Do I Need a Chimney Liner?

What a liner is for

Brick and mortar are good at standing up straight for a long time. They are not designed to absorb hot gases, acidic exhaust and moisture from the inside year after year. The liner is the inner shell, usually clay tile, stainless steel or a poured cement lining, that takes that punishment so the masonry around it does not have to.

A sound liner does four jobs at once:

• Keeps heat away from joists, studs and lath that sit close to the outside of the chimney
• Keeps smoke and exhaust inside the flue instead of seeping through mortar joints into rooms
• Protects brick and mortar from the acids and water in exhaust
• Gives the flue a smooth, correctly sized path so the draft is strong and steady

Which chimneys need one

The rule of thumb is simple: if anything burns into the flue, the flue needs a working liner. That covers an open wood fireplace, a wood or pellet stove, a fireplace insert, an oil or gas boiler or furnace, and a gas water heater. Gas appliances are not an exception. Gas exhaust is clean of soot but heavy with water vapor, and in a cold flue that water condenses and works on bare brick and mortar.

A chimney that serves nothing at all, such as a sealed-off fireplace in a back bedroom, does not need a liner to stay safe. It still needs a cap or a sealed top so rain, nests and animals stay out, and it needs a liner again the moment anyone decides to use it.

Multi-flue stacks need each flue judged on its own. A brownstone chimney can hold a lined boiler flue next to an unlined fireplace flue and a third that has been sealed for decades, and the condition of one says little about the others.

How to tell whether yours is lined

From the ground, look at the very top of the chimney, with binoculars if the house is tall. A clay liner usually shows as square, rectangular or round tile sticking up a few inches above the crown. A metal liner usually shows as a round pipe or a flat top plate under a cap. If all you see is a dark hole framed by brick, the flue may be unlined.

From inside, open the damper or the cellar cleanout door and shine a strong flashlight upward. Clay tile looks smooth with a joint every couple of feet. Bare brick looks rough, with mortar lines every few inches. A boiler flue often shows its liner where the connector pipe enters the chimney in the cellar.

Why older homes in the region so often need work

Housing across New York City, northern New Jersey and Philadelphia includes a lot of masonry chimneys built before liners were standard practice or before modern heating appliances existed. In roughly the order they turn up, these are the reasons a technician recommends relining:

• No liner at all, common in nineteenth-century rowhouses and brownstones where flues were only parged with mortar
• Cracked or broken clay tile from a chimney fire, water and freeze-thaw, or settling
• Eroded joints and flaking tile from years of oil or gas condensation
• A flue left too large after a big old boiler was replaced with a smaller, more efficient one
• A water heater stranded in an oversized flue once the furnace that shared it is gone
• A new stove or insert added to a fireplace flue without a liner connected to it

When a sound liner is still the wrong one

Size matters as much as condition. A flue that is too small for the appliance cannot carry all of its exhaust, so some spills back into the house. A flue that is too large lets exhaust slow down and cool, which makes condensation on gas and oil flues and creosote on wood flues, and it weakens the draft.

That is why appliance changes trigger the question. When an oil boiler is converted to gas, a high-efficiency furnace goes in, or a wood stove insert is fitted into an old fireplace, the new appliance's venting instructions decide the flue size and liner type. An old clay flue sized for a large fireplace or a big oil burner is often far too generous for what replaced it.

If nothing looks wrong

Relining is not automatic. If the existing liner is intact, its joints are tight, and its size suits the appliance, it can keep doing its job. The dependable way to know is to have the flue examined over its full height, because the damage that matters is usually out of sight from the firebox and the roof. NFPA 211 expects an inspection every year, and that visit is when liner problems are normally caught.

Plan the inspection before buying a new boiler, water heater, stove or insert, not after. Finding out on installation day that the flue needs relining is the most common way a simple appliance swap turns into a delay.

Relining options in plain terms

A stainless steel liner, lowered from the top and connected to the appliance, is the usual choice for boilers, furnaces, water heaters, stoves and inserts. It comes in grades and wall thicknesses matched to the fuel, and it can be insulated where the chimney runs cold or the appliance's listing calls for it. Aluminum liners are used only for certain gas appliances whose instructions allow them.

A cast-in-place liner is a cement mix poured or pumped around a form inside the old flue, which also adds some strength to tired masonry. New clay tile is mostly installed when a chimney is being rebuilt from the roofline up. Which option fits depends on the fuel, the flue's shape and offsets, and the condition of the brick around it.

What the visit involves

The technician looks at the chimney top, cap and crown, examines the flue over its full height, checks the base and cleanout, and reads the data plate and venting instructions of whatever appliance the flue serves. If the liner is sound and correctly sized, you hear that plainly.

Signs of trouble found along the way are noted too: tile shards or scale at the base, staining on the chimney breast, rust on the connector pipe, or a draft that is weak when the appliance runs. Each one helps explain whether the liner is the problem or something else is.

If it is not, you get the reasons, the options that fit your chimney and appliance, and a free estimate in writing, so you can decide without pressure.