Question About the Ratio of Flue Size to Smoke Pipe Size

Question About the Ratio of Flue Size to Smoke Pipe Size

It is my understanding that the internal size of the flue, in square inches, shouldn't exceed twice the size of the smoke pipe. My masonry flue, which is 35 feet high, has 74.75 internal square inches ( 11.5" by 6.5") and a 8" smoke pipe has 34.26 square inches, which exceeds the 2 to 1 ratio by 6.23 square inches. Is this a problem or would it still work? Thanks in advance for your reply...Gary
Gary1

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

The actual recommendation that I've seen for the perfect chimney is that it have the same internal cross sectional area as your flue pipe. I have the same situation as you have, only way worse, as my chimney is just like yours, but my boiler has only a 5" flue pipe exit. I fathom that the velocity of the gas traveling up the chimney is greatly diminished by having such a disparity in cross sectional areas. But not knowing the mathematics of draft, I can only speculate (OK, flat out guess) that this velocity decrease would somehow negatively impact the overall draft on windless days, and worse yet, on hot and windless days. But in a big wind it may enhance the draft???
Last edited by lsayre on Sat. Oct. 19, 2013 10:15 am, edited 1 time in total.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

Hey gary--what kind of stove??? what does your set-up look like--where is the stove sittin in regard to the chimney???????? Without any realistic pix/info---didactly all I can assume according to your calculations is, NOPE, it won't work
Last edited by freetown fred on Sat. Oct. 19, 2013 12:21 pm, edited 1 time in total.
freetown fred

Hand Fed Coal Stove: HITZER 50-93
Coal Size/Type: BLASCHAK Nut

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

I'm living proof that you can get by with it, but I often wish my draft was better than it is.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

Lets not forget the warmth of the chimney. How well insulated by running up the interior or exterior. All the colonial houses had interior chimneys, another lesson lost.
franco b

Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

franco b wrote:Lets not forget the warmth of the chimney. How well insulated by running up the interior or exterior. All the colonial houses had interior chimneys, another lesson lost.

Excellent point! Fortunately for me the chimney that my boiler is attached to is indoors. The otherwise pretty much identical chimney that our fire place insert in our basement / family room is attached to runs up the outside of the house, and it is notoriously terrible for draft.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

an 8" pipe into an 8/12 flue (like yours) is ideal and will work fine. The area of the flue is not terribly important. IF you want a chimney that drafts well, build it in the interior of the home and, most importantly, make sure it's 3-4' above the highest part of the home, not just the part of the house where it exits.
Berlin

Hot Air Coal Stoker Furnace: Will-Burt Combustioneer 77B
Coal Size/Type: Ohio BITUMINOUS pea stoker coal

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

I wanted a Glenwood stove, but due to stove and chimney placement problems, as well as poor heat flow due to the stove having to be at one end of the house or the other, I've resigned myself to a multi-fuel furnace, which I haven't selected yet. It would be placed in the basement, have its own dedicated masonry flue, which we're discussing, and be tied into my existing oil furnace's first floor hot air ductwork. Now I'd get heat distributed evenly in every room of the first floor.

The furnace would sit directly in front of the flue. The length of the stove pipe from the back of the furnace to the thimble would be about 4 feet, and have about a 2 and 1/2 inch rise per foot. The thimble is an additional foot in depth.

I have an external chimney at the end of the house which rises 3 feet above the peak of the roof.

Recently I carefully drilled out the portion of the vertical flue that's visible thru the thimble. When you put your hand in front of the thimble you can feel the draft and faintly hear it. I then wadded up a piece of newspaper, placed it inside the thimble about 2 inches from the vertical flue and lit it. The draft sucked the flames almost horizontal into and up the flue. This was done on a day when it was about 60 degrees outside with no wind. Based on this I thought the draft was sufficient, but then began thinking that it was good because of the high heat of the burning paper, whereas the cooler stove pipe gases might not produce as good a draft.

If I had to buy my solid fuel I'd choose coal, since I understand its benefits over wood. However, since I get all my wood for free from my own property its hard to justify paying about \$300 per ton for coal here in Connecticut. Still, I want to experiment with it, hence my reason for not wanting just a wood furnace.

I look forward to your replies...Gary
Gary1

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

I would not buy any multi fuel furnace unless it was EPA rated for wood. Its's very difficult to do both because wood wants a lot of room and coal prefers a compact fire box so one or the other suffers. You want vertical fire bricks and a grate that covers the entire bottom. I would investigate the DS Machine offering first.
franco b

Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

Hi Franco,

Thanks for your input. Funny you should mention the DS Machine offerings because although I've been interested in the Meyer Mfg. Co.'s Woodchuck unit for a long time, I just became aware of what's supposed to be a good wood/coal unit from DS called the Kozy-King. Comes in two sizes. I'm planning on calling them Monday morning and will ask them about the things you mentioned also...Gary
Gary1

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

There is also the option to sell your wood and buy coal with the proceeds which may be easy or not to do.

The trouble with wood is it is half gas that is rapidly released on heating it. Unless it is burned it is wasted as smoke. Once the gas is released what's left is charcoal which acts and burns very much like coal. The problem is how to handle that huge release without excess heat going up the chimney even if you are burning the smoke.

Coal on the other hand is only 10 or less percent gas so a large load sufficient to last 12 hours is much less of a problem to deal with.
franco b

Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Question About the Ratio of Flue Size to Smoke Pipe Size

Thanks for the information franco. I'll keep that in mind....Gary
Gary1