Size counts?

New to this so please bear with me. The wood burner (kaminofen) I have identified for my Austrian home has a 150mm exhaust, but my chimney has a 120mm inlet hole. Do the laws of physics/chemistry permit me to do this with the correctly adapted pipework? I suspect not but greater minds than mine prowl these posts.

Size does matter.
Is the chimney the proper size, just not the inlet? It is usually possible to enlarge the inlet.
Check the specifications on the heater you have in mind. Usually the designers or manufacturers will know the range of chimney sizes that their heater can tolerate.

The proportion of flow in the two sizes (120 mm vs 150 mm) will be:
r = radius of 120 mm diameter pipe = 60 mm
R = Radius of 150 mm diameter pipe = 75 mm
Ratio of cross-sectional area for flow to go through = pi*r*r / pi*R*R
= (60*60/75*75) (the pi's cancel out)
= 3600 / 5625
= 0.64
Conclusion: even though 30 mm sounds small, you will lose more than one third of the flow area.

It would make a difference to my stoves, though I could probably beef up other factors if I had to make it work.

Best option is to re-cut the inlet larger, if the chimney is 150 mm. Or choose a heater that is designed for such a size of chimney collar.

-Erica

Thanks Erica. I am off to work in the Netherlands from Sunday - mid December and will investigate further upon my return.
As an aside, I very much enjoyed viewing the RSMH DVD.
Kind regards,

Nigel

Erica Wisner wrote:
The proportion of flow in the two sizes (120 mm vs 150 mm) will be:
r = radius of 120 mm diameter pipe = 60 mm
R = Radius of 150 mm diameter pipe = 75 mm
Ratio of cross-sectional area for flow to go through = pi*r*r / pi*R*R
= (60*60/75*75) (the pi's cancel out)
= 3600 / 5625
= 0.64
Conclusion: even though 30 mm sounds small, you will lose more than one third of the flow area.
-Erica

Erica,

Is the volume of air flow dependant on the smallest cross-sectional area of the gas path, or does the expanding flue gases heated by combustion draw incoming air through the smallest cross-sectional area at higher velocities dependant on chimney length?

Curbie

After dozens of cities in Europe burned down because of chimney fires, governments established training programs and apprenticeships for chimney sweeps as a requirement for each local district. European chimney sweeps are as highly respected as any government official or educated professional. Consult your local chimney sweep for advice. He'll make sure you do it right.