Brick Chimney Flue

[Andytibbles]

Thats really helpful thank you David

[david s]

For pizza, yes the door is off and you maintain a live fire. This is essential to retain the high pizza temperature required. It also provides a convenient light. Pizzas are only in for a minute before requiring turning and then another minute they're done, so you must keep an eye on them. If the door is closed the fire will go out. For a bread oven the fire is allowed to die, the coals raked out, floor cleaned and then when the desired temperature is reached the bread is loaded and the door placed shut to retain the heat. Having the flue inside the chamber is not a good solution because it increases heat loss. I'm not sure if your oven will be suited to cooking pizza, but it sounds like it was a good functioning bakers oven. I don't think you should be altering it. I also can't see why you want to change the chimney. Bread ovens usually have more thermal mass which means it may take longer to heat up. As they were usually fired every day they never really cooled down.Also as you have no idea how much insulation it has over and under, you may not be able to tell how it performs until you try it.

What is the intended use for the oven?

[Andytibbles]

I really appreciate that David, I am sorry to tap in to your knowledge. The issue really is when the door is closed, the flue is separated from the main oven - it was built as a bread oven not a pizza oven. What we would like to know is whether it would be beneficial to look at relocating the door so that when it is closed the flue is within the pizza chamber. Or do you leave the door open at all times when cooking pizzas?

[david s]

I can't see a door in any of the photos. I see an oven mouth, is this what you meant by removal?
Does the oven look as though it was used? If so there may be absolutely nothing wrong with it. As it appears to be a bread oven, it may have been fired daily for many years.
Also if it was a bread oven it would more likely have a rectangular floor plan rather than a circle. Is this so?
On doors, most bread ovens have steel doors hinged to a steel frame that is mortared into the brickwork of the oven. Most pizza ovens don't have doors because they require a live fire to maintain the higher pizza temperatures. A removable door that just sits against the oven mouth is usually used for roasting and baking with the door in place and the cooking done with the retained heat of the oven as in a bread oven baking.

[Sixto]

I'm an amateur currently building my own oven, and a retired architect, so take my advice with a grain of salt...The only benefit I can imagine for removing the existing door is if the door or some other part of the oven was not built to current specifications for efficient operation, and you need to remove it to fix it. I've been baking for 5 years and just recently figured out how to make bread good enough to bake twice a week. My gas oven does not get hot enough for "really good" pizza, so I decided to build a wood fired oven over a year ago and am about 2 weeks from finishing it. This whole wood-burning thing has already been a huge learning experience for me and I haven't even lit the drying fires yet!

i'll take a guess from your photos that the door-frame is mortared in, and it looks like the mortar at the top of the door is also part of the flue assembly, since you said it runs at an angle, maybe it follows the roof of the dome?. It is possible that the door can be removed at the hinges leaving the frame in place. That might be enough to satisfy the owner, but he or she may want to use a door at some point to keep the oven hot for a day or longer. They can always build a removable door, but if you already have one, then why bother taking that out?

Now, if they want a bigger opening, that could be a problem - both because of the frame being mortared-in, and also because the ratio of door size to oven size is important, and I'm assuming the people who built this oven originally knew what they were doing. The door height is supposed to be smaller than the oven height so the heat above the door height stays inside the oven and doesn't flow out the top of the door.

if you remove the frame, you may have to build a new arch over the entry, and make that flow into the flue. Probably not a huge deal, but i'm sure since you do remodels, you know once you open up Pandora's box, you never know what your'e going to get into.

I hope I'm wrong, but it sounds like the owner doesn't really know much about how these ovens work. If they want to use it, they are going to have to put in some time and effort to learn how this works, and I suggest that starting with the existing conditions may give them the best baseline to judge whether this is worth pursuing or not.... So - I would uncap and extend the flue, then maybe help the owner build a fire (by themselves) see how it goes... it may need some slow drying if it hasn't been used in a long time.) Then you can see if the flue draws well, build slowly to a high enough temperature, and see if they like what they have, can deal with the process day-in-and-out without your guidance, or want to make more changes first.

[Andytibbles]

Hi David , the client is wondering if there is any benefit to taking the door off or not? can these be removed?

[Andytibbles]

Hello , yes I think you are right I suspect it was a bread oven due to its age. Not sure on the thickness of the material above and below. It is difficult to tell.

[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?

[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 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]

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

[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

[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]

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

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.