Chimney Health Center

Firewood that has not dried out is carrying a large amount of water. That water does not disappear when you light the fire. It has to be boiled off first, and the energy to boil it comes out of your fire.

So the fire runs cooler, and the flue above it runs cooler. Smoke leaving a cool fire is full of unburned gas and steam, and when that mixture hits a cool flue wall it condenses instead of exiting. What it leaves behind is creosote.

Creosote is fuel. Once it builds up on the walls of your flue, you have a layer of burnable material lining the inside of your chimney.

The crown is the concrete slab at the very top of the chimney. It is the only thing covering the masonry underneath, and it takes weather from every direction with nothing above it.

When it cracks, water does not simply sit on top. It runs down into the gap between the flue liner and the brick, and it soaks into the masonry itself. Through a Connecticut winter that water freezes, expands, and pushes the face off the brick from the inside out. Mortar joints wash out behind it.

By the time the damage shows up somewhere you can see it, a stain on the ceiling or rust on the damper, the water has been working its way down for a long time.

Flashing is the metal that seals the joint where the chimney comes through the roof. It is two separate pieces. Base flashing runs up the chimney and out onto the shingles. Counterflashing is cut into a mortar joint and laps down over the top of it, so water running down the brick is carried back out onto the roof.

When the counterflashing pulls loose, or when it was only caulked to the surface instead of cut into the joint, water gets behind it. From there it runs straight down the chimney inside your roof, soaking the sheathing and framing on its way.

This is why a ceiling stain near the chimney is so often blamed on the roof. The roof is usually fine. The water is entering at the flashing and traveling before it ever shows itself.

The flue liner is the barrier between combustion gas and the rest of your house. Everything a fire produces, including carbon monoxide, is supposed to stay inside it from the firebox all the way to the top.

Liners crack. Age does it, settling does it, water does it, and a chimney fire does it fast. Once a tile is cracked, gas leaves the flue and enters the void between the liner and the brick, and from there it moves into wall and ceiling cavities the same way any air does.

The part that makes this dangerous rather than just inefficient is that carbon monoxide gives you nothing to notice. No smell, no colour, no taste, no irritation.

Creosote is what condenses on the flue wall when smoke cools before it exits. Every wood fire produces some. How much, and what form it takes, depends on how hot the fire burns and how warm the flue stays.

It arrives in stages. Stage one is dry and flaky and brushes out. Stage two is sticky when it forms and hardens into a crunchy layer that needs mechanical work. Stage three is glazed, a hard shiny sheet that has effectively fused to the flue and will not come off with a brush at all.

Each stage narrows the flue, which weakens the draft, which cools the flue further, which lays down more creosote. It compounds. A flue that reached stage two last winter will get there faster this winter.

A chimney does not push smoke out. Warm gas in the flue weighs less than the cold air outside, so the column rises on its own. That rising column leaves low pressure at the bottom, and that low pressure is what pulls air through your fire and carries the smoke away.

This matters because it explains almost every complaint people have about a fireplace or stove. A fire that smokes when you light it, a stove that will not stay going, smoke that appears when the dryer runs, an odour in the house in July. None of those are the fire misbehaving. They are the pull failing.

Three things break it. A cold flue has no warm column yet. A tight house with exhaust fans running has nothing left to give the fire. And a chimney that is too short, or that terminates in the wind shadow of the roof, never builds enough pull to begin with.

A cap is a stainless lid with a screen that sits over the flue opening. Without one, the flue is an open pipe pointing at the sky, and four separate problems start at once.

Rain goes straight down it. Animals nest in it, and a nest is both a blockage and a fire hazard. Water reaches the damper, smoke shelf and firebox and rusts everything metal it touches. And if you run an oil-fired appliance, the water does something worse than get things wet.

Oil exhaust carries sulfur compounds. When rainwater condenses with them inside the flue you get sulfuric acid, and it attacks clay tile and mortar from the inside. That one catches people off guard, because nothing about it is visible from the ground.

Clay flue liners were not standard in older construction. Plenty of chimneys in this part of Connecticut were built without one, which means the only thing containing flue gas and heat is the brick and the mortar joints between it.

Mortar was never intended to do that job. Over decades of heating and freezing it shrinks, cracks and falls out, and the joints open up. Heat and combustion gas pass through the gaps into whatever is on the other side.

In an old house what is on the other side is usually wood. Floor joists were commonly framed straight into the masonry, so the end of a structural timber can be sitting in the chimney wall itself. Wood exposed to repeated heat does not need to reach its normal ignition temperature. It chars, and charred wood lights at a lower temperature each time.