Chimney volume: Playing with some numbers

I realize there is more to this than I am thinking and having some of you help out is why I posted.

I realize draft doesn’t equate to volume. I’m certain there are other factors at play with draft like barometric pressure, chimney insulation or not, type of chimney liner, inside/outside temps, chimney temps, etc.

My total chimney length is around 15ft from the fire box, but I also need to subtract length because of two 90* turns...going into the thimble and then up the straight run of the main chimney. NOTE: An additional 5ft onto the 10 that is already there is going to look stupid on the house. I need to compromise somewhere.

My current masonry chimney is 7.5” square and 10ft above the thimble. That’s approximately 6750 cubic inches of volume.


That’s darn near the same number as my ideal round 6”x20ft chimney for 6785.85 cubic inches.

OR...

Near the same as round 8”x10ft for 6031.87 cubic inches.

OR...

Near the same as round 7”x15ft for 6927.20 cubic inches.



The 7” round clay flue liners were said to fit into my existing masonry chimney. If I don’t extend the chimney length at all then round 7”x10ft comes out to 4618.15 cubic inches of chimney volume.


Now, let’s say that I don’t change the length of my chimney but I reduce it to 6” round. That’s 6”x10ft for 3393.9 cubic inches.

By Ohio Revised Code I am legally allowed to be 3x the stove outlet size for an interior chimney and 2x the stove outlet size for an exterior chimney.

Obviously this works but is not optimal. Optimal would be a matching stove/chimney at stove manufacturers recommended height. NOTE: Hitzer is the only stove company I have found that mentions in their manual the same information as mentioned in the O.R.C.

I find it real interesting that every single chimney sweep I have spoken to will not install a stove unless they put in a matching sized liner. They’ve all said by law they have to do this, yet...I can prove by O.R.C. they are indeed blowing smoke in people’s ears. Completely false and dishonest information. Interestingly enough I had a forum member on another forum, who happens to be a sweep as well, tell me much of what he does is designed mainly to make sales for his industry. As they say...follow the money.

Anyway, the purpose of all this investigation on my part is to get as much bang for my buck as possible. That is, to get as much draft as I can for the least amount of money.

Obviously, an increase of cubic inches or lack of cubic inches in a chimney... volume ... alone does not increase draft that I can see ... to a point.

It seems there is a fine line, or optimal zone, between chimney diameter and length. I’d like to get as near the sweet spot as possible by changing it as little as possible.

Then there’s the factor of can this be done for a duel fuel chimney, coal/wood? For me the choice is coal, but I realize we may not live here long and it’s highly unlikely anyone else will burn anything but wood, so I am thinking more inline for wood.

Velocity...
Don't over think this...
LOL...

Draft potential is directly proportional to the height of the chimney. The cross sectional area of a chimney does not appear in the draft equation at all, so chimney cross sectional area is completely irrelevant with regard to whether or not a chimney will have the potential to draft.

However, chimney flow rate potential is directly proportional to the cross-sectional area of chimney multiplied by the square root of the height of the chimney.

Due to the flow rate equation requiring that the height of a chimney be considered specifically as to it's square root, chimney internal volume, which is the measure of cross sectional area times height is not a valid concern when comparing one chimney to another. Any valid comparison must take into consideration the square root of the height.

Comparison example for equivalence in flow rate potential:

Chimney A is 8" in internal diameter and is round. It's cross sectional area is 50.27 square inches. It is 14 Ft. in height.
Chimney B is 6" in internal diameter and is round. It's cross sectional area is 28.27 square inches.

For chimney B to potentially flow at the same rate as chimney A (all other factors being equal) what height is required of chimney B?

For chimney A, the square root of 14' in height = 3.742
3.742 x 50.27 = 188.11

For chimney B, therefore:
28.27 x Sqrt(Height) = 188.11
Sqrt(Height) = 188.11/28.27 = 6.654
6.654 x 6.654 = 44.27

Chimney B will only have the same flow rate "potential" as chimney A if chimney B is 44.27 feet high.

But chimney B's draft potential is 44.27/14 = 3.16 times greater than chimney A's draft potential.

As to their internal volumes:
Chimney A's volume = 14 x 12 x 50.27 = 8,445.36 Cubic inches
Chimney B's volume = 44.27 x 12 x 28.27 = 15,018.15 Cubic inches

So flow rate and draft are different? Obviously I can’t have a 44’ y’all chimney with a single story home. LOL!

Yes, I was playing with volume cubic inches of different sizes which triggered the topic.

Sounds to me like there is a sweet spot by slightly increasing my chimney height by no more than 4 feet (one factor) and down sizing to 6” or 7” pipe to get closer to optimal draft.

I’m not as concerned with optimal draft for a given appliance as I am as finding the optimal potential of my chimney to handle (here’s the kicker) stoves from 6” to 8” outlet.

One person told me to add 3’-4’ and drop to 6” as most stoves are 6” today. Makes sense. The wood stove and this coal stove only run to max surface temps of 345 in the coldest temps here to keep the inside of the home a comfortable 72-75 degrees. Most of the time the coal stove is 300.

Downsizing to a 30-95 or 254 Hitzer or downsizing a wood stove would help when considering chimney changes for a better draft. Chimney does decent now with coal, but wood could be better when burning low.

I only plan to burn wood in the shoulder season, but I also realize we may not stay here and it’s unlikely a new owner will burn coal. Just a fact, but they may want to burn wood. So increasing draft for wood burning is the focus, to create a safe well drafting chimney for a potential new owner.

Yes...trying to satisfy too many things at once. If we decide to stay and add-on the home I will build a better chimney for coal and fix this one for wood...and have the best of both.

Velocity...
One person has said that given the same height chimney that 6” chimney the draft can be increased, the velocity, just by insulating the chimney. One person even told me most any stove can run in a 5” chimney...that even if uninsulated the 5” would heat up more and draft harder.

I think the key thought I get from that is the chimney also has to draft well enough to get a cold stove and a cold chimney to start warming up or you get smoke into the room. To me that could mean there’s a point at which reducing too much can go against you.

I am limited now to ten feet above the thimble approximately 14-15’ from the firebox...inside chimney with two 90* turns. Can change one of the 90* to 45* elbows the other cannot change. I can add no more than 2’-4’ ...most likely 3’ and the chimney remain looking nice on the house without looking silly.

First comes checking existing chimney/stove set-up with a manometer. Then changing to a 45* on one elbow is the cheapest and first factor in working towards a better draft. Then adding height. Then reducing to 7” or 6” chimney and changing height.

I could even remove existing 7.5” square I.D. clay liner and replace with 8” round to accommodate any sized stove outlet or even reduce to 7” or 6” round and extend a little.

I sort of wonder if 7” round and an additional 2’-3’ would be the best middle of the road option.

Back to the drawing board H!!!!!!!!!!!!!!!!!!!

freetown fred wrote: ↑

Sun. Sep. 06, 2020 1:19 pm

Back to the drawing board H!!!!!!!!!!!!!!!!!!!

When you going to change that “H” to “B”??? Hmnnn? LOL!!!

Soon as you change your user name to BOYTMAN Billy!!!!!!!!!!!!!!!!!!!!!!!! LOL

90+ percent of buyers wouldnt know the difference in chimneys. Just build what you need yourself and it will be fine.

How about a tall chimney pot ~2 feet high above your masonry chimney to increase draft but maintain proportions?
https://www.woodlanddirect.com/Chimney/Clay-Pots?state=20308

Leo

What a good idea for $1000.00--$2000.00 or there abouts!!! NOT.

freetown fred wrote: ↑

Sun. Sep. 06, 2020 3:56 pm

Soon as you change your user name to BOYTMAN Billy!!!!!!!!!!!!!!!!!!!!!!!! LOL

Bwahahahaha!!!

LeoinRI wrote: ↑

Sun. Sep. 06, 2020 5:21 pm

How about a tall chimney pot ~2 feet high above your masonry chimney to increase draft but maintain proportions?
https://www.woodlanddirect.com/Chimney/Clay-Pots?state=20308

Leo

I considered that for a split second until, like Fred said, until I seen the prices of them. Heck, I could buy the clay powder and make and fire my own for waaaaayy cheaper. LOL!

Yes. Those things are not cheap!!! I’d have a blingin’ chimney though.

Its no pricier than having someone install a liner but for us do it yourself people yes, its quite a bit. They sure look nice though. They must be harder to make than we think or more people would make them.

Good luck with your project no matter what you decide on.

I like Pauliewog's idea back on member
Donkom's McCleary base burner with piggyback oven post.

Stuff a steel pipe with plate on bottom and a cover plate a foot or two down from top of pipe. Cement in place.

Top plate will trap heat in chimney. Force the heat back down to bottom plate. Then up and out pipe.

Would restrict flow out of chimney and help keep it warm.

Not much cost and still look good.