How to Keep the Space Around Your Furnace and Water Heater Safe

Why this matters

A gas furnace and a gas water heater burn fuel in your house. To do that safely they need three things from the space around them: enough air to burn cleanly, a clear path for the exhaust to leave, and nothing that can catch fire close to the flame. The closet or corner they sit in tends to become storage, because it is out of the way, and storage is exactly what takes those three things away. A laundry basket against the furnace door, paint cans beside the water heater, a box pushed against the low air grille: each looks harmless, and each can mean a dirtier flame, exhaust coming back into the room, or vapors reaching a burner.

This article is about keeping that space right. You are not adjusting, repairing or moving anything; you are clearing, looking and reporting.

First: the signs that mean leave or call now

• Gas odor (rotten eggs or sulfur): leave the house with everyone, do not touch switches, do not unplug anything, do not use a phone inside. Call the gas utility or 911 from outside.
• A sounding carbon monoxide alarm: get everyone outside into fresh air and call 911 or the fire department. Do not go back in to look at the appliances. See related: How to Respond When Your Carbon Monoxide Alarm Goes Off.
• Flames outside the burner area, scorching, or a burning smell from the appliance: keep clear and call your heating contractor or plumber; if there is fire, get out and call 911.

Everything below assumes none of those is happening. See related: When to Stop and Call a Licensed Pro Around the House.

What the space is doing

            VENT to outdoors: rises the
        |   whole way, no dips, joints
        |   tight, no gaps at the wall
  +-----|----------------------------+ ceiling
  |     |                            |
  |   connectors                    [=] HIGH AIR
  |   rise to vent                   |  opening, top
  |   /       \                      |  12 in
  | +-------+  +----+                |
  | |FURNACE|  |WATER   label sets   |
  | |       |  |HEATER  clearance to |
  | |burner |  |    |   combustibles |
  | +-------+  +----+   around each  |
  |                                 [=] LOW AIR
  |  [ open working space in front ] |  opening, bottom
  +----------------------------------+ 12 in  floor
    keep clear: storage, laundry,
    paint, solvents, gasoline

Combustion air. A conventional gas appliance takes the air it burns from the room around it. When the room is small and closed, like a utility closet, the model fuel gas codes (the International Fuel Gas Code section 304 and NFPA 54, as adopted by your jurisdiction) require openings to bring more air in. The common indoor arrangement is two openings to the rest of the house, one within 12 inches of the top of the space and one within 12 inches of the bottom, each sized to the total gas input of the appliances in the space. Those openings may be grilles in the wall or the door, or a louvered door.

The vent path. Exhaust leaves through the vent connector on top of each appliance, into a vent or chimney, and outdoors. On many older furnaces and water heaters it rises by natural draft, which means it relies on every joint being tight and the pipe sloping upward all the way.

Clearances. Every gas appliance carries a rating label that states its required clearance to combustibles, the minimum gap to anything that can burn, on each side, the front, the top and the vent. Those figures vary by model, so the label, not a rule of thumb, is what applies to yours. The model mechanical code also calls for working space in front of the controls (International Mechanical Code section 306, as adopted), so a technician can service it. See related: Reading the Labels on Your Furnace and Air Conditioner, and Reading the Label on Your Water Heater.

Step 1: Move stored items out of the clearances and away from the front

Take everything out of the space that does not belong there and put back only what you must, outside the label clearances and not in front of the appliance doors or the controls. A clear arm's length or more in front of each appliance is a sensible habit even where the label allows less.

Why: boxes and fabric too close to a hot cabinet or vent can scorch or ignite over time, and anything in front of the furnace door blocks the air the burner draws in at the front and the technician who needs to open it.

Step 2: Keep laundry and lint out of the closet

Furnace and water heater closets are often next to the laundry. Keep laundry baskets, folded clothes and lint out of the space. Lint is fine and burns easily, and a furnace draws it toward the burner compartment. If your dryer is in the same space, keep its lint screen clean and look at the dryer duct for lint build-up at the joints. See related: Your Dryer Vent and Why It Matters.

Step 3: Never store gasoline, solvents or chemicals near a burner

Gasoline, paint thinner, lighter fluid and similar liquids give off vapors that are heavier than air. They flow along the floor and can reach a pilot light or a burner several feet away. Store them in a detached shed or at least far from any gas appliance, in closed containers made for them. Many gas water heaters built in recent decades are designed to resist igniting floor-level vapors, but that is a backup, not permission to store fuel next to one. Where a gas appliance sits in a garage, codes as adopted commonly require its burner to be raised 18 inches above the floor unless the appliance is listed as resistant to flammable vapor ignition.

Keep bleach, pool chemicals, aerosol cans and paint strippers out of the closet too. Their vapors drawn into a flame form corrosive compounds that eat at the heat exchanger and the vent from the inside.

Step 4: Keep the air openings, grilles and louvered doors open

Do not block the high or low air openings with boxes, shelves or furniture. Do not replace a louvered or grilled door with a solid one. Do not tape over a combustion air opening or a duct bringing outdoor air into the space, even if it lets cold air in during winter. That cold draft is the appliance's air supply. If it is uncomfortable, tell your heating contractor; there may be a correct fix, but closing it is not one.

Step 5: Look at the vent and the top of each appliance, a few times a year

Use a flashlight and look; do not touch the vent, which can be hot. Look for:

• Soot or dark staining around the top of the water heater, at the draft hood (the open bell-shaped collar where the vent connects on many tank heaters), or around the furnace vent connection.
• Rust or white crusty residue at the vent connector, or rust flakes on top of the water heater.
• Scorched or melted paint, plastic or labels near the top of either appliance.
• A connector that sags, dips, has a gap at a joint, or has pulled away from the wall or chimney.
• Water dripping from the vent or stains running down the pipe.

Each of these can mean exhaust is not leaving the way it should. Do not try to reconnect or tape a vent. Report it.

Step 6: Keep a working carbon monoxide alarm near the space

A carbon monoxide alarm outside the utility closet and one on each sleeping level covers the case where something above goes wrong between visits. Test them monthly. See related: How to Test Your Smoke and Carbon Monoxide Alarms, and What Your Carbon Monoxide Alarm Is Telling You.

A worked example: a small closet that had become storage

A hall closet 4 feet by 3 feet with an 8 foot ceiling held a gas furnace and a gas water heater. The labels read 80,000 Btu per hour input for the furnace and 40,000 Btu per hour input for the water heater, both drawing air from the room. Two metal grilles in the closet door connected it to the hallway, one near the top and one near the bottom.

Is the closet big enough to supply air by itself? The standard method in IFGC section 304 treats a space as large enough when it has at least 50 cubic feet per 1,000 Btu per hour of total input.

• Total input: 80,000 plus 40,000 is 120,000 Btu per hour.
• Volume needed: 120 times 50 is 6,000 cubic feet.
• Closet volume: 4 times 3 times 8 is 96 cubic feet.
• 96 divided by 6,000 is 0.016, so the closet holds 1.6 percent of the air volume the method calls for.

So the two grilles are not optional; they are the air supply. For openings to the rest of the house, that same section calls for each opening to provide at least 1 square inch of free area per 1,000 Btu per hour of total input, and never less than 100 square inches. Here that is 120 square inches each.

What the grilles provide. Each grille measured 12 by 14 inches, 168 square inches of face. Free area is the open part between the louvers, and it is less than the face. Where the free area is not marked, the code's default for metal louvers is 75 percent: 168 times 0.75 is 126 square inches, just over the 120 needed. The grille's own marking, if it has one, governs instead of the default.

What the homeowner found. A plastic storage bin had been pushed against the inside of the door, covering roughly the lower half of the low grille. Half of 126 is 63 square inches, about half the 120 required. On top of the water heater sat a folded drop cloth, and on the floor beside it two cans of paint and a can of thinner.

What they did. They moved the bin, the drop cloth and the cans out of the closet; the thinner went to the detached garage. With a flashlight they looked at the water heater's draft hood and found a light ring of soot on its lip. They did not wipe it off. They photographed it and called a heating contractor, who confirmed that the water heater had been spilling exhaust at startup, cleaned the vent connector and checked the draft once the low grille was clear. See related: How to Photograph and Record a Problem for Your Contractor.

What would have changed the numbers. If the furnace had been a sealed-combustion type, with plastic pipes taking its air from outdoors, only the water heater would draw from the closet: 40 times 50 is 2,000 cubic feet needed, still far beyond 96, and each opening would need the 100 square inch minimum because 40 square inches falls below it. A wood louvered door instead of metal grilles would carry the code default of 25 percent free area, so the same 168 square inch face would provide only 42 square inches. In either case, sizing is the contractor's call; the homeowner's job is keeping what is there open.

What to report, and to whom

Call your heating contractor or plumber, and say what you saw, if you find: soot, scorching or melting near either appliance; rust or residue at the vent; a sagging, gapped or disconnected vent; water dripping from the vent; a pilot that keeps going out; or a yellow, lazy flame where you normally see blue through a viewing window. Mention any changes to the house since the last visit too, such as a new tight door, a closed-in closet or a new exhaust fan, since those can change how the appliance gets its air. See related: Smells in the House and What Each One Might Mean.

References

• International Fuel Gas Code section 304 and NFPA 54, National Fuel Gas Code, combustion air provisions, as adopted by your jurisdiction
• International Mechanical Code section 306, access and working space, as adopted by your jurisdiction
• See related: How to Respond When Your Carbon Monoxide Alarm Goes Off
• See related: Carbon Monoxide Testing + Response Reference
• See related: When to Stop and Call a Licensed Pro Around the House