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Post by webbm00 on Apr 23, 2019 21:14:43 GMT
So I've fired up my nearly completed modular oven a few times and have thermocouples in various places to measure the temperature. It's a modular oven with the flue in the front of the dome so I created a flue bung and a door out of thermalite blocks to see what I could do with slow cooking joints. The oven dome and base is 95mm of refractory, then there is a thermal blanket and ~50mm of vermacrete. I've not rendered it yet. Base has 50mm concrete slab and 25mm calcium silicate board on it Build linkBefore I render the oven I wanted to gather some opinions about heat retention expectations as I'm not sure if I have enough vermicrete on the oven and want to get it right. I heated the oven for 3 hours and made some pizzas by which time under the fire bricks was ~275degC and inside the dome was dropping from 600+ to 300 as the fire died down. At 21:00 I put in the lamb and blocked the flue and door when the dome and under the fire bricks were at ~200degC and my measurement of the oven floor was also 200degC. As you can see from the graph below (click on it for a bigger picture) the oven was at 60degC at 08:00 the next morning and although the lamb was cooked lovely it could have had a bit more slow and low cooking to make it pull apart better. I'm sure I've read on the site about peoples ovens that are still hot the next morning so should I be expecting more heat retention?  |
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Post by simonh on Apr 24, 2019 8:16:55 GMT
with the flue closed and my door on I've seen temps of 100+c the morning after a pizza firing.
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Post by oblertone on Apr 24, 2019 8:34:28 GMT
I'm not sure the issue lies with the vermicrete; although, as the saying goes 'you can't have too much insulation' but possibly with your available heat store. My oven is built from half-bricks, a render layer of 'home-brew' followed by a layer of blanket covered in tile cement and tiles. On the morning after a pizza session I remove the ashes and let it cool so I can cook bread (230c) followed by a slow roast tapering from 200c to 80c over 4-5 hours.
The point I make is that my 'heat store' is 4" thick bricks with a barely adequate insulate covering, how thick is your modular shell ?
A further point is that I raise my oven to 500c + before it goes 'soot free' for pizza cooking, however with a slow wood feed I've managed over 70 pizzas in one evening.
I would be tempted to massively over insulate your oven to ensure that I made best use of whatever heat reserve I managed to build up; the benefits of a well insulated door cannot be overstated.
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Post by webbm00 on Apr 24, 2019 11:42:18 GMT
I'm not sure the issue lies with the vermicrete; although, as the saying goes 'you can't have too much insulation' but possibly with your available heat store. My oven is built from half-bricks, a render layer of 'home-brew' followed by a layer of blanket covered in tile cement and tiles. On the morning after a pizza session I remove the ashes and let it cool so I can cook bread (230c) followed by a slow roast tapering from 200c to 80c over 4-5 hours. The point I make is that my 'heat store' is 4" thick bricks with a barely adequate insulate covering, how thick is your modular shell ? A further point is that I raise my oven to 500c + before it goes 'soot free' for pizza cooking, however with a slow wood feed I've managed over 70 pizzas in one evening. I would be tempted to massively over insulate your oven to ensure that I made best use of whatever heat reserve I managed to build up; the benefits of a well insulated door cannot be overstated. The oven is 95mm thick on the dome and base and I used 100mm blocks for the door which wasn't fully sealed but as close as I'm likely to get with a proper door. The flue bung was a tapered cone of the block wedged in so again a decent thermal blockage. The outside of the insulation blanket got to 100degC and the top of the vermicrete only 50degC so you may be right in that the insulation isn't the issue. But the outside insulation blanket temp did drop off quickly so I wonder if I haven't got enough heat into the oven before leaving it? Although I had to wait for it to cool a bit before putting the lamb in as the floor was too hot! I think some of the heat is lost through the floor as I only have 25mm of calcium silicate board but this can't be fixed so will remain on the wish I'd done it differently list This is a complex thermodynamics question but I recon a few more pizzas and lamb joints may help answer it. I could try putting a load of blankets over the oven once It's heated to see if extra insulation helps before committing to buying more vermiculite |
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Post by webbm00 on Apr 24, 2019 11:51:30 GMT
with the flue closed and my door on I've seen temps of 100+c the morning after a pizza firing. Looking at your build photos I'm not sure if I'm heating the oven up enough as I don't have a big pile of embers left over from the wood I'm using and the fire isn't huge. Maybe another test would be to get a really bit fire to heat it up and see what it does then in the cooling down phase |
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Post by simonh on Apr 24, 2019 12:14:27 GMT
I am getting to 450c and above on the floor when cooking pizzas, around 600c at the ceiling
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Post by webbm00 on Apr 26, 2019 16:55:41 GMT
I am getting to 450c and above on the floor when cooking pizzas, around 600c at the ceiling I was measuring 400degC on the floor so I think a test with a really big fire is in order once I get some more wood |
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Post by downunderdave on Apr 27, 2019 2:49:35 GMT
I am getting to 450c and above on the floor when cooking pizzas, around 600c at the ceiling I was measuring 400degC on the floor so I think a test with a really big fire is in order once I get some more wood From your description it sounds to me like your oven is still purging itself of moisture. As water is a much better conductor of heat If you render over it now that moisture will be harder to remove, so just keep using it for now as is and you will find its performance will improve regarding retained heat. As water is way more thermally conductive than air, it will draw heat from the oven impairing heat retention. After your oven has been fired for an hour try throwing some sheet plastic over the vermiculite to see if moisture condenses on the underside. This will not tell you anything about the moisture under the floor however, and as this is always the hardest and last to be removed, will also be the area where temperature drop off is the greatest. A hand held to the outside of the oven is a great indicator of both temperature and moisture so do it often to tune into your oven. Hope this all makes sense and helps you. Dave |
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Post by webbm00 on Apr 27, 2019 18:29:28 GMT
I was measuring 400degC on the floor so I think a test with a really big fire is in order once I get some more wood From your description it sounds to me like your oven is still purging itself of moisture. As water is a much better conductor of heat If you render over it now that moisture will be harder to remove, so just keep using it for now as is and you will find its performance will improve regarding retained heat. As water is way more thermally conductive than air, it will draw heat from the oven impairing heat retention. After your oven has been fired for an hour try throwing some sheet plastic over the vermiculite to see if moisture condenses on the underside. This will not tell you anything about the moisture under the floor however, and as this is always the hardest and last to be removed, will also be the area where temperature drop off is the greatest. A hand held to the outside of the oven is a great indicator of both temperature and moisture so do it often to tune into your oven. Hope this all makes sense and helps you. Dave Makes sense but I wouldn't have thought there was much moisture getting in as the oven is in the corner of a BBQ shelter with two block work walls and i had quite a few "drying" fires since the vermicrete was applied. I like the plastic sheet idea and have added that to my list of things to test. Must get on and get some wood though or I wont be able to test anything |
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Post by downunderdave on Apr 27, 2019 22:05:16 GMT
From your description it sounds to me like your oven is still purging itself of moisture. As water is a much better conductor of heat If you render over it now that moisture will be harder to remove, so just keep using it for now as is and you will find its performance will improve regarding retained heat. As water is way more thermally conductive than air, it will draw heat from the oven impairing heat retention. After your oven has been fired for an hour try throwing some sheet plastic over the vermiculite to see if moisture condenses on the underside. This will not tell you anything about the moisture under the floor however, and as this is always the hardest and last to be removed, will also be the area where temperature drop off is the greatest. A hand held to the outside of the oven is a great indicator of both temperature and moisture so do it often to tune into your oven. Hope this all makes sense and helps you. Dave Makes sense but I wouldn't have thought there was much moisture getting in as the oven is in the corner of a BBQ shelter with two block work walls and i had quite a few "drying" fires since the vermicrete was applied. I like the plastic sheet idea and have added that to my list of things to test. Must get on and get some wood though or I wont be able to test anything On re-reading your thread more carefully it is clear that your insulation is minimal. This will not be able to be rectified without a partial dismantle over the dome or a total dismantle to add more under floor insulation. Don't be dismayed just work with what you've got, you will probably find that your oven will still continue to improve its performance with more firings as it totally eliminates all the water though. Your floor insulation at 25 mm calsil is only half the usual thickness recommended and with one layer of blanket (presumably 25 mm) over the dome. Many builders use 2 or 3 layers of 25 mm blanket. You don't say what ratio of vermicrete you used over the blanket but the amount of cement used has quite a bearing on its insulation value (see attached table) I use 10:1 which makes application a bit harder, but results in good insulation value. Having the flue inside the oven chamber, even though you've plugged it up, also results in significant heat loss, although it provides extra oven area. There is a disadvantage in having the oven really well insulated in that you can be waiting hours for your oven to cool down to roasting or baking temperature. My practice usually is to fire the oven for exactly one hour of flame which does not saturate the oven with heat, but is sufficient for a roast or baking bread. Although my oven inner castings are 50 mm not 90 mm like yours. This regime saves both time and fuel, although the temperature drop off is even faster. Be happy with what you've got, you still have around 3 or 4 good hrs of heat for cooking. You can also roast without the door and an active fire on the side. Do lots of firing and cooking and you will learn your ovens characteristics, rely less and less on your thermocouples which will inevitably fail in time anyway. |
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Post by downunderdave on Apr 27, 2019 22:11:52 GMT
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Post by webbm00 on Apr 29, 2019 9:15:48 GMT
Makes sense but I wouldn't have thought there was much moisture getting in as the oven is in the corner of a BBQ shelter with two block work walls and i had quite a few "drying" fires since the vermicrete was applied. I like the plastic sheet idea and have added that to my list of things to test. Must get on and get some wood though or I wont be able to test anything On re-reading your thread more carefully it is clear that your insulation is minimal. This will not be able to be rectified without a partial dismantle over the dome or a total dismantle to add more under floor insulation. Don't be dismayed just work with what you've got, you will probably find that your oven will still continue to improve its performance with more firings as it totally eliminates all the water though. Your floor insulation at 25 mm calsil is only half the usual thickness recommended and with one layer of blanket (presumably 25 mm) over the dome. Many builders use 2 or 3 layers of 25 mm blanket. You don't say what ratio of vermicrete you used over the blanket but the amount of cement used has quite a bearing on its insulation value (see attached table) I use 10:1 which makes application a bit harder, but results in good insulation value. Having the flue inside the oven chamber, even though you've plugged it up, also results in significant heat loss, although it provides extra oven area. There is a disadvantage in having the oven really well insulated in that you can be waiting hours for your oven to cool down to roasting or baking temperature. My practice usually is to fire the oven for exactly one hour of flame which does not saturate the oven with heat, but is sufficient for a roast or baking bread. Although my oven inner castings are 50 mm not 90 mm like yours. This regime saves both time and fuel, although the temperature drop off is even faster. Be happy with what you've got, you still have around 3 or 4 good hrs of heat for cooking. You can also roast without the door and an active fire on the side. Do lots of firing and cooking and you will learn your ovens characteristics, rely less and less on your thermocouples which will inevitably fail in time anyway. The vermicrete was 10:1 with a bit of powdered clay added to it. Do you have any idea of how much heat is lost in the base compared to the dome? I can add additional to the dome with an additional layer of vermicrete but as you say I will have to live with the base insulation I have already. It would be nice to get another 1 or 2 hours of slow cooking out of the oven but if not I'll just try putting it in a bit earlier at a higher temp. As the drop off is quick it shouldn't make that much difference. As for the thermocouples, I'm an engineer so we like data. I want to use them to change the colour of some lights behind the oven depending on the oven temp but this will be about phase 10 of the build when we are at phase 2 now |
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Post by downunderdave on Apr 29, 2019 12:29:21 GMT
25 mm of calsil underfloor insulation vs 25 mm CFB + 50 mm 10:1 vermicrete means you will be losing lots more heat via the floor than the dome. So adding more insulation over the dome may turn out to be a wasted exercise, as heat rises via convection, but doesn’t care about direction when travelling via conduction, so as your floor and dome are of equal thickness I still think you’ll find that your oven will improve with more firings. Be happy with what you’ve got. You can always build a better oven next time.y
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Post by webbm00 on May 2, 2019 7:29:26 GMT
Tried the "big fire" test yesterday. Four hours of a large fire with flames coming out of the front at times.   It looks like the problem was getting heat into the oven. I don't think my last fire was big enough for long enough as this time I got under the base fire bricks up to 500degC and it was 7hrs from 200 to 100degC rather than 5hrs. Thats enough for my needs  There was also a little moisture on the plastic sheet I put over it when it was hot so there is some moisture still in the oven. I think most of the heat loss is under the base as the top of the vermicrete was ~60degC max and dropped off even though the top of the insulation blanket got to 150degC whereas the top of the concrete slab under the CaSi board was tracking the temperature of the CaSi board and up to 90degC. Could the CaSi board be wet? The oven is undercover and the CaSi board was covered in foil before the oven base was installed. If it is how do you get the moisture out apart from having lots of fires |
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Post by simonh on May 2, 2019 7:56:02 GMT
AFAIK the only way to get moisture out is with lots of fires
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