Smoky woodstove: constant or erratic smoking
Current standards require that unless specified otherwise by the manufacturer, the chimney flue should be at least as large as the flue collar of the stove.
Here's why: The stove collar on any given model of stove is sized in accordance with the stove's design. That model needs at least that much vent capacity to vent the combustion products it emits, properly and safely.
Undersized flues lack this needed capacity, and often spill smoke into the home. Yes, some stoves function with undersized flues without spilling smoke. But even if the stove doesn't smoke as a result, bottlenecking the smoke and gasses into an undersized flue could create a fire hazard, and should be avoided.
If the flue is even partially blocked, reducing the effective venting capacity of the chimney, the stove might spill smoke. Soot and creosote buildup, animal nests, leaves and debris, or internal collapse of chimney liners or brickwork can cause blockages. If you think your chimney may be blocked, or if you haven't had it checked by a chimney professional within the past year, make an appointment for a chimney check. Your chimney professional is qualified to identify and correct chimney blockages, and to check your chimney for other hazards as well.
Multiple Appliances Connected to the Same Flue
In the past, people often connected wood stoves to flues already serving a furnace, fireplace, or other appliance. Newer standards require that the wood stove have its own, separate flue. There are several good reasons for this:
Soot and creosote from the wood stove could potentially block the flue, causing toxic exhaust from another appliance to enter the home.
Example: A gas-fired furnace or water heater could vent odorless, potentially lethal carbon monoxide (CO) fumes into the house if the flue serving it is blocked.
The draft to one appliance can be severely affected by the other appliance.
Example: Each time the furnace cycles on, the stove might emit a large puff of smoke into the room.
Exhaust products, when combined, could react adversely.
Example: High concentrations of water vapor from gas-fired appliances could mix with creosote deposits, causing liquid creosote to seep into the walls of the venting system, compromising the fire safety of the chimney.
Backpuffing is a specific type of smoking problem often misdiagnosed as a wind-induced problem or stove defect.
Backpuffing: Jets of smoke emitted from a wood stove, caused by the ignition of a buildup of combustible gasses in the firebox.
In short, backpuffing is caused by an inadequate flow of oxygen into the firebox. Instead of burning steadily, combustible gasses build up in the firebox and periodically ignite in a small explosion, forcing smoke out of the stove through every available opening, including the air intakes.
What might cause your stove to backpuff? Generally, shutting the air controls down too far, starving the fire of oxygen, AND:
either Using super-dry wood, like pallets or kiln-dried wood blocks, which burn very rapidly, emitting too much combustible gas too quickly.
or Using firewood that is split very small, which also burns too rapidly, creating an excess of combustible gas in the firebox.
Backpuffing is fairly easy to diagnose
Try opening the air control on the stove. If the smoking stops, it is very likely a backpuffing problem. Of course, you shouldn't over-fire the stove, either. If you have to open the air control to the point that the stove will overheat before the backpuffing stops, then reconsider your firewood supply. Either get a new load of properly seasoned wood, or try mixing less dry or larger pieces with the super dry or small cut pieces. (See article “Tips for Woodstove Users” for details on firewood selection.)