Thin-walled brick chimney?

[rsandler]

My oven vent narrows to a roughly 5"x6" opening. Until recently I had a 8.5x8.5" square terracotta flue tile on top of the vent, but it hardly drew any better than the vent with no flue. Between that and the fact that the flue tile cracked from thermal cycling (an ill-timed rainstorm contributed to this, I believe), I recently removed the flue, and I'm trying to figure out what to do next.

My inclination is to just build up the existing chimney with firebricks, and I'm wondering if that will actually work, at least the way I have in mine. The front and back of the existing vent are basically a whole firebrick cut lengthwise to make a 9"X4.5"X1.2" piece. If I kept building up with thin pieces like this, would it hold together? Or would it just crack, same as the flue tile?

As the attached pictures show, I don't really have room to build a thicker chimney, although on my build thread Chip suggested building a decorative arch and using that to support a full-brick, stretcher-laid chimney stack. Main advantages I see to a thin-walled chimney are that it would be faster, easier, and possibly faster to heat up, resulting in better draw.

Thoughts?

[mrchipster]

Re: Thin-walled brick chimney?

I keep with my original suggestion, you have plenty of landing there for a decorative arch and that would easily hold a decorative surround. And an internal clay liner. The flue temp is not the thing that increases flow but the height differential, the thing that causes the increase in flow. Think of water being siphoned from a tub or a water tower. The greater the vertical distance the greater the flow and pressure. You just need to think in the reverse direction with heat.

[brickie in oz]

Re: Thin-walled brick chimney?

If I kept building up with thin pieces like this, would it hold together? Or would it just crack, same as the flue tile?

I doubt it would last, it will just crack over time as it will be too thin.

The greater the vertical distance the greater the flow and pressure.

And of course the greater the distance the more resistance to flow....

[mrchipster]

Re: Thin-walled brick chimney?

And of course the greater the distance the more resistance to flow....

OK....there is a point where the height (distance) does reduce flow.

[rsandler]

Re: Thin-walled brick chimney?

I keep with my original suggestion, you have plenty of landing there for a decorative arch and that would easily hold a decorative surround. And an internal clay liner.

I hear you, the trouble with building something intended to fit a clay liner is that they just don't make clay liners small enough for my vent. I've already discovered that sticking a too-big flue on my chimney is essentially useless.

The flue temp is not the thing that increases flow but the height differential, the thing that causes the increase in flow.

Certainly, but you need to get the flue hot in order to create the temperature differential that pulls cold air in through the top, no? I realize it doesn't increase draw per se to heat up faster, but the things don't start drawing well until they heat up, that's all I'm saying. That's why an insulated flue is ideal, and some people take to lighting newspaper in the flue to reduce smoking at start-up, right? Not that it's a major factor, just something worth mentioning.

I doubt it would last, it will just crack over time as it will be too thin.

Okay; that's what I was trying to figure out. Clearly a ~3/4" flue tile was too thin. Wasn't sure about the half-thick firebrick. Now have to figure out what to do about it. Maybe a decorative arch like Chip suggests. Maybe just shell out for a steel flue. I think a 6" SS flue with anchor plate would fit. Hmm.

[deejayoh]

Re: Thin-walled brick chimney?

Do you think your vent size might be part of your problem? 5"x6" on a 36" oven seems pretty tight. Same size as 6" round flue - but I think I have read that square shapes are not as efficient as round flues.

[rsandler]

Re: Thin-walled brick chimney?

Do you think your vent size might be part of your problem? 5"x6" on a 36" oven seems pretty tight. Same size as 6" round flue - but I think I have read that square shapes are not as efficient as round flues.

Maybe a bit, but my oven opening is ~207 square inches, so I'm right about at the recommended 15% ratio with a 30 square inch vent. Wasn't my intention to have it that small--a bit of joint creep as I was building--but I think it's large enough in principle. But putting a 49 square inch flue on a 30 square inch vent, was, in retrospect, probably not a great plan.

[david s]

Re: Thin-walled brick chimney?

The draw of a chimney is increased if the height is increased, but increasing the cross section or diameter if it's round, has a much more powerful effect on increasing the draw. You can have a larger cross section than your vent opening without effecting the overall draw. The restriction at the vent opening will simply act as a Venturi increasing the smoke speed at that point. The restriction at the entry won't reduce the overall draw until it gets down to around 2/3 of the chimney cross section (like having a damper at that point).Other members have their flues set up like this and report they work really well.

[Tscarborough]

Re: Thin-walled brick chimney?

The 15% number is predicated upon a 15' chimney height all other factors being equal, FYI, so it should be increased for shorter flues. You do not need firebrick to build a chimney, regular brick will do, and making the chimney inside dimensions larger than the actual throat will increase the draw. Go as high as you are comfortable with, and buttress your arch accordingly.

[rsandler]

Re: Thin-walled brick chimney?

The draw of a chimney is increased if the height is increased, but increasing the cross section or diameter if it's round, has a much more powerful effect on increasing the draw. You can have a larger cross section than your vent opening without effecting the overall draw. The restriction at the vent opening will simply act as a Venturi increasing the smoke speed at that point. The restriction at the entry won't reduce the overall draw until it gets down to around 2/3 of the chimney cross section (like having a damper at that point).Other members have their flues set up like this and report they work really well.

Okay, that's good to know. My vent was, in fact less than 2/3 of the cross-section of the 8.5"x8.5" flue tile I had on there originally. But an 8"x8" rectangular flue would probably work.

The 15% number is predicated upon a 15' chimney height all other factors being equal, FYI, so it should be increased for shorter flues.

I remember reading another thread(s) awhile back when you and some others were discussing this, and I thought the upshot was that a 10% ratio is what's recommended based on a much taller chimney, and 15-20% was what was actually a good idea for a WFO with a short flue. Maybe I'm misremembering. Doesn't really matter--I meant to make the vent bigger originally, and there's not much to do about it at this stage.

[Tscarborough]

Re: Thin-walled brick chimney?

Bigger is fine, smaller could cause a draw issue.