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Post by BenAlexanderT on Dec 8, 2018 12:58:23 GMT -8
Hello there again with a new topic When I was looking some builds of masonry heater, they were putting either cardboard or ceramic fiber blanket between the two walls, and I read here that the purpose of this is to have some room for expansion, but I don't understand how the heat will be transferred from the first wall to the second wall when you are using an insulative material. Do they remove it after the build is finished? I was searching the forum but I wasn't able to find an answer. Sorry if it's in the wrong section. Thanks in advance.
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Post by peterberg on Dec 8, 2018 13:21:26 GMT -8
Having some material between the inner and outer skins provides an expansion and slip joint, that's true. The ceramic fibre blanket won't be removed, it just slows down the transfer of heat between the skins. This transfer is all done by radiation, cardboard is burned away or turns to charcoal in the absence of oxygen and fibre blanket stays in there. The thicker the fibre blanket the slower heat is coming through. This way it's possible to a certain extent to "tune" the way the heater is behaving.
In Finland people are using a special make of hard rockwool to this purpose, 10 mm thick most of the time.
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Post by BenAlexanderT on Dec 8, 2018 13:34:04 GMT -8
So, the difference is in the inertia of the heater? Doesn't the heat go away through the chimney because of the delay the blanket gives?
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Post by peterberg on Dec 9, 2018 1:40:49 GMT -8
The difference isn't that great, the expansion/slip joint is the main reason to do it like this. Otherwise the outer skin of the heater will crack for sure. And no, the heat doesn't go away through the chimney because of this. For one, exhaust opening is situated close to the floor for contraflow and bell constructions alike so the coldest gases are exhausted first. In order to stop exhausting to the chimney after the fire is gone there are two solutions: a chimney damper or a tight closing door. The latter, being the middle European solution, is my favourite.
Similar heaters are in use for quite a long time now, it's important to realize those are heavy-weight things. Very, very different in almost all aspects of what we/you/they are used to. It is important to forget all you know about what most people call "normal stoves" and start from scatch. In this respect, mass heaters and especially the larger ones are behaving far from "normal".
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Post by Vortex on Dec 9, 2018 2:18:18 GMT -8
That's the theory. If it's the case the chimney thermometer should show the outside temperature. Really helps if your chimney is not inside your stove, like the one in Peters avatar above. I have an insulated internal chimney, an airtight door and a chimney damper, and the thermometer's showing heat leaking away through the chimney all the time. Admittedly the pipe is steel so it's conducting some.
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Post by peterberg on Dec 9, 2018 3:40:59 GMT -8
I have an insulated internal chimney, an airtight door and a chimney damper, and the thermometer's showing heat leaking away through the chimney all the time. Admittedly the pipe is steel so it's conducting some. At first I thought the door was leaking, so during the first season at some point I closed off the chimney completely at the top but the pipe remained lukewarm. Later on I realized the pipe was conducting heat away. At the point where it's exiting the heater it is markedly warmer, just over handwarm. Logically, the pipe is connected to the inner skin so heat is leaking out quite easily. But since it's a single walled pipe up to the ceiling it's losing it in the living space again.
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Post by treebased on Jan 11, 2019 6:07:58 GMT -8
If one were to build a double skin bell, would an air gap between the two suffice to prevent cracks in the exterior or is some type of medium sandwiched between a must?
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Post by peterberg on Jan 11, 2019 6:38:59 GMT -8
Hi Treebased, welcome to the boards. A gap would be sufficient, yes. Difficult to accomplish since it need to be completely empty. Just small blobs of mortar would connect inner and outer skin, cracking the outer skin when the inner skin heats up sufficiently. I am using half an inch of superwool myself, the insulation value will deminish hot spots as a bonus. The heater will be a tad slower to heat up but I'd think that isn't a disadvantage per se. You could use cardboard to separate the skins, this will char in the absence of oxygen, leaving the desired gap. It's a bit more adventureous, the outer skin need to be completely closed otherwise you'll get lots of nasty smoke in your living space.
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Post by satamax on Jan 30, 2019 0:49:26 GMT -8
I'll reiterate what i have said previousmy.
Steel wool is a very good gap filler.
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Post by Vortex on Jan 30, 2019 2:28:14 GMT -8
That's a good idea Max, but I've never seen it for sale in thin sheets or rolls big enough, they're all tiny. How do you get around that, or do you have a source?
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moose
New Member
Posts: 2
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Post by moose on Feb 23, 2019 8:00:43 GMT -8
Hello all, first off many thanks for sharing all your abundant and exciting wood burning knowledge! On the subject of keeping mortar from bridging the gap between bell skins, if my bell is bench height/blind ended, can I use standard Portland/Lime/Sand mortar for my brickwork?
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