How much class A chimney pipe on outside of house?

The length of outside chimney is only one of many factors affecting draft, and I'm not sure that when using a Class A chimney it is the primary factor. Assuming it is installed to meet all horizontal and vertical requirements from an adjoining roof, other factors can include other buildings, trees, obstructions; wind directions, intensity and variability; outside temperature; the wood furnace itself and flue gas temperatures; forced draft/induced draft vs natural draft; and I'm sure the list goes on.

As to the outside factor only, I have 17' of Class A through an unheated and very well vented attic space, plus 5' of black stove pipe from my Tarm to the Class A at the ceiling. In my situation I was obtaining very strong draft, regardless of outside temperature, and I had to damper the forced draft of the Tarm to bring down the flue gas temperature.

Note I also live in northern MN. We just got 16-18" of snow Sunday and Sunday night, with more in the forecast later this week. It's a real mess out there now.

Call it creosote or just a fine ash, I get a little at the chimney cap and then a very fine layer inside the chimney. I started burning in Sept, almost all dry pine (no green wood), and I'm still burning. Have not had to clean the chimney or the cap, but will do so this summer. Interior probe thermometer flue temps run 400-500 average. Achieving this performance is a no-brainer once the draft was properly adjusted and turbulators added to the heat exchanger tubes.

Keep in mind what I have is Tarm gasifier wood boiler. If you go with a gasification type boiler, I would think your experience should be similar. If you go with a non-gasifying wood burning appliance, then your experience may be much different, and I would suggest carefully watching the chimney/creosote until you know how your wood burner performs.

We also have a free standing wood stove, about 5' black chimney pipe to the ceiling, and then Class A through an unheated attic, about 11' of Class A total. We burn almost only well seasoned aspen in the wood stove. This chimney accumulates more creosote, not ash, than with the Tarm, but still only need to clean it once a year. We maintain our burns for 300-350 exterior flue temp as measured by a magnetic thermometer. Maintaining this temperature requires some attention, and both my wife and I are pretty careful not to burn too cool and not to overheat the stove. A simple draft adjustment occasionally is all that's needed, mostly on start-up and when adding wood.

I'm a firm believer in maintaining proper flue temp, both from a stove/chimney safety perspective and from a clean burning, minimum creosote perspective.

BTW -- I'm about 30 miles SE of Walker.

huntingbuck101 said:

Looking at installing a wood furnace and need to know how much class A chimney pipe on the outside of the house, I can get away with before I run into draft problems? I would like to run about 12'-14' outside so it's 2 feet higher then the roof peek. there will be about 7' of pipe in the heated house. The house is in cold northern MN.
thanks.

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I doubt you'd have problems, but if so, you can always build a chase. btw, we have that much pipe running up the side of the house and have no problems. Be sure when you install the horizontal that it has a minimum of 1/4" raise per foot. We have even more raise than the recommended 1/4" and have no problems.