All about chimneys

A good chimney and system design produces desirable performance characteristics:

• Fires are easy to light and draft builds quickly
• Smoke does not fill the room when you try to light a fire
• No smoke spillage when you open the door to tend the fire
• No foul odors or cold air from the hearth when it is not in use

When planning a woodburning system, the first thing you need is reliable advice on matching the appliance to the right type and size of chimney.  Most wood heat retailers and chimney sweeps can guide you and there may be government agencies and publications you could get locally. We strongly recommend having your chimney professionally installed by someone whose references you have checked.  You never want to lie awake at night wondering if an incompetent chimney installation is putting your house and family at risk.

This is good wood stove performance:

(hint: it's really the chimney that's doing the performing!)

• When no fire is burning and you open the stove door, air flows into the stove, not out.
• When you light a kindling fire, the smoke immediately flows up the chimney, not into the room.
• A properly built kindling fire burns bright and hot very quickly.
• When you open the stove door to add more wood, smoke does not spill out.
• If you are careful, you can run the stove so that you never smell wood smoke in the house.

Understanding how chimneys work

Think of the chimney as the engine that drives the wood heating system.  Think of its fuel as heat.  Think of the power it puts out as draft.  The more fuel (heat) you give this engine (chimney), the more power (draft) it will deliver.  So, the hotter the exhaust gases, the more draft is produced.  Draft, by the way, is good.  It's the suction that keeps the smoke from coming into the room.  Insulation in the chimney is important because it helps to keep the exhaust hot until it is expelled outside, and so, increases draft.

The chimney works with the stove or fireplace in a kind of feedback loop.  Heat in the chimney makes draft, which pulls in more combustion air, which makes the fire burn hotter, which delivers more heat to the chimney which makes more draft and so on.  An insulated chimney makes more draft with less heat.

Wood-fired stoves and fireplaces operate on natural draft, meaning that no fans assist the flow of exhaust. For the chimney to function correctly, it needs to receive about twenty percent of the heat energy produced by the burning wood. Past experience has shown that when wood stove efficiency is much over 80 per cent, water vapor condensation can begin to occur in chimneys, especially outside ones made of masonry. Never starve your chimney of heat. It needs heat to function properly.

In winter, a well-designed and properly installed chimney makes some draft and flows some air upwards, even when no fire is burning.  When you build a fire in a stove connected to such a chimney, the kindling ignites easily, draft increases rapidly and you have a nice bright, hot fire right away—and no smoking.  This is the kind of system you want in your house.

Understanding how houses work

Straight systems produce some upward flow, even when no fire burns. When it is cold outside, the warm air inside the house wants to rise, producing a pressure difference: low pressure low in the house and high pressure high in the house. 

Houses are being built more tightly sealed for increased comfort and lower energy costs.  This is done by using doors and windows with gaskets and walls with a continuous air barrier (usually plastic film).  If you turned on a powerful range hood or downdraft kitchen exhaust in a relatively small, tightly sealed house, it might suck so much air out of the house that the pressure inside would fall enough to overcome chimney draft and suck the smoke out of the stove.  It's not that common yet, but it can happen.

Typical problems and ways to avoid them

1. Cold hearths and odors: when no fire is burning, cold air and/or odors seep from the stove.

• The air in a chimney that runs up the outside of the house gets chilled, so the draft in the chimney is less than the stack effect of the house, and the chimney backdrafts, making the hearth cold and causing unpleasant odors
• Cold backdrafting can also happen if the chimney penetrates the warm part of the building below its highest level, which it will if the hearth is installed at the low side of a cathedral ceiling, or in a single story section of a two-story house
• Install the chimney inside the building and have it penetrate the warm space at its highest level and it will make draft, even when no fire burns

2. Open door smoke spillage: when you go to reload, smoke rolls out the door.

• When you open the stove door, a lot of dilution air must flow through the opening to keep the smoke inside; if the exhaust flow is restricted, smoke will roll out into the room

• Go straight up, if possible; avoid 90 degree turns in the flue pipe and offsets in the chimney

3. Sluggish performance: smokey fire, hard to get a hot fire burning.

• Large, cold chimneys, like old brick ones, suck up the heat from the exhaust, causing slow draft build up.

• Size the flue to match the stove and use an insulated chimney to keep exhaust hot and moving quickly; never use an air-cooled chimney

Summary of design guidelines

1. Put the chimney inside the warm building environment
2. Go straight up, no elbows or offsets
3. Insulation around the flue liner
4. Flue sized to match stove

A word about climate and altitude

If you live in an area that has a real winter – the ground freezes and you get some snow – or if you live at high altitude – say more than 4,000 feet – you'll need to follow these design guidelines exactly in order to get perfect performance.  You people at low altitude with mild winters may not need to be quite so fussy, but, then again, good design always pays off in better performance.