Shape of a brick chimney

[Jochen]

I am at the final stages of my oven build. I have now started with my chimney which is a simple brick chimney that sits on top of the front arch.
See picture.

So far the chimney is about 30cm high and it was recommended to me (from a non oven builder ph..) to make the top of the chimney narrower towards the end to increase the ?upwards draft?.

Is that true?

Also, the inside of the chimney is very rough due to different sized bricks and my lack of brick laying skills. Does the rough surface have a negative effect on draft ability of the chimney? Is it worth rendering the inside of the chimney with fire mortar?

Any thoughts are very much appreciated

[dmun]

Re: Shape of a brick chimney

Also, the inside of the chimney is very rough due to different sized bricks and my lack of brick laying skills. Does the rough surface have a negative effect on draft ability of the chimney? Is it worth rendering the inside of the chimney with fire mortar?

I don't think it's worth messing with the inside of the chimney. The folks who build masonry heaters have largely concluded that a smooth transition isn't worth cutting a bunch of bricks. On the other hand, the rumford fireplace folks think a smooth curve is essential. It's a matter of opinion: mine is that smoke is a gas and will flow where it flows despite mini-turbulence caused by roughness in your smoke funnel.

[dower.bill]

Re: Shape of a brick chimney

@DMUN.

Kudos for the excellent quality of your work and the documentation to help put it out there for the rest of us to learn from and enjoy. I especially enjoyed the geodesic dome and your comments about it. I was thinking of trying that, but have decided to go conventional. Of all the things you have posted, this comment on smoke flow in a smooth vs rough chimney is the only one I might offer a second perspective on.

People who design pipes and airplanes spend a lot of time reducing the rough edges because this can have a surprising effect on the flow of fluids (gas and liquid). It will never be as important as having the proper size, but could make the difference between 'almost big enough' and 'big enough'.

bd