Hi all, We have recently purchased a farm in the mountains in the Western Cape, South Africa and are planning to build our first RMS. We are having difficulty in translating the words "Stove Pipe" into buyable materials. Please could someone assist in being more specific as to what exactly this is as this term is not used in SA. The common chimney flue pipe used here is either galvanised (which I have read is not suitable) or a mild steel pipe 1.6mm wall thickness, 1,2metre lengths, more commonly 150mm diameter. Also available are 45 and 90 deg bends.
We are perhaps being a little ambitious so please comment on whether our plan is feasible. The idea is to setup a 8"(20cm) firebox and to run 18-20cm pipes (depending on what is available) through 3 rooms - see diagram. This will run a total of about 14 metres of pipe (2 pipes for each mass) and will include 2 "U" bends and 6 x 90deg bends + a bend to the chimney. Since the standard flue piping is not that freely available and is very costly, we thought it might be a better idea to get the pipes cut to length and to have the U-bend and the right angles fabricated. In this case, is mild steel the correct material and what should the wall thickness be.
The building has the original house with a pitch roof and a flat roofed section that was added on. The plan was to have the chimney exit through the flat roof section. We have been told that the chimney should extend above the highest point of the roof. If as in our proposal the chimney would be on the flat roof section which is substantially lower than the pitched roof it is said that a backdraft can occur, caused by thermals when wind is held back by roof sections higher than the chimney cowl. Any comment on this?
An early winter is upon us so any assistance would be appreciated.
Don't need it unless code requires. But seeing most don't get rmh inspected, there is no need. The exhaust is cooled alredy. So unless you are trying to tell an inspector a white lie, no need for Insulative exhaust systems. Ps. If your mass is built correctly, even if the metal degrades, the cob bench will have cured
into mud pipes.
The diagram didn't come through, but you will definitely not be able to run ducting the way you describe. The standard recommendation is that an 8" system can power up to 50' (15m or so) of horizontal duct, MINUS 5' for each 90 degree bend. You can see the difficulty.
What you might possibly be able to do is create a series of bells, largeish chambers where the hot air enters at the bottom and rises, then falls as it cools, with only the coolest air leaving via an exit at the bottom. These have been done in sequence before, though I don't know about three of them. A major benefit of bells is that friction is much less, only involving the connecting ducts and minor entry/exit losses.
You are right that a higher roof some distance away could affect the draft in a chimney. Each situation is different, but a general rule in the US is that a chimney must be at least 2' higher than any part of the roof within 10'. How much difference in height do you have, and what horizontal distances?
"Stove pipe" in the US generally means a black sheetmetal pipe which comes in standard lengths (2', 3', 5'...) and is heavier than ordinary galvanized metal ducting (used for heating and air conditioning). 1.6 mmm is heavier than "stove pipe" we see.
Chad is right that you don't need pipe that heavy for your RMH exhaust. The galvanized duct if it is the cheapest option would work fine for most of your mass run or connections. If you want bench-style masses, search the forum for Matt Walker's "half-barrel bell" concept. Old 55 gallon barrels cut in half lengthwise and laid flat side down can be joined to create the volume for heat absorption, and may be cheaper than duct - especially having duct custom-fabricated.
Forgot to mention that the galvanizing is only an issue when exposed to high heat, as in or near the barrel. By the time you get a meter or so from the barrel, the temperatures will be fine. Pre-burning (outdoors) any galvanized duct will convert the shiny galvanizing to a dull gray, stable form.