Brick Chimney Flue

[Andytibbles]

Hi all , we are restoring an old internal pizza oven on a listed building
The pizza oven opening is around 900mm wide by 450mm tall. (arched)
The existing chimney stack has an internal measurement of 250x250mm and has been sealed over internally before roof level
We have been tasked with extending this chimney up and out through the roof so that the customers can use again.
The foundations have been checked and are ok , but we are wondering if the flue diameter will be too narrow for the pizza oven to work effectively
They would like to extend the existing stack however we have said rebuild from scratch at a wider diameter ,
Are there any experts that can calculate for us if we are right!

[MarkJerling]

250x250mm sounds about right, but it would be good to know the internal measurements of the oven to do a few calcs.
I notice from your photo that there appears to be an extract hood in the picture. Regardless of the flue, it would make sense to have an extract hood as well. In many jurisdictions, it will be a requirement in any event. Consider too fresh air supply for combustion as well as height of chimney and potential for down-draft.

A potentially better solution than a 250x250mm square flue would be a 300mm dia. round flue, or a 250mm dia. round flue, in stainless steel. These will be far easier to sweep than a square flue.

[Andytibbles]

Thank you , I think the extractor is for something else.
We were thinking of rebuilding and installing a pumice liner 300mm however client would prefer to use the existing chimney and build up from there.

[david s]

Some more pics would be most useful. The dimensions shown on the drawings attached would help. The attached photo shows the typical set up. It is a 1600mm internal diameter. Your 250 x 250 mm chimney should be quite adequate. Can you provide
A. oven internal diameter
B.flue gallery width
C. flue gallery depth
D. chimney cross sectional area (yours 250 x 250mm)
E. chimney height
What currently supports the chimney? Any bends required should be no flatter than 45 degrees.
Also check local building code as modern requirements for chimneys may supersede the original.
Also what fuel is the oven going to use?

[Andytibbles]

Hi David , that is very helpful thank you. We will take some more measurements and pics and report back. Thank you

[Andytibbles]

Hi David

A= 141cm door to rear (width of the dome is about 126cm)
B = 46cm
C = 60cm height of flue gather , 46cm width gather


The config of the existing pizza oven is that the dome oven has one flue gather in at the entrance which then leads up and across by 1.5m to the chimney.

[MarkJerling]

Why is the dome supported with some wood? Is it in danger of collapse?

[david s]

Hi David

A= 141cm door to rear (width of the dome is about 126cm)
B = 46cm
C = 60cm height of flue gather , 46cm width gather


The config of the existing pizza oven is that the dome oven has one flue gather in at the entrance which then leads up and across by 1.5m to the chimney.

Those numbers sound ok, but I forgot to also ask how high is the internal height, at the crown of the dome?
When you say "the flue gather leads up and across", it should be no flatter than 45 degrees or you may have smoke issues.

[Andytibbles]

Hi David , Dome height about 350mm in the centre , its a slight gradient to the chimney from the gather but certainly not 45degrees as you can see from the internal picture.
If the chimney is sufficent diameter would it be prudent to perhaps install a fan to aid draw , triggered by a heat sensor?
Are there any calcs that can be used to work this out ?

[david s]

Hi David , Dome height about 350mm in the centre , its a slight gradient to the chimney from the gather but certainly not 45degrees as you can see from the internal picture.
If the chimney is sufficent diameter would it be prudent to perhaps install a fan to aid draw , triggered by a heat sensor?
Are there any calcs that can be used to work this out ?

Something's not right with these measurements. Can you check them to confirm, including the additional measurements as per diagram?

A is internal oven diameter
B is width of flue gallery at its base
C is the depth of the flue gallery at its base
D is chimney (cross sectional area)
E is internal oven height (at crown)
F is flue gallery height from oven floor
Also how high is the chimney?

I've seen a number of internal ovens in restaurants that have a similar slope to a chimney that is flatter than 45 degrees. They work ok so you'll probably be fine.Any smoke issues occur at start up when the chimney is cold. This problem can be largely eliminated by simply keeping the fire really small until the chimney has heated sufficiently to create the required draw.

[Andytibbles]

A is internal oven diameter 141cm
B is width of flue gallery at its base 60cm internally
C is the depth of the flue gallery at its base
D is chimney (cross sectional area) 46cm
E is internal oven height (at crown) 40cm
F is flue gallery height from oven floor 40cm
Also how high is the chimney - currently blocked off we have been tasked with the rebuild so it can be as tall as we like

[Andytibbles]

A is internal oven diameter 141cm
B is width of flue gallery at its base 60cm internally
C is the depth of the flue gallery at its base (I dont have this , its not sat directly above)
D is chimney (cross sectional area) 46cm
E is internal oven height (at crown) 40cm
F is flue gallery height from oven floor 40cm
Also how high is the chimney - currently blocked off we have been tasked with the rebuild so it can be as tall as we like

[david s]

The flue gallery collection at its base should have a cross sectional area larger than the cross sectional area of the chimney to assist adequate smoke extraction. The chimney appears to be square so I assume D is 46 x 46 cm which = 2116 cm2. B x C should give you the area in cm2 of the flue gallery at its base. This area is normally greater than the chimney cross sectional area. Generally cross draft ovens (as in my drawing) have a lower flue gallery than the crown of the oven. This allows the flame to rise, travel across the roof of the dome then down and out the flue gallery. In operation this results in retaining all the flame within the chamber, retaining heat at the crown and provides excellent circulation. If your flue gallery is the same height as the crown of the dome then flame will be lost as it jumps straight to the exit. This will result in a less efficient operation, but will also reduce any smoke issues out the front.

[Andytibbles]

Thank you , the pizza oven itself is listed hence the protection and timber. The clients want to make it operational again which means us extending the chimney. Would the use of a fan built into the chimney be wise. We would ideally like to build from the existing stub stack which you will see in the photos above however our initial thought was that the chimney diameter would not be sufficient.

[david s]

The power of the draw in a chimney is produced largely by its cross sectional area and to a lesser extent by its height. That’s why I asked how high the chimney was. I don’t think you’ll have a problem with inadequate draw. An extractor fan could always be added later if required. With such a low rioof the oven sounds to me more like it was a bread oven. Any idea of the thickness and material used for insulation both over the oven and under the cooking floor?