Garage Water Heater Replacement: T&P Discharge and Ignition Hazards
A 50-gallon atmospheric gas water heater is replaced in an attached garage in a climate with a design low of -5 F. The new heater is set directly on the slab next to the furnace. The T&P discharge is 1/2-inch Schedule 40 PVC threaded into the valve, run through the rim joist and out the sidewall, terminating 10 in above grade with a threaded cap on the end. The valve is marked 150 psi / 210 F. A new PRV at the meter has created a closed system, and no expansion tank was installed. The garage also stores a lawn mower and paint thinner. The inspector red-tags the job.
1. List the T&P discharge violations and explain the reason behind each requirement.
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1. Reduced size (1/2 inch on a valve with a 3/4-inch outlet). The valve's rated relieving capacity assumes full-size discharge. Any reduction chokes the flow and the valve cannot pass the BTU input it was selected for.
2. PVC as the material. PVC is not rated for 210 F. During a relief event it softens, sags, and collapses - the discharge fails exactly when it is needed. Use copper, CPVC, or another material listed for the temperature.
3. Threaded cap on the outlet. The discharge must be open, gravity-flowing, and incapable of being capped or plugged. A cap also creates a pressure trap at the valve.
4. Termination detail. The discharge should terminate within about 6 in of the floor or at a floor drain/receptor, or outdoors 6-24 in above grade pointing down and away from the building. Ten inches above grade outdoors in a -5 F climate is a freeze-and-block hazard - water left in the pipe freezes and plugs it. Best practice here is to terminate to a floor drain or an air-gapped receptor inside the envelope.
5. Also check: no traps, no valves, no uphill runs, and the discharge visible at the point of relief.
2. What ignition-source and physical-protection requirements apply to the heater in this garage?
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- Elevation: the ignition source must be at least 18 in above the floor to stay above gasoline vapors, which are heavier than air. A heater listed as FVIR (flammable vapor ignition resistant) may be installed on the floor per its listing, but the listing governs - check the label rather than assuming.
- The stored mower and paint thinner make this a live issue, not a theoretical one. If the listing does not permit floor installation in a garage, build a platform at least as wide as the heater and 18 in high.
- Physical protection: the heater must be protected from vehicle impact - a bollard, or placement out of the vehicle path. An unprotected heater next to a parking space is a common red tag.
- Seismic: in a seismic zone, two straps - one in the lower third and one in the upper third of the tank - with the lower strap at least 4 in above the controls.
- Clearances and venting: maintain listing clearances to combustibles and verify the draft hood, vent connector, and vent termination are correct for an atmospheric heater.
3. Is an expansion tank required here, and how would you install it? What else must be verified after the T&P piping is corrected?
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Yes. The PRV makes this a closed system, so water expanding as the heater fires has no path back to the main and drives pressure toward the 150 psi relief setting. Without a tank, the T&P becomes the expansion device - which is what the dripping discharge tube was reporting.
Installation: a listed diaphragm or bladder tank on the cold-water inlet, between the PRV and the heater, pre-charged to the line pressure downstream of the PRV with the system drained. Not between the T&P and the heater. Sized by the manufacturer's method using system volume, temperature rise, and the 150 psi relief setting.
Then verify:
- PRV setpoint below 80 psi and holding at no-flow.
- T&P uncapped, full-size, gravity-drained, terminating at an acceptable location.
- Whether the existing T&P has been cycling long enough to be weakened - replace it if the seat is damaged rather than reusing it.
- Gas connector, sediment trap, and shutoff at the appliance.
- Flue draft and vent connector slope on the atmospheric heater; a garage install with a blocked or short flue is a carbon monoxide risk.