Newbie question about oven floor

[Neil2]

Re: Newbie question about oven floor

"extending well beyond my hearth floor,"

Well, to the limit of the dome insulation anyway. Do not "daylight" to the edge of the slab as this will create a broad entryway for moisture.

[david s]

Re: Newbie question about oven floor

I recently inspected a brick oven for a home owner who inherited a home built oven on purchasing a house. The one metre internal oven had been built on a timber stand. On close inspection the timber support in the middle was badly charred. Suspecting no insulation under the floor, on lifting a floor brick (common reds had been used) there was a 1" layer of clay under which a second layer of bricks sat on a thin sheet of steel. Because the dome had been built on top of the floor, removal of both layers of floor bricks and the clay layer in between was going to be a difficult job, let alone the replacement of the supporting timber beam. I did not investigate how the dome had been insulated because I could see that becoming a second nightmare. Needless to say I declined the job of fixing it. I bet they had some fun trying to get the thing up to temperature, but they obviously did judging by the burnt support underneath.

[Tomweb]

Re: Newbie question about oven floor

Good visual, thanks for the clarification, Gulf.

Adequate slab insulation, extending well beyond my hearth floor, coupled with my correctly insulated dome will do the job.
Scanning the threads and reading about other builds, I can see the importance of heat retention to carry out post-pizza baking/ roasting on occasion....

[Gulf]

Re: Newbie question about oven floor

Regarding heat loss from the hearth slab, one other question, from one who has yet to get his paws dirty on a WEO......but can't wait~ If we are to insulate as described, why do we not consider the uninsulated sides of the hearth slab, which represent a fairly large surface area to dissipate heat also?

If we insulate as recommended, there will be a measurable distance of insulation between the oven and the hearth slab. If that distance is not vertical, then the insulated foot print area should be wider than the foot print of the oven.
The firebrick (floor and dome) should be totally entombed in insulation. The concrete hearth should be seperated by, depending on the type of insulation used, 2" for most commercial insulations and 4" for V/Pcrete. If the concrete hearth is seperated from the oven by these distances then there is nothing outside of the tolerances to rob the oven of heat.

[Tomweb]

Re: Newbie question about oven floor

Regarding heat loss from the hearth slab, one other question, from one who has yet to get his paws dirty on a WEO......but can't wait~ If we are to insulate as described, why do we not consider the uninsulated sides of the hearth slab, which represent a fairly large surface area to dissipate heat also?

[Giulio]

Re: Newbie question about oven floor

Will the 2" thick Alumina Silicate board be enough insulation between the firebrick and the solid base?

YES IT WILL just biult an oven with 3 in calcium siliucate board and it is as good or better tnan my first with verm concrete,
they use the boards to ins blast furnaces at 1000 c
cheers

[Gary Mots]

Re: Newbie question about oven floor

This site may help concerning heat loss and info on your insulation problem with your contractor. GlobalSpec - Engineering Search & Industrial Supplier Catalogs , there's a lot of varied info on many subjects.

[WJW]

Re: Newbie question about oven floor

Fix it.

The guy that gave me the idea to have a wood fired oven has no insulation in his floor. He bought a kit from FB and had a contractor "experienced in building pizza ovens" build his oven.

The guy laid out a masonry stand with no insulation...lots of concrete block and rebar with a modular FB kit sitting on top of it. I don't know which one but it appears to be about 30 inches in diameter. He can cook pizza in it. That's pretty much it. And even with pizza he needs to move the pizzas around constantly or the floor gets too cool to cook well.

The massive masonry stand acts like a giant heat sink which sucks the heat from the floor into the stand and then into good old planet earth. His wife is a trained chef and a fantastically good baker. Yet she has never successfully baked a single loaf of bread in the oven in the twelve years since they built it. The only way they could bake bread would be to burn a fire for hours and hours and hours to the point that all that mass below the hearth was so saturated with heat that the heat loss below made no difference because the area saturated was so large.

I did my first big pizza and bread bake a week ago and they came over. He was blown away over what a difference the floor insulation made.

My advice is as said previously...pull out the bottome layer of firebricks and replace it with a correspondingly thick layer of ceramic fiberboard insulation to isolate that hearth. Then go with a single layer of firebrick for your hearth.

Bill

[Bacterium]

Re: Newbie question about oven floor

Jay,
Too add to what has been said so far........I can speak from experience on this.
Search for an oven build post for "Redgum Roarer" and you will see on my first oven build that I didnt insulate underneath the oven floor. Basically it was concrete slab then a small layer of sand then the oven floor bricks. Yes the oven worked to a degree but it clearly got hot underneath as it lost heat through the floor.

About a year or so later I helped a mate with his oven which he built with an insulating layer under the floor. Having cooked in both ovens the comparison was....
His tended to retain better pizza temps on a long pizza session from less wood. Next day his oven was around 300deg celcius whereas mine was 100deg at best.

Basically my oven could do pizza and bread baking all on the same evening, so it was dropping down to the 250deg celcius before the night was out. His oven had a decent amount of retained heat the next day which meant he could do a roast or 3 decent loads of bread.......whereas I had to refire mine the next day. His wasn't hitting the 100deg c mark until well into the third day.

Sure my first oven did work, used more wood, was more challenging to keep the heat up for slow roasting and bread baking. Now that I have a chance to build a second oven ( see Bacteriums 2nd build) I'm definitely insulating much better and trying to shoot for 3 days of retained heat..............the constant echo on here insulate...insulate....insulate is true and tested.

All the best with you planning and hope the detail helps you work through it.

[Puckncrazy1]

Re: Newbie question about oven floor

Thanks everyone... I really appreciate your responses!! I'll let you know how things go..

[Faith In Virginia]

Re: Newbie question about oven floor

I will offer my phone number to your mason if he needs to talk about his error in thinking. Let me know and I will send it in a private message. I have been talking with an experienced refractory mason with many years of experience. When we talked about the necessary insulation under the oven he was unaware of the heat loss in a downward direction. As the others said stop the progress now. Fire the guy only if he refuses to fix the error.

[Neil2]

Re: Newbie question about oven floor

"My guy thinks that ...."

I second Shuboyje and RT. Get a new "guy".

If he is going to screw up and argue on this, He is likely to screw up and argue down the line.

[RTflorida]

Re: Newbie question about oven floor

Pull up both layers of the firebrick , add the 2" board and then top it with one layer of firebrick. The 2" board is the equivelent of 4-5" of insulated concrete, you should then be fine.

Fire the idiot, if you leave it the way it is you will have the largest heat sink known to man. It would NEVER, EVER get up to temp and certainly not maintain a temp. He may me a fine mason, but he certainly is not an oven builder.

RT

[shuboyje]

Re: Newbie question about oven floor

Fire the mason and get one who can listen to simple direct instructions from his customer. I work in large commercial construction for a living, if I ignored the direct request of the customer I would no longer have a job. That's the easiest part of construction, the guy with the money calls the shots. If you are an expert in your field you may sway their thoughts, but they still make the final call.

[Puckncrazy1]

Re: Newbie question about oven floor

Will the 2" thick Alumina Silicate board be enough insulation between the firebrick and the solid base?