flue height and bends

barbbiggs

New Member
Jan 21, 2021
2
Italy
Hello,
I'm about to buy an old wood burning majolica stove. I need a 90 degree bend to attach the flue at the back that goes up the internal wall for 3m.
Then I have a 45 degree bend to get to the outside.
How much flue should I put outside to create a good draw for the fire?
Any advise would be much appreciated.
Thanks
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,180
NE PA
If you can't find a manual, the following is what is used in the US; General rule is minimum 15 feet (4.572 m) from stove top to chimney top. I see EU / UK is 4.5 m, so that is a universal height. In some cases this is due to a 15 foot chimney used for testing, so that height is required when "installed as tested", not always due to what is physically required for proper operation.

The more bends, or horizontal runs, the higher the resistance, so more height is needed to increase draft. The exterior chimney needs to be insulated prefab metal or masonry, not single wall pipe.

The top of chimney must be 2 feet (.61 m) above anything, including roof within 10 feet (3.1 m) measuring horizontal. It also has to be at least 3 feet (.92 m) above roof penetration when it goes through a roof. When this criteria is met, it is normally high enough.
Your local codes may vary.

There may be an offset limitation for the center line of inside and outside pipe when a manufacturer gives that maximum in the manual. If exceeding that, a higher chimney may make up for the added resistance from the horizontal run. A higher chimney creates more draft, but exhaust gasses cool as they rise, so a shorter chimney benefits more as height is added. The taller you go, the less increase in draft it creates. There are tables that give the efficiency curve of chimney heights. But you need to know the BTU capacity and how much heat you are letting up the stack.

There are many factors that affect the height requirement. Outside air temperature, altitude (atmospheric air pressure on any given day) Chimney construction being inside or outside, masonry or insulated flue. And the resistance created by connector pipe configuration. Even house design with a stove in a lower room with heat rising away from it upstairs competes with draft in the chimney. This is called stack effect, so what works in one installation may give problems in another.