Chimney question

[mga]

House has two fireplaces, one up, one down. When making a fire in the upstairs fireplace, it seems like the lower one draws the smoke from the fire back into the house, from the top of the chimney.. The flues at the top are side by side and only maybe 8 inches apart.

. My theory is that when there's a fire, the negative air pressure is drawing smoke back in from the unused one, and since they are side by side at the top, it kinda makes sense...no? Downstairs, seems to get filled with the smoke odor....not so much the smoke itself.

The dampers are not the best, they're the old steel ones that close but don't seal air tight.

Any other ideas as to why this would happen?

 

[Short timer]

You could test your theory by opening windows to relieve any negative pressure.

 

[Be Stihl]

You could test your theory by opening windows to relieve any negative pressure.

Yep, need a fresh air inlet


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[mga]

i went up and covered the flue for the unused fireplace downstairs to seal it off. i'll do a fire tonite and test it. if it works, fine, i'll figure out where to go from there.

if it doesn't i'll try the window being open.

thanks!!

 

[moresnow]

This is not unheard of and you are likely correct. Adding chimney to the side most used has been a common recommendation. Easier said than done in some cases! Hopefully sealing the unused side well will fix it.

 

[NSMaple1]

i went up and covered the flue for the unused fireplace downstairs to seal it off. i'll do a fire tonite and test it. if it works, fine, i'll figure out where to go from there.

if it doesn't i'll try the window being open.

thanks!!

I'd try cracking a window first before you block the flue. Easy troubleshooting info you might be able to use later sometime.

 

[bbxlr8]

This happens on my chimney that has a first floor FP and the adjacent stack that handles my oil burning furnace (which is not on much )
On a really cold day, it will draw down the other stack and smell up[ that area of the basement. Goes away after I get it cranking.

Interestingly, tt does not happen on the other end of the house with a chimney that has my stove in the basement and a FP on the first floor.

 

[Mustang71]

You need a good source of make up air. A chimney is a good non restrictive source of make up air.

 

[Huskybill]

You can’t have the house too air tight. The woodstove can be finicky to start the draft on a windless night. We hold up a lighted piece of news paper to the chimney pipe till the draft starts then the cold air draws the hot air. The stove is on a 8” x10” flue. The fireplace is a 10” x 10” flue the furnace is a 8” x 10” flue .

I argue this with the misses we have three flues. One fireplace, one woodstove in basement, and the oil burner. We installed new double pane windows. We have larger picture windows in the kitchen and living room. Now I can run all three chimneys with no backup from smoke. My back door and front door aren’t that air tight for that reason. My misses wants to get that free inspection for air leaks. The woodstove in the basement heats the large ranch style house no problem the furthest bedroom is 67/70 degrees on the 0 zero nights.

Rule of thumb for woodstove chimneys the stove has a 6” pipe the chimney must be no smaller than 8” for draft on windless nights.

 

[NSMaple1]

You can’t have the house too air tight. The woodstove can be finicky to start the draft on a windless night. We hold up a lighted piece of news paper to the chimney pipe till the draft starts then the cold air draws the hot air. The stove is on a 8” x10” flue. The fireplace is a 10” x 10” flue the furnace is a 8” x 10” flue .

I argue this with the misses we have three flues. One fireplace, one woodstove in basement, and the oil burner. We installed new double pane windows. We have larger picture windows in the kitchen and living room. Now I can run all three chimneys with no backup from smoke. My back door and front door aren’t that air tight for that reason. My misses wants to get that free inspection for air leaks. The woodstove in the basement heats the large ranch style house no problem the furthest bedroom is 67/70 degrees on the 0 zero nights.

Rule of thumb for woodstove chimneys the stove has a 6” pipe the chimney must be no smaller than 8” for draft on windless nights.

I dont think that's a rule.

 

[moresnow]

Rule of thumb for woodstove chimneys the stove has a 6” pipe the chimney must be no smaller than 8” for draft on windless nights.

Um? Chimney diameter should match stove outlet diameter for best draft performance in every manual I've used.

 

[Huskybill]

When the chimney is the same size as the stove pipe you can get back drafts on wind less nights. I installed many woodstove using 8" pipe stainless chimney with a 6" stove pipe. I have a book put out for woodstove safety and installations by the government. I know I see many woodstovrs with 6" chimneys. They complain about back drafts its dealer installed. Inspected by the towns code people too. If there's a fire the insurance company will inspect the installation with the fire department. I taught my towns code guy.

 

[laynes69]

Contrary to belief, the larger the diameter of chimney on a modern woodstove, the worse the draft. Large chimneys were just fine with older non efficient stoves due to the amount of heat they sent up the chimney. A modern woodburner puts less heat up a chimney. The larger the diameter, the worse the draft due to the lesser amount of heat. I had a 8x11 chimney (32') and when our damper would close, the chimney draft would drop to almost .01" of water. We always would have draft issues. Now we have a 5.5" rigid stainless liner and when the damper closes we maintain .04" of water column. Yes a 8" chimney will work on a 6" outlet, but a 6" chimney is best for the system.

 

[Huskybill]

I never had a problem with a 6" stove pipe with a 8" chimney. I do put a damper on the 6" stove pipe and two temp gauges one on the stove and one on the stove pipe. I control the amount of heat and burn time in the stove and what temp goes up the chimney to keep the creosote down.

Some stoves have a 5" outlet.

 

[NSMaple1]

There's more to it than just diameter.

 

[mga]

i covered the basement fireplace flue opening on the roof and started a fire upstairs. No smoke in the basement! So, now i just have to figure out a decent air intake that i can install without punching holes in the blocks.

thanks for all the feed back!!

 

[Ted Jenkins]

There's more to it than just diameter.

That is correct as there is a balance of size and heat. A typical situation is when the stove pipe is the same size all the way to the out side. If much of the pipe is double or tripe wall the draft will be much better because when the pipe is very hot it will eliminate creosote build up and draw better. Thanks

 

[Northerner]

I run a 6” chimney with a 4” fresh air. Works very well as long as the dryer isn’t going when u try to light a fire. Then I just crack a window until the chimney is warmed up.


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[Marine5068]

When the chimney is the same size as the stove pipe you can get back drafts on wind less nights. I installed many woodstove using 8" pipe stainless chimney with a 6" stove pipe. I have a book put out for woodstove safety and installations by the government. I know I see many woodstoves with 6" chimneys. They complain about back drafts its dealer installed. Inspected by the towns code people too. If there's a fire the insurance company will inspect the installation with the fire department. I taught my towns code guy.

My 6" woodstove outlet is tapered right to an 8" stainless insulated Selkirk chimney about 30" out from the stove top and it works great. My setup starts with a 6" black single wall pipe with the 6"-8" reducer( it's actually an expander) and a cast iron damper in the black pipe section.
In 17 years I've never had any backdrafts and fire always starts easily on windy and no wind days so I guess I'm lucky that way.
There's a lost of physics to the design and construction of chimneys but there are also many site variables too.
It's not a static science so the system needs to be flexible.
You may have to adjust the height of one of the chimney tops.
Hopefully you come up with a good plan that's not too financially painful.

 

[moresnow]

My 6" woodstove outlet is tapered right to an 8" stainless insulated Selkirk chimney about 30" out from the stove top and it works great. My setup starts with a 6" black single wall pipe with the 6"-8" reducer( it's actually an expander) and a cast iron damper in the black pipe section.
In 17 years I've never had any backdrafts and fire always starts easily on windy and no wind days so I guess I'm lucky that way.
There's a lost of physics to the design and construction of chimneys but there are also many site variables too.
It's not a static science so the system needs to be flexible.
You may have to adjust the height of one of the chimney tops.
Hopefully you come up with a good plan that's not too financially painful.
View attachment 867694

Top notch stove you have there. Easy breather as well.