High Heat Loss

[jcampo]

We have just finished building a wood burning oven and I wish I would have found this forum prior to building it. What we are finding is that even if we have a good fire going for a few hours prior to cooking, without a big fire going the heat loss in the oven is very high. We can barley manage to cook 1 pizza. The fire bricks just don't seem to retain much heat for long.

Anyone have any ideas as to what we can do? Can we add additional insulation or another layer of fire brick?

[jengineer]

Tell us more about your oven - have any pics?

What form did you follow?

Rough guess is it sounds like you need more insulation if it is not holding the heat. Is the underside of the oven warm after you have fired the oven? Is the dome of the oven warm to the touch after you have fired the oven? If the answer to one or both is yes then you will need more insulation.

Did you use fire brick or insulating fire brick? - there is a difference? If you brick is insulating fire brick then it will not hold the heat?

we need more information

welcome aboard

je

[CanuckJim]

Jcampo,

JE's probably correct, but without more information, it's a difficult situation to call. I'm to the north and east of TO, about 70 kms from downtown. If you'd like to have a look at what I've done here, send me an email through the forum and we can work something out. I'll reply with my phone number, too, in case it's possible for me to have a look at what you've done and make some recommendations.

Jim

[jcampo]

Hi Guys,

Thanks so much for the responce. I will try and provide you more info....
The oven is constucted of all brick/concrete with blocks used as the foundation. I am not sure what type of fire bricks we used, we went to the local supplier and ask for fire brick. Didn't realize there was a difference.

After a couple of hours of burning a fire the exterior bricks do become warm to the touch. If I need to insulate how should I go about doing it?
I really don't have any access to the interior of the structure (as it has been sealed with bricks) anymore.

[jengineer]

Did you observe the construction?

the hearth lays on top of the bock foundation walls. What was the construction of the hearth?

Did they first lay down a insulating concrete (vermiculite or perlite mixed with cement) and then a more plain concrete reinforced with re-bar on which the firebrick floor was placed? Or is your hearth reinforced concrete and then a layer of insulating concrete? it may be a moot point if you cannot get access to the underneath since your walls are enclosed. Any chance of punching a 6 inch access hole? If all the blocks are filled with concrete this will be tough withouy a concrete boring machine.


A quicker way around this is describe your oven - is it similar in style the Forno Bravo line of oven, round like a ball cut in half, or is it more like a small barrel cut in half? We may be able to deduce how this oven was built with this information. Or you may be able to bribe Jim to do a road trip in his copious free time.

As for the outside of the oven if you can feel heat it probably needs more insulation. Jim has been doing some legwork on getting local materials that closely match those that Forno Bravo can supply (Import duty alleviation)

A quicker way around

[jcampo]

I did witness the construction. I will try and describe it as best I can....

The oven interior is round inside with an opening for access. The chimney exhaust is actually constructed just inside from the oven's door. There is about a 3-4" gap between the door and the actual oven in which the exhaust is situated.

The oven is then sealed in a brick structure that resembles a small house.
(Sorry...I don't have a pic with me)

The hearth was built ontop of concrete blocks with a layer of "normal" concrete (they purchased at Home Depot) acting as the base in which the fire bricks where then placed. The oven interior is lined with fire brick.
I am pretty sure no insulating concrete was used in any part of the construction.


Does this help any? If a pick would be better I will get some this weekend and post it.

Thanks,

John

[RTflorida]

I keep reading about Jim and his copious free time Still can't figure out how he does it Must be a Canadian thing. Is there a Canuck school of time management?

[jengineer]

The concrete slab that the oven floor is placed on is a monster heat sink. Without any insulation underneath, it is radiating heat into that area like an oven - only you can't get to it. If one wall had been partially built you could have used it for short term wood storage, as in the photo below.

Without any insulation on top of the dome and the dome being enclosed in a brick house you are radiating heat into the "attic".

As Pink Floydd says - Break down the wall.

You definitely need to insulate the dome. If there is zero insulation on top of the dome and your oven enclosure is similar to this


then you can open up part of the roof or get in through a hole cut in the upper wall and "blow' in perlite. As for insulating under the floor you need to create an access hole that is structurally sound. What is needed underneath is insulation board (Cal-Sil) probably 3 to 4 inches thick bonded to the underside of the floor.

[jengineer]

Originally Posted by RTflorida

I keep reading about Jim and his copious free time Still can't figure out how he does it Must be a Canadian thing. Is there a Canuck school of time management?

Ah copious free time is what a mentor told me about 20 some odd years ago. He had a pile of papers on his desk and I asked if he was ever gonna get to the bottom of it. His rely was "In my copious free time". Another of my favorites when someone sends me something to work on - this is a quote from Rolls Royce by the way and can be modified to fit the description - it was for a contract..." Your package has been received and its contents duly noted"

[jcampo]

My oven looks very similar to the picture. So if open a hole in the roof is perlite something i can fill in myself or do I need to call somone? What type of perlite should I be asking for?

Also I have the same wood storage access as the photo. Can i bond an insulation board from underneath that? Is that close enough to the slab to make a difference?