Menu:

All about chimneys

The chimney is the engine that drives a heat system

No woodburning stove, fireplace, furnace, or cooking equiment can function properly without a good chimney.  A good chimney is:

A good chimney and system design produces desirable performance characteristics:

When planning a system, the first thing you need is reliable advice on matching the appliance to the right type and size of chimney.  Most exhaust professionals can guide you; there are government agencies and publications available. 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 you at at risk.

This is good system performance:
(hint: it's really the chimney that's doing the performing!)

Understanding how chimneys work

Think of the chimney as the engine that drives the 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.
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 system in a kind of feedback loop. 

Heat in 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.

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

Understanding how buildings work

When it is cold outside, the warm air inside the structure wants to rise, producing a pressure difference: Reference: 1.

  • STACK EFFECT The pressure difference is called stack effect. The colder it is outside, the greater is the temperature difference, so the stronger is the stack effect. A chimney installed in the middle of a  building naturally overcomes stack effect by being as warm, but taller than the building.

  • For further study of Stack Effect and home ventilation

    Homes and buildings 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, or turned on your clothes dryer, or your central air unit,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 back down the chimney.

    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 unit.

      • 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
      • Install the chimney inside the building, keep it as warm and as tall as the building and it will make draft, even when no fire burns

    2. Open door smoke spillage: smoke rolls out the door.

      • When you open a 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 system and use an insulated chimney to keep exhaust hot and moving quickly; never use and 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. People at low altitude with mild winters may not need to be quite so concerned, but, then again, good design always pays off in better performance.