high chimney

[stovexxx]

I would like to further elaborate on the issue regarding very high flues,
which notoriously can have an excessive draft,
but at the same time what I read little here,
can present an excessive cooling of the fumes, in the final phase of combustion,
when the air is rather closed,
with consequent difficulty in expelling the fumes.

 

[stoveliker]

I think that a 200 F column of exhaust fumes still will draft significantly if the flue is 25 ft tall. It's "excessively cooled" (below condensation temperature), but the pressure difference is sufficient for good flow.

Both are true (excessive draft and excessive cooling), but the "threshold" for one is not the same as the other.

 

[begreen]

30' flues are often coming from the basement via exterior chimneys. This causes excessive cooling unless the stove is run hard enough to keep the exiting flue gases above creosote condensation range. Stoves in this situation can be balky to start until the flue heats up. Then they can draft too strongly and need a key damper to tame the draft. Another complication of this setup can be weakening draft as the fire cools down. Sometimes this can lead to draft reversal which one wants to avoid.