Atmospheric Water Heater at 6,500 Feet in a Closet

Yes - and the arithmetic matters more than the label.

• Volume of the closet: 3 × 4 × 8 = 96 cubic feet. At the standard indoor-air allowance of 50 cubic feet per 1,000 Btu/hr, a 40,000 Btu/hr appliance needs 2,000 cubic feet. The closet is roughly 5% of what is required, so it is a confined space under any interpretation.
• Can the basement be counted? Only if the closet communicates with it through permanent openings. With louvered doors, the question becomes net free area, not gross door size: the openings must provide roughly 1 square inch per 1,000 Btu/hr (about 40 square inches here) and must not be defeatable - a door that can be closed, blocked, or weatherstripped is not a reliable permanent opening. Calculate free area from the louver schedule, not from the door dimensions.
• Do not count the basement casually. A finished basement with a bath fan, dryer, or kitchen exhaust upstairs can depressurize the space and reverse the draft even with adequate openings.
• Best answer for the homeowner: convert to a direct-vent/sealed-combustion or power-vent unit, which removes the combustion-air and chimney problem in one move. If keeping atmospheric, install properly sized high and low combustion-air openings and verify with a draft test.

No. Two problems compound each other:

1. Oversized flue. The 8 × 8 inch tile has roughly 50 square inches of area serving a single appliance with a 4-inch connector (about 12.6 square inches). A Category I draft-hood appliance needs the flue sized to it; an oversized, exterior, unlined masonry flue chills the flue gases, slows the draft, and promotes condensation of acidic flue products that attack the mortar. That is why unlined exterior chimneys are a classic source of winter spillage and CO complaints, and why a clay flue serving a single gas appliance often cannot be relined with a smaller tile without breaking it out.
2. Unlisted condition. The connector also has to be sized per the gas code tables for the appliance's listed draft-hood outlet and rise, and the termination must be correct - a screened masonry cap that was chosen for a fireplace is not a listed vent termination for a Category I appliance.

Practical options, in order of preference: install a listed flexible liner sized to the appliance inside the existing flue; install a power-vent or direct-vent unit vented with PVC per its listing (a liner then is unnecessary); or, if neither is possible, do not install the atmospheric unit. A spillage/draft test at start-up is mandatory, and a CO alarm on each level should be part of the handoff.

Altitude. At about 6,500 ft, atmospheric gas appliances over-fire unless they are derated. Manufacturers typically publish an altitude conversion table requiring an orifice change or kit, of roughly 4% input reduction per 1,000 ft above 2,000 ft - on the order of 18% here. Check the rating plate, the kit number, and whether the model is listed for this elevation at all. A unit installed without the altitude kit will run rich, produce more CO, and void its listing and warranty. Never drill an orifice as a workaround.

Relief valve piping. The 1/2-inch copper is fine in material and size (it must be full size, rated for 210°F, and run by gravity), but the threaded cap is a violation - it blocks the discharge and defeats the valve. Remove it, leave the end unthreaded, and terminate not more than 6 inches above the floor or to the exterior in a location that will not scald anyone. If it discharges near the floor drain, maintain an air gap.

Also verify the T&P rating matches the appliance (150 psi / 210°F typical), the heater is properly supported and drained, and a thermal-expansion tank or other expansion control is present if the system is closed by a PRV or check valve.