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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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]

Re: High Heat Loss

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?

[CanuckJim]

Re: High Heat Loss

JCampo,

The best you can do is the best you can do. I'd really follow JE's advice and use high heat glue to install four inches of rigid calcium silicate high heat insulation board under the oven. There will still be radiation into the walls, but there's little you can do about that now. At least you will greatly reduce the heat loss you're experiencing now. Can't believe your builders did it the way they did. Not good, won't work.

I should know on Monday about a local source for the board and blanket insulation. High temp glue can be found at fireplace/woodstove suppliers.

As for the loose insulation for the enclosure, you can get large bags of vermiculite from pool installers or suppliers. It's called PoolPac. I got mine from Bathe & McClellan in Whitby. If you can actually climb inside the enclosure, the very best thing would be to wrap the dome in high heat blanket insulation, then pour in the vermiculite.

I don't think another layer of firebrick will have much of an effect, and it might adversely affect draught.

It's a fair bit of work, and some expense, but it will be worth it. Whoever built your oven should go into another line of work.

Jim

[RTflorida]

Re: High Heat Loss

If you have a structure similar to the photo your battle is half way won.
As Patrick (jengineer) has noted; you need to access the area above/around the dome (under the roof of the 'house').
Horticultural grade perlite or if more readily available, vermiculite. Try your local garden centers, you will probably need several 6 cubic ft. bags. Fill the area with at least 4", better yet - FILL IT UP!!! Depending on how much access you give yourself will determine whether you would need to 'blow it in' or merely dump in the bags....DRY, no need to mix with cement in an enclosed dome. Remember only use the horticultural grade products...it may also be available in different grades or courseness...the courser the better-don't use the fine grade unless you have too.

As for the hearth insulation. ideally you would want the insulating layer directly under the firebrick and on top of the support slab.....obviously that is not possible at this point. Again, as mentioned, adding one of the insulation board products such as SuperIsol (sold by Forno Bravo) or Cal sil to the underside of the support slab is your best option; accessing through your wood storage area.....the thicker, the better.
This won't be ideal, you will still be heating the entire hearth slab at every firing, but you should see a huge improvement over what you currently have.

Any chance of your posting a few photos, so we can be sure of our recommendations??

RT

[jcampo]

Re: High Heat Loss

Thanks for all your help! I will post some pics on Monday so you can see what I have bean venting about.

If you can post any local sources for finding the materials that would be great as well!

[dmun]

Re: High Heat Loss

Can i bond an insulation board from underneath that? Is that close enough to the slab to make a difference?

Although you still have a big heat sink in the slab and what it sits on, insulating underneath can't hurt, and it might help. That far away from the fire, you probably don't need refractory insulation - a couple of inches of the pink foam stuff from HD might make a difference. A warning-if it's too hot to hold your hand against when the oven is running it's probably too hot for the pink foam.

The original pompeii ovens had the perlite insulation under the support slab. If your support slab isn't bonded to the block base, you may be able to lift it with post jacks in the corners, and add four inches of perlite concrete after the fact. Might be easier that tearing it down.

Too bad you got here when you did - the oven builders motto is insulate! insulate! insulate!

[jcampo]

Re: High Heat Loss

Hi Guys,

Finaly got around to posting some pics of the oven. Did some additional research and I have access underneath the oven via the wood storage area. However above the dome is pretty much filled in with concrete there is no room to add any perlite. I guess my only option is to mount some insulation to the bottom anh hope that helps? As for the the heat loss from the dome am I stuck?