Long duct, short chimney.

Hello,
I am planning a RMH with a 20cm j-tube with metal barrel.
It will have a straight 10m long duct inside a single bench so no bends, only clean out in the beginning and the end. Then a straight vertical chimney of insulated double wall stainless steel pipe, around 2.5m inside the building and 2.5m outside.
If I take the calculations from Erika Wisner it must be possible. But I am not sure if the chimney will cause enough draft to make the system work good. It is impossible to make a bypass because of the single bench
Anyone have experience with some simular long bench and short chimney?
Or anyone can give me some advise on this?
Thanks in advance.

Your sketch shows a nice clean, simple system and should work just fine. An 8" J-tube (equal to 20cm) can handle up to 45-50 feet of straight duct even without a really good chimney, so 10m (33') of straight duct with 5m (16') of chimney, half inside the space, is likely to actually run hotter and stronger than ideal. I believe based on my and others' experience that you could use a 6" or 15cm insulated chimney which would be distinctly less expensive.

I have an 8" J-tube in a masonry bell, with a temporary 6" stovepipe running up 7' and out the sidewall, then up 9' to above the roof with 6" insulated chimney (so fairly similar to your proposal). It draws well in all conditions, and I still need to partially cover the feed to avoid too much airflow through the system.

The only time you might have an issue with draft would be when it is warmer outside than in with a stone cold system. Then you might need to prime the chimney with a small fire or other heat at the base of the chimney. A bypass or reheater from the chimney being close to the barrel would not help much in that case anyway.


By the way, you don't need the insulated chimney all the way up inside the building, only within about a half meter of the ceiling (your local building code may be slightly different) and above the roof.

Gert Gerard wrote:
Anyone have experience with some similar long bench and short chimney?

Gert, your planned system will be almost identical to mine, when I account for the drag of two 180 turns inside my bench. I've never needed a bypass.

Your natural draft can be tested before the stove and bench is built. I.e. lay the 10m long duct on the floor and connect it to the 5m chimney. You should be able to feel air flowing into the open end of the 10m duct. The draft may even be strong enough to pull the flame off a lit match or lighter, when the room is warmer than it is outdoors. A very good sign to say the least.