Mama Bear flue question

[dentside]

Hello All!

New to the forum, so greetings to everyone. I've poked around on the forums already and have enjoyed reading other's comments and suggestions on keeping these old stoves running at their best.

I'm in the process of building my house. As a "house warming" present, a good friend has given me a near mint condition Mama Bear stove, complete with all new fire bricks AND a custom baffle (per Coaly's instructions).

I am asking the age-old question of how to adapt the 6" O.D. flue of the Mama Bear stove. I've read where some have suggested using an adapter (e.g. Dura Vent #1672) and others have suggested over-crimping the male end to fit inside the outlet.

Since I'd like to install a T fitting (with removable cap), I'm not quite sure how to go about this. I've seen a photo that Coaly posted showing a T that slips OVER the flue outlet. I haven't found a brand that measures the necessary 6 1/8" or so to do this. Maybe Coaly will read this and let me know what brand of T he's using?

Any and all comments welcome!

Cheers,

Roy

 

[coaly]

Welcome to the Forum;

Nothing special, Imperial name brand from Ace Hardware fit mine. The female end of pipe, el or Tee fit over my flue outlet pipe snug. Pipes on my stoves here measure 6 inches OD.

 

[dentside]

Thanks for the prompt response, Coaly...much appreciated!

Did you use anything to attach or seal the T to the flue outlet...high temp cement or some kind of clamp or band?

Cheers,

Roy

 

[coaly]

No, the small amount of leakage into the pipe isn't enough to cool the rising gasses enough to make a difference. If you have a large enough leak, like a crushed piece of pipe at the end or rust hole allowing a lot of air in, (I'd say 1/2 inch or larger) the smoke particles can ignite in the pipe making it glow just above the opening it uses as an oxygen inlet. That's another good reason for a baffle to keep the outlet temperature down and reduce the smoke particles available in the case of a leak like that.
3 screws at each pipe joint are required, so drilling three 1/8 inch holes in the outlet pipe for self tapping screws prevents the connection from moving. At the pipe joints where there is any gap in the joint, I put a screw at the gap first to pull it tight to make a better sealed joint. The only time you should see any leakage come out is if you put cardboard over coals allowing it to preheat and it lights with a poof. Kindling over a coal bed can do that if you let it get hot enough with little draft (draft causes oxygen to get into firebox) If you open door and let oxygen in before it has a flame to ignite gasses, it can light explosively blowing smoke out of every joint. That's operator error. Opening air inlets wide when starting on a pile of coals should ignite before the firebox and pipe can load up with combustible smoke particles.

 

[dentside]

Thank you, Coaly...I appreciate the detail.

I understand that besides your baffle modification you use a flue damper. I would consider installing a damper if I felt there was some benefit, but I don't know how a person decides if their draft needs to be controlled via a damper.

If chimney length is a factor, I can tell you that the chimney will consist of a T on the stove's outlet flue, then about 5 feet of 6 inch single-wall black pipe going into 9 feet of 10" insulated pipe (for an overall chimney length of approximately 14 feet)...all straight shot out the roof, other than the initial 90 at the T.

The chimney punctures the roof close to the ridgeline of the house...about 18 inches to one side, and terminates about 3 feet higher than the ridgeline.

Any thoughts?

 

[coaly]

You can not use a 10 inch diameter chimney flue.
Flue gasses expand when allowed to increase. From 6 to 8 is almost double the area inside and cools by almost 1/2. Expanding to 10 simply will not work.

You would need far more wasted heat than that stove has to give up the chimney. The more you have to send up the chimney, the less it has to heat the area inside. You also can not install a baffle in your stove increasing the diameter that much.

Square inch area of 6 inch flue and your outlet is 28.26. 10 inch flue is 78.5 square inches. You must keep the inner flue temperature above 250*f. to the top to prevent water vapor from condensing which allows smoke particles to stick. (while smoke is present) You will form creosote rapidly and the increase slows the velocity causing more problems. The damper controls velocity of rising gasses. You would need that with the correct 6 inch flue all the way. Opening the air intake will roar up the stack letting you know to slowly close it until the roar stops. That gives you an idea of the correct draft.

 

[dentside]

Pardon me, Coaly...let me try that again!

The insulated pipe sections are 10" O.D. (i.e. 6" flue with 2" insulation). Thus, I will have 6" all the way.

I've operated many wood stoves in the past (including Fishers) that did not have flue dampers. All air control is/was from the intake. If the stack was roaring, then the intake was reduced. Are you saying that (assuming a damper was installed) this kind of air control should be done from a flue damper instead?

Cheers,

Roy

 

[coaly]

Ah, that's better, I was ready to suggest hooking a couple more stoves to that monster chimney (no, don't do that)

No, only when a chimney has too much draft. Basically with no other resistance like elbows or horizontal runs that slows a chimney down.
One of the times overdraft occurs is when you're starting a fire with kindling. It can roar before igniting larger pieces, so then you want to slow it down with the flue damper, since you need more heat in the stove at that point to ignite larger pieces. That is not the time to slow it down by depriving it of oxygen at intakes. Another use is when you're fighting that unruly piece that will only fit through the door one way, it's been in there cooking awhile and things look like it may take some time to get it unstuck...... You close the damper slowly until smoke starts to roll in at top, then open slowly until smoke rises into stove. That gives some control with door open. All double doors require the damper for open door burning with screen in place for that reason. Other than that you may only need to damper it down slightly when extremely cold when you have more draft than necessary. Where you are there may be no such thing as too much heat.

 

[dentside]

One of the times overdraft occurs is when you're starting a fire with kindling. It can roar before igniting larger pieces, so then you want to slow it down with the flue damper, since you need more heat in the stove at that point to ignite larger pieces....Other than that you may only need to damper it down slightly when extremely cold when you have more draft than necessary.

Okay, thanks!

So, the first bit about the flue damper being useful when starting a fire so that the stove's interior temp can rise quicker...that part I understand.

The second bit I'm not quite clear on. Is the flue damper useful when extremely cold outside because of air pressure changes/differences, thus creating an overdraft? Or, is there a concern around very cold outside temperatures only if I have an existing overdraft issue?

It can get down into the minus 30s Celsius around here, and strong winds are not uncommon.

Is an overdraft predictable? Is there an ideal chimney length for the Mama Bear?

I'm not opposed to installing a flue damper if I know it's worth the hassle of removing it every time I clean the chimney. Otherwise, I'd leave it out of my build. Actually, I suppose if I installed it directly above the T, it might not need to be removed. I'm imagining it might just be hot enough in that position to remain relatively creosote free. Just pull the cap off the bottom of the T and clean from above with the flue in place...?

Cheers,

Roy

 

[coaly]

Yes, if extreme temperature differential inside and outside chimney causes excessive draft.

I clean mine with damper installed, cleaning down to damper only. A soot eater (whip) goes around the damper if you can clean from bottom up.

No recommended height given. Only minimum found in codes depending where you are. As you go taller it does create more, but the cooling towards the top and resistance of more length requires more heat to overcome the detrimental factors. For instance, doubling chimney height results in a 40% increase in capacity. 10% height increase increases by 5% capacity. There are many factors such as chimney construction since if heat is conducted out through the chimney walls, the increase is diminished. So insulated chimneys benefit more increasing height. It is all about the net effect which is not very large compared to chimney diameter. A 50 foot chimney is more than 8 times taller than a 6 foot, but it's capacity is only 2 1/2 times greater. Info from Chapter 5 "Chimneys" and tables in The Woodburners Encyclopedia.

 

[dentside]

Thanks again, Coaly...

If I can effectively clean the chimney with the flue damper in place, then I believe I'm convinced enough to install one...even if it's only used once in awhile.

I've never used a "soot eater", just wire brushes from above. I see there are a few brands of these whips online, and from reading a few reviews it seems that some might be more durable than others. Any recommendations?

Cheers,

Roy

 

[coaly]

I've got a soot eater and I really like it. Picture a nylon ball spinning like the head of a weed wacker with extra tough thick line. I close my damper and run it up to it, then open it and push it past without spinning, then spin the rest of the way to the cap. It even knocks any creosote sticking to the screen off without going up on the roof when it's nasty.