2nd oven. New materials - lots of insulation....performance issues

[Aburner]

Hi - second attempt. I built a 42 inch Pompei as per the forum plans. I havent put the final surface layer on yet but all insulation is on. 6 inches vermiculite mix all over and ceramic heat blanket on top of that....I tested the oven at the weekend and really had a large fire going for a couple of hours but 1) the oven did not get hot enough, especially the base for pizzas. and 2) the heat loss after was quite dramatic. I even lit another one the next day with a warmer drier oven and still the heat retention was poor
Another thing I noticed is quite a bit of smoke came out the front. And there must be some gaps at there was smoke coming out the side of the arches.
Reading through other people's issues, I may have not cured the oven for long enough. But would that make such a big difference?
Do any experts have an opinion on my arch and chimney construction?
All help appreciated. Having used three loads of logs I was hoping more heat and more efficiency. Not sure I can go through a third rebuild!
Photos attatched.

[Gulf]

Hi Aburner,

Welcome to the forum. For those of us that are uncomfortable about opening some types of zip files, could you attach real photos through the regular forum aps?

[fox]

Refractory materials are like sponges they can soak up moisture and store it deep inside, a damp oven will not perform well.
It is very important to spend time slowly drying out your oven before you lite large fires, it can take many small fires depending on the moister content.
If you heat it to quickly steam can form and as it expands it can easily crack your new oven or the outer plaster.
Wet or damp insulation will hinder the oven and not allow it to work properly.

[Aburner]

My apologies. the zip file was unintentional and I was unable to amend. Photos as promised:

[Aburner]

Thank for the tip on moisture. I can see I was in a bit of a rush to try it out and should have spent more time drying it out first. I will do this. This all makes me realise given the positioning of my oven that it is always going to suffer from rain. I was going to cover it with a waterproof render. But then maybe I need a roof or cover to really stop water coming in. This also made me realise that rain is going to come in from the slope. So I think I am going to gid a trench around it and put some drainage in....Any other ideas/tips to keep it from being a constant issue in the UK weather?
Also do you think the performance is just a damp issue looking at the construction?

[fox]

It could take weeks of fires to dry out an oven like that and if you don't completely seal it up against the elements you will always be fighting a battle!
You need to top coat the dome and get a some weather shield paint on it, perhaps dig out around the back and add a waterproof membrane (pond liner) agains the casement.
With so much vermiculite you will need a steam escape like a small hole in the top of the dome with a cover over the hole.

[MarkJerling]

Hi - second attempt. I built a 42 inch Pompei as per the forum plans. I havent put the final surface layer on yet but all insulation is on. 6 inches vermiculite mix all over and ceramic heat blanket on top of that....I tested the oven at the weekend and really had a large fire going for a couple of hours but 1) the oven did not get hot enough, especially the base for pizzas. and 2) the heat loss after was quite dramatic. I even lit another one the next day with a warmer drier oven and still the heat retention was poor
Another thing I noticed is quite a bit of smoke came out the front. And there must be some gaps at there was smoke coming out the side of the arches.
Reading through other people's issues, I may have not cured the oven for long enough. But would that make such a big difference?
Do any experts have an opinion on my arch and chimney construction?
All help appreciated. Having used three loads of logs I was hoping more heat and more efficiency. Not sure I can go through a third rebuild!
Photos attatched.

Welcome to the forum. What type of under-floor insulation have you used? How much insulation do you have over the top?

[david s]

Thank for the tip on moisture. I can see I was in a bit of a rush to try it out and should have spent more time drying it out first. I will do this. This all makes me realise given the positioning of my oven that it is always going to suffer from rain. I was going to cover it with a waterproof render. But then maybe I need a roof or cover to really stop water coming in. This also made me realise that rain is going to come in from the slope. So I think I am going to gid a trench around it and put some drainage in....Any other ideas/tips to keep it from being a constant issue in the UK weather?
Also do you think the performance is just a damp issue looking at the construction?

All new ovens tend to be somewhat smoky when fired. It is because the chamber still contains moisture. As the water is driven off, often a persistent ring of black soot remains around the base. I see that this is the situation in one of your pics. This means there is still moisture there and as the higher part of the chamber is dry the fire often wants to gallop away. Be careful, this is often the time that cracks develop because the temperature difference between the top and the base of the dome and in turn differing thermal expansion set up stresses the structure can't handle an cracking results. allowing the oven to cool and letting some of the moisture migrate higher and bringing the temperature back to ambient for the whole structure allows you to start the drying process again. This is the basis of the 7 fires in 7 days drying method.

[Kvanbael]

What a nice location but water and moisture will indeed be a concern. It also looks like your chimney pipe is on the small side. What’s the diameter?

[Aburner]

Thank you for all the advice. Very good tips indeed. The chimney pipe is 100ml diameter. I actually had a wider one but switched to this chimney was a special pizza designed one with dampner control (or so I thought). Is it really too small?
I have 6 inches of vermiculite mix as the floor insulation and over the top i have the same and a ceramic blanket.
The damp and its affects are clearly something I have not taken enough into consideration. I aim to build a channel round the back and line it so stop damp getting in from the ground, I will coat the top render with a waterproof paint and I think I will then build a small wooden structure to house it to really ensure rain does not get in.
I will also drill a hole for the steam escape and ensure this is covered.
Going to also try to fill the cracks in the arch to ensure no smoke or heat can escape from there...
Once all this is acheived I will do the 7 fires over 7 days....
Anything else you think I might be missing or other advice?
I've resigned myself to no pizzas for the next few months so want to get this right for when it is ready

[david s]

Builders have come to the conclusion that a 42” internal diameter oven requires an 8” diameter flue. 4” is way too small. My ovens are 21” and have a 5” diameter flue with no smoke issues. So your small diameter pipe is certainly the reason you have smoke issues. There is no problem with smoke out the front, the only reason you need a flue is to keep the smoke away from your face, but if you want to fix it you’ll need a bigger pipe.
Do you have underfloor insulation?
Don’t waterproof the exterior until the oven is properly dry, as it will hold the moisture in as well as holding it out.

[Aburner]

Thanks David. Ill go see how much flexibility i have on the flue now. To be honest the smoke doesnt bother me. I was more worried about performance issues. I have 6 inches of insulation under the floor. The first oven I did not have this, so I made sure this time round. Apporeciate the tip on the waterproofing.

[Aburner]

Finally built a wooden structure around the oven to keep everything dry, along with a channel and drainage around the outside to stop water access from the ground. Have also changed the chimney to a larger diameter.(Pictures to follow) I had several days of fires to dry everything out and followed with two days of cooking. There has been a great improvement in performance. There is still smoke coming through the front which does not worry me so much. I was able to get the oven to around 500c. However the base of the oven is still cooling down very quickly. I would say in one hour it has dropped 100c. There is 6 inches of vermiculite mix insulation on the bottom which should be sufficient I thought. The roof of the oven is retaining its heat better. I have read other reports of much slower rates of temperature decreases. Any tips clues would be appreciated. At the moment I can move the fire to the side and have a few pizzas working well but after an hour it was not enough to bake bread and I had to load up the fire considerably to roast meat in there....

[Aburner]

[SableSprings]

That much vermiculite mix for the base will hold a lot of moisture. With no easy path for it to escape it will take a long time to properly dry out. The big drops in temp on your cooking floor is a strong indication of considerable moisture still being retained by the floor base. I am assuming you used the 5:1 (vermiculite: cement) mix ratio. You should see continuing improvement with every firing as that moisture slowly is driven out (and you have insured no new water will seep in). I have just about 4" of perlite mix under my cooking floor and can easily bake 15-20 loaves of bread on a single firing. Relax, you'll be baking bread soon & your build looks terrific!