Chimney height in addition/wing with lower roof than main house

[Skinflint]

IMHO - as yoda says - 'Size matters not.' You are basically building a giant "U". The stove flue is one upright and the 2-story house is the other. If the connection between the two - your doors or pass-through opening - is bigger than the stove pipe (which either is by multiple orders of magnitude) then there is effectively free-flow between the two legs of the U. All that matters then is which one drafts stronger.

Also bear in mind that "burning fine" and "easy starting" and "doesn't set off CO detectors when the fire dies down" are three completely different modes of operation. The first is easy - the second two are where problems can arise. Your actual flue will have some effect too. If you have a "mostly interior" insulated pipe, only peeking out a few feet at the top of the roof (which it sounds like, given the cathedral ceilings), that would be ideal. If you plan to go 'through the wall' and have a full exterior pipe, fully out in the cold, then you are increasing the slope of your already up-hill battle.

I have a question similar to @nhwoods, where I have a 1 1/2 story main residence with a gas fireplace and chimney above highest point AND a 1 story addition with a gas fireplace and chimney above highest point of the addition’s ridge. The termination point of the addition chimney is lower than the main house but a good 30’+ horizontally separated from each other.

My main fireplace performs fine, however, the addition fireplace suffers from backdraft. I have closed the damper in the main chimney but the backdraft continues.

Both chimneys are clean and clear.

Can the additional indoor living space of the 1/2 story in the main house still act as a draw on the lower chimney height, even though it being outside?

To match the elevation of the main house chimney, the addition chimney would need to be extended about 4’ higher. It might appear odd but is this what I need to do to correct this situation?

The house was this way when we bought it.

Thanks ahead of time for any response you may be able to provide.

 

[gthomas785]

I have a question similar to @nhwoods, where I have a 1 1/2 story main residence with a gas fireplace and chimney above highest point AND a 1 story addition with a gas fireplace and chimney above highest point of the addition’s ridge. The termination point of the addition chimney is lower than the main house but a good 30’+ horizontally separated from each other.

My main fireplace performs fine, however, the addition fireplace suffers from backdraft. I have closed the damper in the main chimney but the backdraft continues.

Both chimneys are clean and clear.

Can the additional indoor living space of the 1/2 story in the main house still act as a draw on the lower chimney height, even though it being outside?

To match the elevation of the main house chimney, the addition chimney would need to be extended about 4’ higher. It might appear odd but is this what I need to do to correct this situation?

The house was this way when we bought it.

Thanks ahead of time for any response you may be able to provide.

Is the addition's chimney on the outside of the wall or fully contained in the structure? Exterior chimneys are prone to draft reversal when cold.

 

[Skinflint]

The chimney is contained in the structure. I’m thinking about somehow extending the flue temporarily to see if it resolves the problem. Before I do that I’ll call a chimney sweep to get their thoughts as well.

 

[begreen]

There are several possible causes, some are easy to test and isolate, some are not. There could be negative pressure in the room. Does the backdraft stop if a nearby window in the room is open an inch?

One story additions often suffer negative pressure due to the chimney effect of the taller main house. If there is a window open on the upper floor of the main house, or an attic door that is not sealed, then it can create a vacuum effect in the rooms below. It can also happen if the lower chimney top is in a positive pressure zone.

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