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Flue Chimney Tips - Forno Bravo Forum: The Wood-Fired Oven Community


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Flue Chimney Tips

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  • Flue Chimney Tips

    Here in the UK we have stringent building regulations regarding solid fuel heating appliances (boilers, stoves, fireplaces etc). These regs have evolved over the years to make sure the chimney / flue is constructed safely. Here the thinking behind the regs comes from years of people and appliances burning bitumus coal, coke and wet timber.

    As most of the appliances slow burn the fuel in the mistaken belief that it lasts longer a considerable amount of un burnt gasses and moisture are admitted into the chimney / flue system and the out side environment. If the un burnt gasses and moisture cool before leaving the chimney / flue system they will turn in to a corrosive liquid that doesnít do the chimney / flue / system any favourers.

    With a wood-fired oven or masonry heater the burning of the fuel is fast and furious thus capturing most of the calorific value of the fuel within the oven / heater and emitting very little or in most cases no smoke and un burnt gasses. To be able to burn wood in the way above it must be dry.

    So what I am getting at is that the requirements for a chimney / flue for a wood-fired oven is quite different than the solid fuel appliances that the regs were drawn up for. In my experience here in the UK we have to abide by the regs when applying for planning etc, however, often in conversation with the relevant authorities commonsense can prevail in the end.

    Where planning / permits arenít required work on the principle that the chimney / flue is only used for as a chimney / flue for part of the baking / cooking period, you donít want to set anything on fire so use common sense as to the termination of the chimney / flue, and last donít upset the folks next door by positioning the chimney flue where it will cause a nuisance.


  • #2
    code requirements

    I have looked into the code requirements for chimneys in residential construction. The applicable code can be found on the Rumford site:


    International Residential Code 2003 is not a simple document. I don't know how international it actually is, because all the dimensions are in inches. Here are the highlights:

    The footings of the chimney are below the frost line, which here in New Jersey is three feet deep. They are twelve inches deep, and protrude six inches on each side beyond the outside of the chimney. The chimney is made of four inch thick masonry, and surrounds a tile liner, with an airspace equal to the width of the liner thickness. As far as I can tell the tile liner is layed up with refractory mortar and is free standing for the entire height of the chimney until it gets to the chimney cap, which, if poured from concrete, has a flashing to allow it to slide for vertical expansion. This airspace is sufficiently unobstructed that code stipulates that other appliances cannot be vented through this space. Code allows a thirty degree angle if you need to offest the flue, but no mention of how the angled section is supported on the bottom. The masonry chimney itsself is free standing inside or outside the structure, and requires a two inch clearance to combustables, like wooden structural members. A non-combustible fire block is to be provided at certain intervals, but as far as I can tell, it doesn't tie the structure to the chimney.

    The flashing where the chimney meets the structure is extremely complicated, and is interleaved with the shingles on the sloped side. No mention of whether asphalt shingles are considered combustible, and subject to the two inch gap.

    There's a lot more. I assume that if you are building a masonry chimney you need to hammer out your own requirements with your building inspector.

    My geodesic oven project: part 1, part 2