Smoke Spillage with my Grandpa Bear..

I just recently put a Grandpa bear in my shop(mentioned in earlier thread below) to replace my homemade stove that is based off of a Papa Bear design, in hopes of getting longer burn times and more heat in my 40'x 40' shop. My flue set up is 6" pipe horizontal through the wall 4' long with it angled up towards the chimney. And then 21' of 6" vertical outside stack, 8' of pipe above the peak of my Quonset hut. I come off of the top of the stove with 3' of 8" stove pipe, 8" elbow into a 6" reducer.

The stove drafts very well and you can hear it roaring. It will easily climb to 500F on the stove pipe (18" above the stove) and is very controllable using the pipe damper and door vents. Which is part of the reason I like this stove is I can control it, my old stove was a free for all which is another story for another time. When I crack open the door and let everything start to flame up I don't get any spillage. It's not until I stoke the coals and move around what burning wood is left and load wood that I get smoke coming out the top of the door opening. All of my stove pipe and chimney is clean and clear. The one thing I question is my Chimney cap. It is just a rain cap with no screen.. But I have it pretty close to the top of the chimney( 3"-4"). Is it possible this is creating enough restriction with the larger volume stove to cause my issue? I know having an external chimney and horizontal run is not ideal but it is what I have to work with. Any input is appreciated..

Thanks,
Fred

 

Possible to remove the cap temporarily? Give that a shot to see how it runs, or is it possible to raise the cap a couple more inches?

 

That is going to be the first thing I do before I fire it up tonight. I'm hoping it helps but I don't believe I will get that lucky. Another option is I have room to move the stove towards the wall. I could remove 12-18" off of the horizontal. I'm not sure if there would be a noticeable gain from doing that.

 

Changed out the cap and noticed a slight improvement. I'm still getting some smoke rolling out the door during first fire up with the stove or on reloading when there's no fire present and just coals. Other than that the stove works great.

 

I’m guessing that stove needs to breath more. In fact, I’d bet on it. A cap too close to the pipe outlet would not likely affect it, except to restrict it enough to help the pipe hold more heat...which should help it draft better.

I had a similar problem from a 7” outlet stove necked down to a 6” from the stove top up only 3 feet then into an 8”x8” flue on an inside chimney. You wouldn’t think having it choked down only a couple of feet would matter, but it’s like it was starved for air even with both spinners all the way open. Took forever to get it going, but ran decent when you finally got it warm with some coals in her belly. What caused me to reach the conclusion is with a belly full of coals she wouldn’t take off unless both spinners were maxed out and the load door cracked. Open the door all the way and she would smoke. With open doors it’s essentially a fireplace...and a well designed fireplace won’t throw smoke into the room.

Smoked like a freight train on startup and when adding wood to coals...when new wood is just starting to burn.

recently burned this stove with the proper pipe matching stove outlet...7”...and it seemed to make a lot of difference.

 

Eric VW the stove is in my shop that has its fair share of drafts. I believe Hoytman hit the nail on the head. Too much volume of stove and not a large enough exhaust. I'm making the stove work for right now I'm learning its quirks and now I'm getting no smoke in the shop. I still want to go to a 8" pipe all the way up and out. I will say that stove puts out heat and has burn times like I've never seen before.

 

I may have to retract that bet, and just say give it a try. Not going to retract my possible solution, but I will add another.

Based on some things I've learned (just had a long discussion with a really nice fellow forum member here...some might say a guru...and I'd call him that too) about these Fisher stoves since this thread was made, I will ask you two questions...then I'm going to defer you to Coaly for help.

1. Does your stove have a baffle in it? Check for a baffle in that stove. If it doesn't have one this may be the fix.
2. Is your stove pipe insulated all the way to the top?
3. Coaly will likely give you better advice, and I am certain more thoroughly explained information.



Try reading this thread: Papa bear help

 

-My stove does have a baffle. I've tried it with and without the baffle and had no change.
-Here is where I may have an issue and I may get crucified. My chimney is 6" well tile 1/4" thick. I have approx. 13ft of well tile with an additional 6' of 6" insulated pipe on top.

I know this is not ideal by any means. But I will say this when the well tile gets hot it stays hot. I ran this set up before with my other stove for 2 years and never had any creosote build up.

 

Are you talking about clay well tile, or steel well casing? Doesn't really matter other than to know. If it gets hot, it's hot. Get your stove going like you generally like it and if you can take an IR gun and get a reading on the inside of your chimney pipe just near the top of it. I'm just curious. Yes...up on the roof at the top...just inside. If the pipe is smoking that will be fine...even better.

Since your stove outlet is 8" and you're reducing it perhaps you should check the baffle opening "square area" as well. From what I understand that area opening needs to match the area of the outlet, if that makes sense. Since it's reduced...rather than the are of the 8" circle it might need to match the area of the 6" reduction.

Another possibility is you said 8" pipe into an 8" elbow and then reducing to 6"...this is not counting your horizontal run. The elbow is a slow down point. Perhaps after the elbow it may need to be straight again for a bit before reducing it to 6". This a guess at best....I am thinking the draft velocity slows down in the elbow and might need some time to pick up speed some before being reduced. It's hard to say. I'm just thinking you might have a pinch point there. I certainly would not increase your chimney size completely...just yet...as I indicated before my discussion with Coaly. Our discussion has changed my perspective some on chimney's...a discussion not related to this thread.

I still wonder...and think...you need a bigger pipe. Try the cheapest alternatives first, obviously.

Pictures of your baffle might be helpful should Coaly stop by on this thread.

 

You have a point I hadn’t thought of, Dave.

Obviously I didn’t realize he was dealing with two doors. I had a similar issue with my wood stove before I removed it. The thought of using only one door never even crossed my mind. Might be a simple solution.

Does this stove have a smoke flap/shield that pivots above the door opening? How about your stove, Dave? This might be a modification that could help...effectively reducing the size of the opening when the doors are open. I like the one door option better really.

 

I know they can. That’s why I want a couple to try out.

 

I found out only opening one door to stoke and load helps a lot. When I have a really nice bed of coals and not a lot of unburnt wood in the stove I can open up both doors and have no issues. I have 2 elbows and I know that is not helping me but it is my only option. If I knew switching to 8" through the wall and up the stack would solve the problem I would spend the money. But that is an expensive shot in the dark!

And Hoytman it is a steel well casing. I checked it at the outside of the top of the casing and had temperatures in the 125-150 range on the outside of the 1/4" thick casing. I did notice with putting the 6' extension of insulated pipe on top of my well casing that it seems to hold more heat in well pipe. I'm really thinking its a whole intake and exhaust volume situation. I might try and swap out some of the 6" horizontal to gain a little more volume after 90.

 

Take a reading on the inside of the casing at the top and see what you get. It would be interesting to see the internal pipe temps at the top of chimney. I say this after a discussion with Coaly, but I won't go into detail for fear of misstating some things I've learned from him.

I may have to back up on saying you need more volume after a discussion with him, so I hope he chimes in here. I think you'll get better input from him than me.

@Coaly!!!!!

 

Sounds more like well casing needs insulation wrap and an outer cover to prevent cooling. That is no more than heavy duty single wall pipe. You don't want outside air cooling it. As Hoytman suggested, you need to check the temperature of flue gasses near the top of chimney flue. With smoke present, 250* is the minimum.
If this well casing is exposed, weather will affect draft more since wind and water cools the single wall considerably. (draft is the measurement of rising gasses. 4 parameters determine this; Temperature differences between inside flue gasses and outside air, chimney height, chimney diameter, and whole system resistance coefficient).

If you have 8 feet above peak of exposed well casing, that is detrimental to draft as well.
Height does increase draft, but not proportionately, since height added allows more cooling as it rises.

If all things remain unchanged, a doubling of chimney height would result in an increase of 41% capacity. A 10% height increase would improve performance by 5%. BUT things change, the higher the chimney, the more flue gasses cool before emerging out the top. Thus the average temperature of flue gasses decrease, which has a negative effect on chimney capacity. The amount of improvement is diminished if heat is conducted out through chimney walls. Insulated chimneys benefit the most from height increases.
Draft tables illustrate the net effect which is the most important. As an example, a 50 foot chimney is more than 8 times taller than a 6, but its capacity is only 2 1/4 times more. This means higher than needed chimneys yield diminishing returns. (Ref; Woodburners Encyclopedia, Jay Shelton Chapter 5; Chimney Fundamentals)

8 inch diameter flue gives you more capacity for higher heat output. It is not required at lower operation temperatures. (with a correctly constructed, insulated chimney) You won't get the full BTU out of an 8 inch stove reduced to 6 at wide open setting, but firebox cubic inch size is ok with 6 providing the net draft created is acceptable. Using 8 inch connector pipe slows velocity through pipe allowing more cooling, so should be avoided. Reduce at stove, using 45* elbows if possible. Any heat leaving stove is waste heat used to heat chimney flue above the required 250*f. Let the stove do the radiation, not the thin walled connector pipe.

The Papa design allows higher velocity through firebox with more oxygen mixing with flammable gasses exiting the fuel. It is a generally better design for log lengths than a square double door stove designed for fire viewing. You lose efficiency with that capability.

Can you use 45* els instead of 90's? A 45* pitch instead of horizontal is a huge difference that maintains velocity.
Lower velocity = less draft. Draft is a measurement of the net amount of rising gasses due to temperature differential, volume (diameter) and velocity.
As stated above; If you are using 6 inch flue, it should be 6 inch inside as well.
Using 8 inch inside slows velocity, allowing more heat to be lost through connector pipe that should be used in the flue. Putting a fan on pipe to extract more heat compounds the heat loss needed in the chimney.

I'm assuming the baffle is factory Grandpa III, or if added, it is not flat horizontal. The smoke space where the exhaust travels through is critical as well as angle. The baffle adds resistance inside the fire box, just like els, tees, connector pipe, a variable resistance damper and screen / cap takes away from draft (negative pressure) generated by chimney. Angle decreases resistance as well as not absorbing more heat. A flatter angle increases flame impingement, absorbing heat from flame tips, where you don't want to quench the flame from burning smoke particles. Angling upwards more increases velocity evacuating firebox easier. ALWAYS open doors slowly, if you have a Grandpa III it will have a double action latch to prevent from opening too fast.

The factory baffle in later models was very minimal to allow plenty of heat into an existing larger flue. They could not be designed for a more efficient chimney and have customers connecting to chimneys requiring much more heat. Your smoke space can be reduced to the flue size, providing it is insulated to stay hot enough inside. If you're making your own baffle for a Series I or II, start with a cardboard template resting on rear brick retainers. Angle upwards toward lower bend, allowing the same square inch area as flue and vent pipe. Set on bricks on sides so it has adjustable height in front for increasing smoke space as required for less efficient chimney.
You should only be using flue damper for open door burning. Control burn rate with intake air dampers. The closer the baffle to optimum, the less flue damper would be needed. The damper is only for an over drafting chimney to slow velocity, reducing draft.

The Grandpa was designed with an 8 inch outlet for 2 reasons. They were a freestanding fireplace capable of using an existing larger fireplace chimney. so they were built with 8 inch to allow more heat to escape to heat the larger chimney. Second was for open door burning with screen in place to prevent smoke roll in. The flue damper is slowly closed until smoke starts to roll in at top, then open slightly to retain as much heat while evacuating smoke.

That said, when a smaller, more efficient flue is used, they normally work fine. You don't have an efficient flue that stays hot enough, larger diameter being the best way to accomplish this. (increasing 1 size having the same effect as increasing chimney height by 50%) Higher velocity in your case is needed to increase net draft.
What you have to consider as I explained to Hotyman, is increasing capacity also increases the need for more heat to be left up. Even though a larger chimney diameter has more capacity, more heat is required to create the same draft measurement. Saying a larger chimney drafts better is incorrect, they draft better using more heat. They won't draft as well using the same amount of heat a 6 inch flue requires.