The closet is a confined space by a wide margin, which triggers the two-permanent-opening method: one opening near the top and one near the bottom, each with a free area computed at 1 in² per 1,000 Btu/h, or about 199 in² each. The installation has one louvered door at 200 in² gross, and gross area is not free area: metal louvers are derated and wood louvers more so, so the single opening fails both the free-area test and the two-opening requirement.
Flue: single-wall pipe requires clearance to combustibles (commonly 6 inches), and it is touching framing at the ceiling penetration, so that is a violation regardless of draft performance. Dumping a single-wall connector into an unlined masonry chimney is also not acceptable; the appliance needs a listed vent appropriate to its category, or a listed liner properly sized and installed in the chimney, verified for the appliance's input.
Require corrected combustion air openings with documented free area, proper clearances and a listed venting arrangement, and inspect for backdrafting at the draft hood after the corrections.