The Chimney

YOUR CHIMNEY IS THE ENGINE FOR YOUR WOOD HEATING SYSTEM

If your chimney is not working properly; neither will your wood stove, fireplace or furnace.  Without a good chimney the rest does not much matter.

WHAT IS A GOOD CHIMNEY?

  • the right chimney for the appliance is is serving;
  • sized correctly for the appliance – most often the same size as the appliance outlet collar;
  • located and positioned properly – generally within the heated space of the home (or if exterior then at least insulated);
  • installed properly – as per manufacturers instructions, Ontario Building Code and CSA B365 – which ever apply for the particular chimney.

A GOOD CHIMNEY WILL

  • allow fires to be lit easily and have a good draft
  • will not fill room or house with smoke when lighting a fire
  • will not spill or puff smoke back into room when door of wood stove is opened
  • will not emit foul stinky odors or a cold draft into the room when not in use

If you are planning to have a wood burning system installed.  The first critical component is the chimney.  Get reliable advice on the type and location of the chimney to be installed.  Most wood heat retailers and chimney sweeps can provide guidance in this area.  For most people it is preferable to have a professional install the chimney versus doing it yourself.  Once installed you MUST have a WETT Inspection – and you will need to find a WETT Certified Chimney Sweep, WETT Certified Technician or WETT Comprehensive Inspector to do this – the SITE BASIC Inspector is not permitted to perform inspections on new installations.

GOOD CHIMNEY PERFORMANCE = GREAT STOVE PERFORMANCE!

  • If there is no fire burning and the door is open, air should flow into the stove – not out.
  • When you light kindling the smoke should flow toward the chimney, not back into your face.
  • Kindling when lit should burn with a bright hot flame very quickly.
  • A properly built kindling fire burns bright and hot very quickly.
  • If operated properly there should never be any smell of smoke in the house.

HOW DOES YOUR CHIMNEY WORK?

If the chimney is the engine, then the fuel it needs to work is heat.  The more heat the better and more it works, however a good chimney really will not need much heat to work properly.  The more heat you put into it the more draft or power the chimney will deliver.

The stove helps the chimney produce more power (draft) and the chimney in producing the draft helps the stove work better.  The two of them in a good system work hand in hand.  A chimney that is interior to the home or one that is insulated is able to product more draft with less heat in is essentially a more efficient producer of energy/power (draft).

Chimneys do need heat to work properly and in doing so they do rob the home from heat.  Most chimneys need around 20% of the wood heat, which means that the stove is unable to send more than 80% into the home.  However, the more efficient the chimney is, the better the system functions and the more heat you have inside the home for each stick of wood burned.  Because the chimney needs the heat, stoves running much over 80% tend to have water vapor condensation inside the chimney, especially on exterior masonry chimneys.  Do not starve your chimney of heat as it does need heat to function properly.

In winter, a well-designed and properly installed chimney will always make draft and upward air flow, even without a fire burning.  When a fire is lit, the kindling ignites and burns easily, draft increases quickly and there is a nice, bright, hot fire immediately without any smoke in the house.

UNDERSTANDING HOUSE SCIENCE

Warm air inside the house wants to rise, and pull in cooler air at the lower points of the home.  This is a naturally occurring air flow in any home and is called stack effect.  The cooler the outside air, the stronger this air flow and pressure differential becomes.  The top of the house has a higher pressure and the basement a low pressure, in the middle of these two zones where the pressure is equal to outdoors is called the neutral pressure plane.  This pressure plane will move up and down in actual location in the home depending upon temperature differentials between inside and outside, the operation of exhaust fans in the home, etc.  If the home is tightly sealed this pressure can build to a greater degree and turning on exhaust fans in a tightly sealed home only exaggerates this further.  If the wood stove is situated in the basement, the lowest pressure point location and this pressure differential becomes high enough, air can be drawn in through the chimney in a reverse direction – sucking smoke and flue gases into the home with it.  In this example, you would need a very good chimney and wood stove to enable overcoming this situation at all times.

COMMON PROBLEMS & AVOIDING 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

DESIGN GUIDE SUMMARY

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

OUR CANADIAN WINTERS

Follow the design guide carefully or during our cold winters you will be sorry. Good design will pay off.

Author: Chimney Sweep

WETT Certified Advanced Chimney Sweep - Loving It Every Day