Heating with Wood: Stoves and Chimneys

A good wood stove can heat a Wisconsin house on a few cords a winter. A badly installed or badly run one is one of the most common ways a house burns down. Both outcomes come down to a handful of rules: how far the stove sits from things that burn, how the smoke gets out, and how dry the wood is.

Clearances

Stoves heat the walls around them by radiation, and wood framing behind drywall can char and ignite at temperatures far below a flame. Purdue’s extension bulletin, written from the National Fire Protection Association’s rules, gives the unprotected minimums for an unlisted radiant stove ¹:

• 36 inches to combustible walls and to the ceiling above.
• 18 inches from single-wall stovepipe to combustibles.
• 36 inches for firewood, curtains, furniture, and paper ¹.

A listed stove, one tested to a UL standard and labeled, comes with its own clearances on the back plate, often much smaller. Follow the label and the manual; they’re what an inspector and your insurer will check.

Clearances can be reduced with wall shields of sheet metal or noncombustible board spaced an inch out from the wall on noncombustible spacers, open at the bottom so air circulates behind them. Brick or stone laid flat against a wall protects very little, because masonry conducts heat straight through ¹. A quick check: with the stove running hard, you should be able to hold your hand on the nearest wall ¹.

Floors need protection too, unless the stove sits on concrete: a noncombustible pad extending 12 inches past the sides and back and 18 inches past the loading door, where coals fall out ¹.

Stovepipe

The stovepipe (connector) runs from the stove to the chimney ¹:

• Same diameter as the stove’s outlet; 24-gauge steel or heavier.
• Any horizontal run must rise at least ¼ inch per foot toward the chimney.
• Each joint overlaps about 2 inches with the crimped (male) end pointing down toward the stove, so liquid creosote runs back inside instead of leaking out.
• Three sheet-metal screws per joint.
• One 90° elbow is best, two at most.
• Never run a stovepipe through a wall, ceiling, or closet. Use a listed thimble or chimney section where you go through anything combustible.

The chimney

Use either a lined masonry chimney in good condition or a UL-listed all-fuel prefabricated chimney; the insulated double-wall type usually needs 2 inches from combustibles and stays warm, so it collects less creosote ¹. Give each stove its own flue ¹.

Height follows the 3-2-10 rule: the chimney must be at least 3 feet above where it passes through the roof and at least 2 feet higher than any part of the roof within 10 feet horizontally ¹. A short chimney drafts poorly, smokes into the room when the wind blows, and drops sparks on the shingles.

Dry wood and hot fires

The single biggest factor in a clean, efficient, safe stove is dry wood. The EPA’s guidance ²:

• Season split wood at least six months, stacked off the ground with only the top covered; covering the sides traps moisture.
• Burn it under 20% moisture. Split a piece and press a moisture meter into the fresh face.
• Build hot fires. A smoldering, air-starved fire makes smoke, wastes wood, and coats the flue.
• Keep the doors closed except to load, so carbon monoxide stays out of the room.
• Never burn trash, cardboard, plastic, painted or pressure-treated wood, plywood, or particle board.

Creosote

Smoke is fuel that didn’t burn. As it rises and cools in the pipe and chimney, it condenses on the walls as creosote: first a black liquid, then a crust. Given enough of it, a hot fire ignites it, and a chimney fire burns hot enough to crack masonry and ignite framing and the roof ¹.

You can’t stop creosote completely, but you can keep it small ¹:

• Burn seasoned wood.
• After starting a fire or loading, run it hot for 10–20 minutes before turning the air down.
• Look up the flue after every cord or at least once a year and brush it when deposits build. Modern airtight stoves can build creosote faster than old leaky ones ¹. EPA also recommends a yearly inspection by a certified professional ².
• Empty ashes into a covered metal can and set it outside on a noncombustible surface ².

If you have a chimney fire (a roar like a train, the pipe glowing, sparks out the top): shut the stove’s air supply, discharge an ABC dry-chemical extinguisher into the stove, and get out and call the fire department ¹. Have the chimney inspected before using it again.

Try it

Measure your stove’s clearances against its label, or 36 inches if it has none. Check that every pipe joint has the crimp pointing down and three screws. Go outside and measure the chimney against the 3-2-10 rule. Then buy a moisture meter, split a piece from the middle of your woodpile, and see if it reads under 20%.