Chimney Flue Size: How Big Should a Flue Be?

What a flue has to do

A chimney draws because hot exhaust is lighter than the air around it. The rising column pulls fresh air into the fire behind it, and the taller and warmer that column, the stronger the pull. All of that depends on the flue fitting the fire it serves.

When the flue is too small, it cannot carry the smoke away as fast as the fire makes it, so smoke rolls out of the top of the fireplace opening. When it is too large, the exhaust spreads out, slows down and cools before it reaches the top. Cool, slow exhaust is behind most of what people blame on a bad chimney: weak draft, smoke at startup, heavy creosote from wood fires, and acidic condensation from gas and oil boilers that eats away at mortar and clay tile.

Height plays into it too. Every extra foot of chimney adds pull, so a tall rowhouse chimney running up four stories can work with a flue that would struggle on a short chimney over a one-story addition, where the fire needs every bit of flue area it is given.

Matching a flue to a fireplace opening

For an open masonry fireplace, building codes and fireplace design tables tie the flue's inside area to the area of the opening, width times height. The required ratio depends on the shape of the flue, because a round flue moves smoke more efficiently than a square or rectangular one of the same area, and on the height of the chimney, since a taller chimney draws harder.

That is why a single chart rarely settles it. A wide, tall opening on a short chimney may need a bigger flue than a rule of thumb suggests. Tall, shallow fireplaces of the old Rumford style, found in some early houses, are designed around their own proportions. When a fireplace smokes, comparing the opening with the flue's real inside size, measured at the top and at any narrow point on the way, usually explains it.

Sizing for stoves, inserts and heating appliances

With appliances, the rule comes from the appliance. A wood stove or insert lists the liner size in its installation manual, usually tied to the flue collar, and the liner generally should not be smaller than that collar. Some makers also limit how much larger than the collar the flue may be, because a stove's modest exhaust cannot warm a huge flue. Most insert manuals call for a full-length liner sized to the insert's outlet rather than venting into the open fireplace flue.

Gas and oil appliances are sized from venting tables that account for the appliance's input, the chimney's height, the length of the connector pipe and whether the chimney is on an outside wall. When a boiler and a water heater share one flue, the common vent is sized for both together. A wood-burning fireplace or stove needs a flue of its own and is not combined with a heating appliance.

Why old flues are so often the wrong size

Much of the housing stock across New York, New Jersey, the Philadelphia area and northern Delaware was built before modern heating. Boiler flues were sized for coal furnaces or big early oil burners that sent a large volume of hot exhaust up the chimney. A modern boiler or water heater in that same flue is a common mismatch, and a correctly sized liner is the usual fix.

Fireplaces have their own history. Many nineteenth-century brownstone and rowhouse fireplaces were built for coal grates, with small fireboxes and small flues to match. Open them up and burn a full wood fire, and the flue simply cannot keep up. Elsewhere the opposite happens: a large fireplace converted to take a small insert or gas logs, left venting into a flue several times too big.

Common sizes you will hear about

Flue and liner sizes are quoted in inches. These are common examples, not a sizing guide.

• Square clay flue tiles such as 8 by 8, 12 by 12 or 13 by 13
• Rectangular clay flue tiles such as 8 by 12 or 8 by 13
• Round stainless liners such as 6, 7 or 8 inches for many stoves and inserts
• Smaller round liners for many gas water heaters and boilers, sized from venting tables
• Clay tile is named by its outside size; the inside area that carries smoke is noticeably smaller

Signs of a size mismatch

The symptoms differ with the direction of the mismatch and with whether the flue serves a fire or a burner. Size is not the only possible cause of any of them, so they point toward a measurement rather than prove a mismatch.

• Smoke curling out of the top of a fireplace opening once the fire is going well: often a flue too small for the opening
• Smoke at startup that clears once the flue warms: often a cold or oversized flue
• Heavy, tarry creosote from moderate fires: exhaust cooling in a flue too big for the fire
• Rust, soot or water marks at a water heater draft hood: possible spillage, worth a prompt check
• A wet cleanout and crumbling mortar at the base of a boiler flue: condensation in an oversized flue

Measuring a flue properly

A real measurement is taken inside the flue, not on the outside of the tile sticking up through the crown. The technician measures the inside dimensions at the top, checks for any narrower point lower down, such as an offset or a damper throat, and notes the height from the fireplace or appliance connection to the top. A flue that changes size partway up is sized by its narrowest section.

You can help from inside the house. Measure the width and height of the fireplace opening, note the make and model of any stove, insert, boiler or water heater, and say which flue each one uses if you know. Having those details at booking lets the visit start with the right comparison.

Fixes when the size is wrong

An oversized heating flue is the easy case: a liner sized from the appliance's instructions and the venting tables goes inside it, often insulated on an outside chimney. An insert gets the full-length liner its manual calls for.

An undersized fireplace flue takes more thought. Sometimes the opening can be made smaller, by raising the hearth or adding a metal smoke guard across the top of the opening, so it matches the flue. Sometimes a taller chimney or a draft fan is the better answer, and sometimes the fireplace is better suited to a stove or insert. You get the measurements and the options in writing, so the choice is made on numbers rather than guesswork.

Whatever the fix, the flue is swept and inspected first. A new liner or a smaller opening will not help a flue that is partly blocked by fallen tile, debris or a nest.