Orphaned Water Heater on an Oversized Common Vent

Room volume test.

• All-air-from-inside allowance is 50 cu ft of space per 1,000 Btu/h of input.
• Required: 40,000 / 1,000 x 50 = 2,000 cu ft.
• Available: 12 x 10 x 7.5 = 900 cu ft.
• 900 < 2,000, so the room is a confined space. It must have permanent openings, and because the basement was spray-foamed it cannot be treated as a leaky space.

Makeup air requirement.

• Two permanent openings, one within 12 in. of the top and one within 12 in. of the bottom of the enclosure.
• All-air-from-outdoors: 1 sq in. free area per 1,000 Btu/h = 40 sq in. each.
• Vertical ducts to outdoors: 1 sq in. per 2,000 Btu/h = 20 sq in. each.
• Horizontal ducts to outdoors: 1 sq in. per 4,000 Btu/h = 10 sq in. each.
• Louvers and grilles get sized on free area, not frame size, so a 40 sq in. requirement is often a much larger grille.

Conclusion: as it stands the room is a confined space with a solid door and no openings. That is a violation and, with a draft hood appliance, a spillage and CO hazard. Correct it before anything else.

The vent problem.

• The table shows 5 in. at 20 ft of rise is good for 200,000 Btu/h. The connected load is now 40,000 Btu/h.
• A vent that oversized never warms up enough to establish a steady draft. Flue gases cool, the water vapor condenses, and the condensate is acidic - it eats the vent from the inside and drips at the joints. During warm-up and standby the draft hood spills into the room.
• The 4 ft connector is also now sized for the old combined load; the connector may not be smaller than the appliance draft hood outlet, but it should be sized for the new single load.

Corrective work:

1. Reline the existing chase with a listed 3 in. liner - the table gives 3 in. = 60,000 Btu/h at 20 ft, comfortably above 40,000 Btu/h - or install a 3 in. vent, properly supported and terminated.
2. Keep the connector as short and as direct as possible, sloping at least 1/4 in. per ft upward toward the vent, with a listed thimble at the wall.
3. Provide the combustion air openings from the previous step.
4. Cap or seal the old furnace breeching so it cannot leak flue gas into the room.

Verification:

• Draft gauge at the connector, smoke test at the draft hood, and CO readings in the flue and in the room after the unit has run 10-15 minutes.
• Spillage test at the draft hood at start-up and again after steady state. If the hood spills, the vent or the air supply is still wrong.

Refuse the location as proposed.

• Appliances with draft hoods cannot take combustion air from a bedroom or bathroom, and a closet opening directly into a bedroom is treated the same way for air-supply purposes.
• The closet volume is 4 x 4 x 8 = 128 cu ft against the 2,000 cu ft the 40,000 Btu/h input requires. It would be a confined space with no acceptable air source.
• A solid or louvered door to a bedroom is not a compliant air path, and a bedroom cannot be the relief path for spillage.

Options that do work:

• A direct-vent or sealed-combustion water heater, which takes air from outdoors through a two-pipe system and is not dependent on room volume.
• A power-vented unit vented through the sidewall with outdoor air piped to the appliance.
• Keeping the atmospheric unit where it is, with proper openings and a correctly sized vent.

Also mention: if the appliance is converted to a sealed-combustion or direct-vent type, the venting and air-supply analysis changes entirely - rerun the vent sizing for the new appliance and keep the CO and draft testing at final.