Help! Trying to make the best of a bad situation

[Sam]

My Father had someone build him a WFO. After some investigating I came to the conclusion that it was created with a major flaw - no insulation.

Thanks to this forum, I have been reading as much as I can over the last couple of days and I am trying to make improvement to the current design. I know there are few options but I hope this is not a lost cause.

Any recommendations would be appreciated.

First the design...

The Oven was created on a cement slab of about 5-6 inches thick. The fire brick was laid with mortar directly on the slab. The bricks were joined with a high temp premixed glue. The dome was constructed directly on the fire brick bed. Once again the joints on the inside of the dome are being held together with the high temp glue, the larger gaps to the outside of the dome are filled with mortar.

Once the dome was completed it was covered with 2" of mortar.

Currently the dome is exposed (phew!) and this will allow me to insulate the dome. I purchased locally in Toronto a 1" ceramic blanket that I plan on placing directly on top of the dome. I also bought 1 bag (4 cubic feet) of vermiculite (I will probably need more) that I plan on placing on top of the blanket. I am not sure if I can place it as is or if I should use some type of mix configuration. Once this is done, I will ask them to continue to close off the oven.


As for the oven floor, I realize that the floor will not maintain heat in its current state. I am trying to figure out the best solution.

I can think of two options....

1) insulate under the concrete slab. This may help a little but my limited research suggests that this might not be that helpful.

2) purchase 2" insulating board and place it above the current firebrick and then place another course of fire brick on top. Cutting the firebrick to fit perfectly might be a problem but I think I will be able to manage. I also realize that I will be eliminating aprox. 4-5 inches of height. I think the oven can easily accommodate this without causing a problem.

Any suggestions would be greatly appreciated.

Oh, I almost forgot... What is the best way to secure the insulating blanket to the dome? I see some people use chicken wire. What other methods are there?

[dmun]

Re: Help! Trying to make the best of a bad situation

Insulation under your slab will only help slightly, if you have a high dome oven the raised floor with the cal-sil board is the way to go. Cutting firebricks is not that hard with a wet saw. A combination of blanket and vermiculite is the standard dome covering, and yes, chicken wire helps keep the blanket in place, and gives the vermiculite concrete something more to grab on to.

Good luck, keep us posted.

[mfiore]

Re: Help! Trying to make the best of a bad situation

Sam, how much height within the oven do you have to work with? I agree, raising the floor is likely your best option. Cutting the fire brick to fit precisely should not be too difficult.

[Les]

Re: Help! Trying to make the best of a bad situation

Sam,

It sounds like insulating the dome is not a problem - I used baling wire to hold the blanket in place, then covered the dome with chicken wire for the vermiculite. The floor is your ugly. If you can go vertical, option 2 sounds best to me. If it's a squeeze - you may get away with splits for the brick.

Also, I would try and sue the dude for being and idiot or for trying to impersonate someone that knows what the hell they are doing! You are in Canada so I have no clue as to what the law is all a boot.

Les...

[Sam]

Re: Help! Trying to make the best of a bad situation

I am not sure of the dome height. I will try to visit my parents in the morning to measure and take some pictures. If I were to guess...about 24 inches.

I was leaning towards loose vermiculite. Should I change this to a vermiculite and portland mix?

Thanks

[Les]

Re: Help! Trying to make the best of a bad situation

WOW! While I was composing - 2 others chimed in, you are in good hands.

Les...

[maver]

Re: Help! Trying to make the best of a bad situation

Just for a point of reference for what you are trying to avoid, my father also has an uninsulated floor placed on a concrete slab with an uninsulated dome. His oven is able to be heated to pizza temps, but it take 2-3 hours vs about an hour for mine. During those 2-3 hours he has to feed the oven quite a bit of wood. Incidentally, his vent is from the top of the dome rather than over the opening as most use here. His dome is (I think) 46" and he needs the extra room because he has to keep a very big fire. All that said, his pizza is very good and was enough to inspire me to build my oven. I believe the dome insulation (and avoiding a vent in the dome) are the most important steps. If you can also insulate under the floor bricks as well you are golden.

[CajunKnight]

Re: Help! Trying to make the best of a bad situation

onething to keep in mind when adding board and brick to floor... is door going to be high enough? and if dome isnt covered yet would it be possible to relocated vent to arch opening?

[toddj]

Re: Help! Trying to make the best of a bad situation

Can you lift the slab/oven off the stand with a forklift, then pour a new concrete slab, insulate it with some ceramic board, then set the old slab/oven on top. It would give you a lot of mass to heat but it would work.. Then you would insulate the dome like normal..

[Sam]

Re: Help! Trying to make the best of a bad situation

onething to keep in mind when adding board and brick to floor... is door going to be high enough? and if dome isnt covered yet would it be possible to relocated vent to arch opening?

My Dads vent is located at the arch opening. That they got right (Mavers Dad had the vent issue). As for the door opening... It should be fine...

[Frances]

Re: Help! Trying to make the best of a bad situation

Sounds as if you've got it well worked out, and whatever you do it'll improve the oven's performance.

Please let us know how it turns out (with pictures maybe?) - because I'd love to hear about it, but also as inspiration for people in a similar situation.

[Wiley]

Re: Help! Trying to make the best of a bad situation

CajunKnight is on track regarding the height of interior dome to height of entrance problem caused by simply adding new insulation and firebrick to the existing floor. If one runs some numbers the problem becomes apparent.

The approximate interior dome height was given as 24", 63% of that is 15.25" so that is the design entrance height. If one adds insulation and firebrick to the floor one reduces the interior height as well as the height of the entrance. 4 to 5 inches of insulation and new firebrick was mentioned. 24 minus 4 leaves 20 inches and 63 % of that gives us a design entrance height of 12.6". Removing 4 inches from the 15.25 leaves us with an entrance of 11.25 inches which is a height of entrance to dome ratio of 56%

Anyone have any experience with an oven with such a ratio? The books all stress keeping close to the 63% ratio for proper airflow so one assumes it is important. I think we need some dimensions of the oven as it exists.

Wiley

[asudavew]

Re: Help! Trying to make the best of a bad situation

I think, the 63% is the MAX that the oven door height should be in relation to the dome. It's not necessarily the ideal height.

The idea is to have a good performing oven, heat wise, and a functional oven with a large enough opening to operate it - cooking wise.

If the opening is taller than 63% of the interior dome height, then heat loss would be so great that the oven would have a hard time hitting operating temps. But I think a bit lower would be just fine, if not better.
Maybe just harder to use.

Just my 2 cents

Dave

[Sam]

Re: Help! Trying to make the best of a bad situation

OK I just got back from my dads place and took some photos. I also documented key measurements for discussion purposes.

The oven is 36" in diameter, the ceiling height (at its high point) is 27". The door opening is 20" wide with a height of 16".

I guess if I would add 2" insulation board and another layer of firebrick I would still have 23" of oven height, with a door opening height of 12". I hope this is enough :-)
On with the photos

Exterior View


Interior Dome... I am not sure why the first course of fire brick was applied vertically


Exterior of the dome... Without insulation. I will be adding an insulating blanket and vermiculite.


Dimensions of current setup

[Les]

Re: Help! Trying to make the best of a bad situation

If you raise the floor 4.5 inches the opening is only going to be 11.5 inches. The interior height is not your problem. That may be tight to cook with. Anyone cooking with an opening that low?

Les...