Let's design a smoke pipe setup

I have a chimney with terra cotta lining and a round clay thimble (8-3/8" ID) passing through the chimney wall into my furnace room. Out of the furnace, the smoke needs to move 1 foot further from the furnace, rise 4 feet up, and run 3 feet toward the wall into the clay thimble.

The furnace was installed this fall with 6" 24ga black single wall. We used two 90 elbows and a 6-8" jump right before the thimble. I'm not getting enough draft when the fire is low, and after three months I have 1/8" of creosote in the smoke pipe. The crimped ends are pointing the wrong way and that will be changed. I'm willing to replace all of it.

Is it worth jumping right to 8" from the furnace and doing all 8"?

Should I consider double-wall or stainless?

What's the best way to resolve the gap between 8" pipe and the 8-3/8" ID of the thimble?

Just fishing for opinions.

 

Use 45 instead of 90 to get rid of horizontal. how large is the terracotta internally? Can you slip a 6" single wall down it ? if so then lining with a single wall 6" ( course if it is big enough to insulate or double wall would be better) to a T at the current inside opening would solve the disparity , additionally so much easier to clean . Never had much luck with the barometric damper you have there- just cooled things down and created a ton of creosote. I equate those about 2 steps off of the Magic heat reclaimer boxes.

 

The terra cotta pipe is a bit more than 8"inside, probably 8-1/4 based on the fit to the 8" black pipe.

So you're suggesting I go with a 90 out of the furnace and then two 45s to get to the clay pipe? Not sure of the benefit since the clay pipe is about a foot deep and horizontal.


David

 

Stay with 6" as far as you can. Double wall stove pipe is a good idea, it is usually stainless inside so it will last much longer. But if you want single wall stainless pipe/parts, I can get it for about the same price as regular ole cheap steel stovepipe. (plus shipping) Its cheap enough that you could line the chimney with it as chris has suggested. Local Amish shop will make it up for me...

 

Does the manufacturer recommend or require the same size flue as furnace outlet?
You didn't mention flue height or indoor or outdoor chimney. That is needed to calculate BTU required for height and diameter of flue. The chimney creates positive draft (or negative pressure) and the connector pipe, spark screen, elbows, increaser, and firebox resistance (most at intake area opening) subtracts from the negative pressure. A damper is a variable resistance used when the draft is too strong. Using an arbitrary number of 100 for the pressure chimney creates, each foot of vertical pipe, horizontal pipe and all the other fittings are subtracted using tables that give the number to subtract for each restrictive part. This gives total resistance which isn't a guess whether or not you have enough heat loss up the chimney to provide correct draft which controls air flow through fire.

If it is an efficient heater, you won't have enough wasted heat to heat twice the diameter chimney flue all the way up. You must stay above 250* f all the way up to prevent rapid creosote build up which is difficult if this is an efficient heater allowing flue gasses to expand and cool drastically at the increaser. The flue temp drops approx. in half when allowed to expand from 6 to 8 which is almost double the square inch opening area. Multiply that by chimney height for cubic inch area you are trying to keep above 250*. Insulated liner may need 1/4 the heat you currently require giving you that much more usable heat. You need a thermometer on the pipe where it connects to chimney. A surface mount thermometer will read 1/2 the actual inside flue temp. Depending on height, it is then a guess how much it will cool to the top. Once you know how much creosote you produce you can fine tune your stack temp to what it needs to be. This flue temperature to maintain is only when smoke is present.
The damper is a variable resistance to slow draft by reducing velocity. You already have too slow of a draft increasing from 6 to 8. The larger the chimney, the larger the capacity at any given temperature. The larger the chimney, the more heat required to maintain the same draft. You don't need a high capacity chimney, you want a efficient chimney that works with less fuel.
The barometric damper works great for coal where flow through coal bed is much more critical than wood. Since there is no creosote with coal, admitting air into chimney is not a hazard. Burning wood, if the creosote catches fire in pipe or chimney, the damper is going to close, which is flapper OPEN to slow draft allowing oxygen into chimney fire. It should be sealed shut burning wood.

The horizontal flue liner should be cut off with a masonry blade in a circular saw as close to chimney as possible.
Be sure any horizontal run is pitched at least 1/4 inch per foot uphill towards chimney.

 

Yukon is UL certified and requires the baro to maintain the UL cert...that said, I have my baro covered with HD foil most of the time. I also have a manometer mounted permanently on the wall and I always check it when loading, generally, the only time I need to uncover the baro to let it work when it is really cold out or some crazy high winds.

 

As a precaution or to control the draft?

x2

 

The chimney is "indoor" as I understand the term.

Counting bricks on the short side of the exposed chimney suggests about 4-1/2 feet flue height.

I considered doing a pair of 45 elbows and no vertical or horizontal sections, but I believe at least one angle needs to be greater than 45. Three cleverly placed 45s might do it though...

Interesting points about keeping to 6". I hadn't thought about the benefits of higher exhaust velocity but it makes sense.

Not sure about dropping the baro damper, as Yukon requires it and we had a windy night where draft was excessive and the unit got hotter than necessary.

 

To control high draft.
Yukon makes quite the deal about not letting the draft get over -0.03" WC...and from experience (I had the same model furnace before) high draft will make these things go nuclear pretty quick!

Exactly! The perfect storm for lighting off a chimney fire with the already "pre-loaded" chimney he has there! (scary stuff, especially with an "internal" chimney!)

 

The current setup has two 90's. I took that to mean maybe I could replace the two 90's with two 45s if I ran a straight section between them at an odd angle.

Now i'm sure I could manage 4 45's to do the job of the two 90's. But that wasn't how the original suggestion sounded.

 

The stuff I can see is dry and crusty. I had a few drips of creosote come through a pipe seam early on. I haven't torn it down yet - I don't want to wait long but I want to have the replacement parts on hand so I don't need to make this mess twice.

We talked about lining our whole furnace chimney but our sweeps told us it's in great shape no need to spend the money yet.

I was just reading on another hearthy forum a person with the same oversized terra cotta horizontal flue fitted a length of stainless 8" liner and mortared in between them. This way he wouldn't have to break the mortar to work on it.

 

From the furnace manual, I can use up to an 8" chimney.

It kinda looks like this diagram from the manual except:
1) Mine has a regular 90 off the furnace instead of the tee
2) My chimney is indoor

Should I add the cleanout tee? Seems like it would be helpful.

 

It will get your stove pipe started male end down too...

 

I can see no reason why not to it makes cleaning soo much essier... with set up off back of stove I think space to do 2 45s might be a challenge.

 

You may get away with it...hafta fix the rest of the issues first and then see where your at.
Just FYI, round 8" chimney (liner) will flow better than square (which it looks like you have) and round has about 25% less volume too.(and 6" round is almost half the volume of 8" round)
From what I have been told by professional chimney sweeps/repair guys, VERY few chimneys are built right as far as CTC (clearance to combustibles) is concerned (and how would you ever verify it anyways?) ...myself I would sure sleep a lot better at night having a insulated (if possible) SS liner in that chimney...6" will be cheaper and provide the best draft too.

 

Coaly-very informative post and I agree with what you posted 100% but from my real world experiences I have found that it may not pan out like that. I have run a couple of stoves on 2 different chimneys and removed two 90's and put the correct size chimney for the stove and it basically worked the same. The height of the chimney makes a bigger difference IMHO.
Unless you really need it good single wall pipe will last many many years with the correct burning habits, I am using some that is 30 years old and looks like new.