Trouble shooting a baffle.

  • Active since 1995, is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

Spotted Owl

Nov 5, 2009
Oregon, coast range
Trying to help diagnose over the phone. Besides pictures I have not seen this myself, yet. I’ll be on site next weekend to help. I advised a pro to sweep and inspect because the stove was not burning correctly with smoke backing into the house. Sweep report, about a coffee can of powdery debris removed and the top 12" to 18" a start of a thin layer of level 3 glaze started. Sweep advised to burn a chimney sweep log every two weeks for the rest of the season to help with the glaze removal when final brushing at end of season. Burn season will be over in approximately mid May. Sweep did not mention anything about the baffle installation.

Papa bear stove. 8" rear exit. Approximately 14’ exterior masonry chimney with clay liner. Approx 4’ single wall from stove connector to chimney connector. One bend approx 22* at the single wall to masonry connection and a Tee connection at the stove. Baffle installed by other family prior to this burn season. 3/4 plate, yes, 3/4" steel plate. I don’t know the angle of the baffle but from bottom of 8" outlet to just above lower step bend.

Will a baffle added to a Fisher Papa lower stack temps enough to start build up level 3 in the top 18” of a clay lined flue? I know the wood is as dry as it can be in our climate don’t have a meter reading but it’s 3-4 year shed stacked fir. Approximately 4 - 5 cords burned to date.

My thoughts. I’m thinking the baffle is too close to the stove body and not allowing enough flow. I don’t know the math on this right now. Thinking it needs the choke point opened for better natural exhaust flow. The extra heat directed to the stove top from the baffle is causing an earlier air intake shutdown which is creating more smolder time. Need smaller, hotter fires instead of filling to the gills. I know the baffle angle can affect things, but I don’t know how right now. No thoughts on this right now.

Any thought or idea are gladly welcomed. She wants to keep the big baffle. I will gladly change it for 3/8 I have laying around. I’m not ready to unnecessarily wrestle 3/4 plate in that small space. I’d like to eliminate as much as possible before removing and starting the dreaded trial and error process.

We installed a new Lopi Liberty in another family members house. We are trying to steer her that same direction but she’s a tough sell. She bought her Fisher new in Springfield and isn’t warm to the idea of replacement.

You want the baffle to heat up, not be a massive heat sink. 5/16 is a good compromise to prevent warpage and even heating.

Surface temperature of baffle plate has to do with the hot gases giving up their heat on that side of the flame. This is known as the "quenching distance".

When a flame laps against a solid surface, combustion can be hindered, depending on the surface temperature. Contact of burning gases with a cooler surface quenches the flames. Gases within the "quenching distance" of the surface are below their ignition temperature even with hot burning gases near them. The thickness of which no flame exists depends on surface temperature, being larger for cooler surfaces. Surfaces hotter than the ignition temperature enhance combustion. This is the reason for firebrick having a hot surface compared to a single wall steel plate having a much cooler surface.

Another effect on flames near large surfaces is lack of oxygen. Wood fire flames are diffusion flames. The oxygen needed is diffused into the gaseous fuel from the air around the flame. A flame against the surface restricts oxygen to that side of the flame causing incomplete combustion, soot on the surface, and higher emissions. The baffle should remain clean with no trace of soot (unburned carbon) or other deposits.

The angle is critical to allow the correct velocity, tilting upward speeding the flame and velocity up chimney. This increases NET draft.
Controlling the loss up the stack is done by raising and lowering the baffle at front. This increases or decreases the square inch opening as well as affecting velocity. Start with the same square inch opening above baffle as chimney diameter square inch area. Check creosote frequently until you know how much cooling each venting system has. 250*f is the critical temperature to stay above before exiting while smoke is present to prevent creosote formation.
  • Like
Reactions: Piney