Relationship between chimney draft and height

[ohlongarm]

Hey,

Admittedly my install is not bulletproof. I have about 14 feet from the bottom of my unit to the top of my chimney so probably like 12" of actual chimney liner. My insert heats fantastically, efficiently and burns cleanly. I try to burn good dry wood as best as I can.

My problem is every time I open the door I get smoke in the house. And not just a a quick whiff, like it pours in the room. I have an offset box that attaches the insert to the chimney. I'm fairly sure the offset box is the culprit, but it's a necessary evil without extensive masonry work. Last year I replaced the offset box and used furnace cement to seal any suspicious air gaps. It had no improvement.

Here's my question: will increasing the length of my chimney by 3 feet help?

Here's my version of the math: 12 ft chimney + 3 ft (25%) increases draft by 25%. Is this correct? Will adding three feet really increase my draft and help with this? And if so what is the best product to transition a 6" SS flex liner to class A chimney (or whatever is best here)? Thanks so much.

I had 12 feet for my BK Ultra the minimum required,this fall I added 4 more now at 16 incredible increase in efficiency 12 to 18 hours in sub zero wind chills with house at 73* and stovetop always at 500 to 550 minimum,PS my wood is primo nothing less than 3 years.To answer your question you are correct an increase of 3 increases a 12 foot run by 25%.

 

[oconnor]

It occurs to me that 14 feet with an offset box is essentially a 9 foot flue. The rule of thumb for direction changes is a flue is a loss of five feet for each 90 degree bend.

 

[oldspark]

It occurs to me that 14 feet with an offset box is essentially a 9 foot flue. The rule of thumb for direction changes is a flue is a loss of five feet for each 90 degree bend.

That sounds good but with two stoves and two chimneys going from removing 2 90's to straight up the difference was marginal I question those figures.

 

[brenndatomu]

Yeah, 5 ft. sounds a lil strong. I've always heard it was more like 3-4 ft. per 90.

 

[oldspark]

Yeah, 5 ft. sounds a lil strong. I've always heard it was more like 3-4 ft. per 90.

I even question the 3 to 4 feet, if I had to guess with my real world expeirence it would be 1 to 2 feet, I removed 2 90's and it make a slight difference.
Of course it depends on how crappy the draft is in the first place.

 

[dafattkidd]

Ok. I spoke with a chimney guy I work with. It's probably a bad idea to cut out that steel box. He said there's a good chance it will make my fireplace be out of code and probably void my homeowners insurance.

I think the only options are: 1 get a different insert with an exhaust system that is more suitable for my insert. 2 install an exhaust fan at the top of my chimney. Crap!

I'll probably opt for 2 as he offered to give it to me at cost saving me $700. Honestly, I'm probably not going to do much about this any time soon.

 

[oconnor]

Yeah, 5 ft. sounds a lil strong. I've always heard it was more like 3-4 ft. per 90.

Regardless of 2 or 5 feet per 90, the offset box is two 90* turns, so between 4-10 feet of effective chimney height reduced. He only has 12 feet to start with, so it's not going to work well, and as the OP has stated isn't.

 

[oconnor]

Ok. I spoke with a chimney guy I work with. It's probably a bad idea to cut out that steel box. He said there's a good chance it will make my fireplace be out of code and probably void my homeowners insurance.

I think the only options are: 1 get a different insert with an exhaust system that is more suitable for my insert. 2 install an exhaust fan at the top of my chimney. Crap!

I'll probably opt for 2 as he offered to give it to me at cost saving me $700. Honestly, I'm probably not going to do much about this any time soon.

What did he say about modifying the throat of the fireplace to take the offset box out of the equation? Seems to me it's done fairly regularly.

 

[mellow]

I would double check to see what code it would void, I would try to find something in writing before discounting that. If it is clearance to combustibles and we are talking about a ZC insert I would understand but pure masonry it shouldn't be an issue.

 

[oldspark]

Regardless of 2 or 5 feet per 90, the offset box is two 90* turns, so between 4-10 feet of effective chimney height reduced. He only has 12 feet to start with, so it's not going to work well, and as the OP has stated isn't.

Just saying losing the 2 90's might not make it into a hoover, the 12 feet is probably the issue.

 

[dafattkidd]

What did he say about modifying the throat of the fireplace to take the offset box out of the equation? Seems to me it's done fairly regularly.

The problem is the lintel sits directly above the exhaust of the stove. I'm not sure I follow how modifying the throat of the fireplace would help. Can you further explain how you think this would help? Thanks.

 

[dafattkidd]

Just saying losing the 2 90's might not make it into a hoover, the 12 feet is probably the issue.

Even with the added 4 foot of stove pipe it was little improvement. I'm pretty convinced it's the offset box. 16 foot interior chimney should draft pretty well. Do you have any suggestions? I'm thinking my only option is a very expensive exhaust fan.

 

[oconnor]

The problem is the lintel sits directly above the exhaust of the stove. I'm not sure I follow how modifying the throat of the fireplace would help. Can you further explain how you think this would help? Thanks.

Sometimes by cutting away some of the metal associated with the original damper you can make more room for the liner to connect to the stove. You need to pay attention to clearances, but usually there is only the masonry of the fireplace nearby. You can wrap the liner with ceramic fibre blanket to keep heat from radiating thru masonry to any combustibles.

 

[brenndatomu]

Sometimes by cutting away some of the metal associated with the original damper you can make more room for the liner to connect to the stove. You need to pay attention to clearances, but usually there is only the masonry of the fireplace nearby. You can wrap the liner with ceramic fibre blanket to keep heat from radiating thru masonry to any combustibles.

^^^ This and a 15 degree stainless elbow could give you some flexibility?

 

[dafattkidd]

^^^ This and a 15 degree stainless elbow could give you some flexibility?

Nope. I have 2 3/8" from the top of the insert to the lintel and the lintel is directly over the exhaust of the insert.

 

[oconnor]

Could a custom fab offset box be made that would give more flow? Have it made from 316 stainless.

 

[dafattkidd]

Could a custom fab offset box be made that would give more flow? Have it made from 316 stainless.

I was thinking the same thing. It may be a slight improvement. I'll see. I have a buddy who has a decent set up. We could possibly fabricate something, but I still think ultimately I will need an exhaust fan.

 

[Oregon aloha]

What if you used one of those chimney caps they use in high wind areas that are closed on one side and rotate with the wind.. They improve draft even when there is no wind.