Second-Floor Water Heater: Relief Discharge, Thermal Expansion, and the Damp Pan
A 12-unit apartment building installs a 100-gallon, 199,000 Btu/h gas-fired storage water heater in a second-floor mechanical closet. The plans show a 150 psi/210°F temperature and pressure relief valve with a ¾-inch CPVC discharge pipe, trapped, ending 6 inches above the closet floor; a 1-inch-deep drain pan with a ½-inch drain running 20 feet to a floor drain in the adjacent corridor; and no expansion tank. The city supplies 85 psi through a new reduced-pressure-zone assembly and a 60 psi pressure-reducing valve at the service. Two months after occupancy the relief valve discharges intermittently and the building manager reports the pan is always damp. The heater is not strapped.
1. Explain the intermittent relief valve discharge. What is missing, and what would you require as the fix?
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This is a closed system, and nothing in the design accounts for thermal expansion. The RPZ assembly and the pressure-reducing valve both act as backflow preventers, so heated water has no path back to the main.
Rough number: a 100-gallon tank heated from about 50°F to 140°F expands on the order of 1½ gallons. Forced into a rigid system, that volume raises pressure until the relief valve opens - which is exactly the intermittent discharge reported, and it recurs every heating cycle.
Require:
- A listed thermal expansion tank sized for the system volume, pre-charged to the downstream static pressure, installed on the cold side between the backflow preventer and the heater.
- Verification of the actual downstream pressure with a gauge, because a failed or mis-set PRV would produce the same symptom. Testing tells you whether you have an expansion problem, a pressure problem, or both.
- The relief valve itself must be checked for the correct rating and for a proper tag and setting; a leaking relief valve that has been cycling should be replaced.
An expansion tank is the standard remedy; a thermal expansion relief valve is a fallback only where the code permits it.
2. Evaluate the relief discharge piping and the drain pan against code requirements. List each deficiency and the correction.
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Relief discharge
- The trap must go. A relief discharge must be gravity-flow and may not be trapped. A trap holds water and debris and can block or conceal a discharge.
- Material. The pipe must be rated for the discharge temperature - on the order of 210°F. Verify the CPVC listing; many CPVC systems stop well below that at working pressure, so copper is the defensible choice. State the rating you relied on.
- Size and termination. The discharge must be at least the full size of the valve outlet, must not be reduced, must not be threaded at the end, and must terminate in a location where discharge is visible and cannot cause injury or damage - typically a receptor or outdoors. Six inches above a closet floor with no receptor is acceptable only if the floor is drained and the discharge is visible.
- No valves or obstructions anywhere in the discharge line, and the valve must remain accessible.
Drain pan
- The pan must be at least 1½ inches deep (1 inch is deficient).
- The pan drain must be at least ¾ inch (½ inch is deficient), with a gravity run to a receptor, and the pan must be at least 1½ inches larger than the heater on all sides.
- A listed pan with a water-level alarm or shutoff is an acceptable supplement, never a substitute for the drain.
The 20-foot ½-inch run is both undersized and likely to silt and clog, which explains the persistently damp pan.
3. What else must be confirmed before this heater can pass final inspection?
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Walk the whole install, not just the relief:
- Seismic/restraint strapping where required: two straps, lower one in the lower third of the tank and upper one in the upper third, at least 4 inches above controls, anchored to structure.
- Combustion air and venting for a 199,000 Btu/h appliance in a closet: verify the air openings and the vent category, terminations, clearances, and that a louvered door is not being counted as combustion air unless the free area is documented.
- Access and clearance to the relief valve, drain, controls, and the flue for service.
- Isolation: a shutoff valve on the cold inlet and a union or flange at the connections, with the relief valve not isolated between two valves.
- Temperature control - verify the delivery temperature and any master mixing valve serving the dwelling units.
- Condensate or drain termination if the appliance is a condensing type.
- Confirm the gas supply sizing and sediment trap for the new load and that the appliance regulator matches the manifold pressure.
Final inspection is a system check; a clean relief valve on a badly vented, unstrapped heater still fails.