Grandma Bear retrofit with secondary air

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mark cline

Minister of Fire
Dec 20, 2012
Cattaraugus, NY
I have a Granma Bear , that I had since new , about 1980 . I want to add a secondary air system to clean up the burn and reduce the creosote build up . Has anybody done this ? I had planned to experiment with 3/4" pipe , about 4 rows and a baffle resting on top of the secondary tubes. Top vent stove .Any help or advice is appreciated . Thanks
Definitely want to see how this turns out.

My napoleon only has 1 flat tube up the middle for secondary burn and a fiber baffle covering all but the front inch or so. how do you plan on supplying air to the tunes and monitoring it?
I guess, I was looking at copying the layout similar to my Mansfield. Sort of like a ladder made from 3/4" pipe and elbows with the intake air heated from the outside back of the stove. Then setting a ceramiuc baffle board on top of the tubes.The firebox is big so I would not be loosing much interior space. Grandma Bear retrofit with secondary air
Well, i didn't get the secondary system built that I had planned , but did come up with a different design. I had one 3/4" pipe that I drilled holes in for the sec air . It pulls in hot air from the back side of the stove . I have the baffle installed with about a 2" gap in the front, then I put a baffle on top of the sec pipe so all the smoke needs to pass over the pipe and fed with heated air . Grandma Bear retrofit with secondary air
I test fired it this past week , new paint needed to be cured and 64F outside. I installed 36' of double wall 6" pipe from the stove in the basement to the roof. The draft was phenomenal even with 64F outside. It held 600F for a little more than an hr., you could here the air rush in the sec inlet with a roar. Now I need some cold weather and see how it really performs. I'll up date in a week or so.
Grandma Bear retrofit with secondary air


  • Grandma Bear retrofit with secondary air
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If you put a shield on the stove back like a Grandma III you will have hotter intake air. Shield the bottom with opening in front, under ash fender, and up the back, open at top. (shown in VI manual) Your intake won't mix as much cooler room air.
Another way I've toyed with is a double baffle plate, (1/2 inch apart) hollow inside with air holes drilled in rows through the bottom plate. The entire baffle is a preheat chamber. (the hottest part of the stove) Pipe flanges on the top plate with 3/4 pipe going up the stack about 2 feet and out the side of pipe preheats it very well without drilling holes or modifying the stove itself. (the more intake pipes, the more surface heating area, and the slower the flow and longer duration time heating air in pipes. You can also add a twisted steel spiral inside the intake pipe to rotate air against inner pipe walls and increase "dwell time" to increase air temperature - corkscrew fashion)

I didn't comment last year when you started this thread since I never operated a factory built secondary stove to compare my results. I thought mine was poor until the experience I had last winter with a neighbors new Lopi.
Yes, it burned cleaner and less wood. With less heat over an 8 hour duration. For me that = cold house. As my only heat source, I don't have time to babysit it to keep the secondaries burning. I need to load and have 8 hour cycles hot enough to keep the house between 68 and 72. Could be I didn't play with it enough, but as my only heat, and being away 4 to 6 hours, I'd rather burn more wood and come home to a warm house. Plus it makes it more of a space heater with no cooking ability. (the stove was a Mama Bear in the middle of kitchen and convenient to save propane from using range)

Since then we went to the large Kitchen Queen with so much mass and water heating capability we have a very steady house temp. Plus it adds an oven always ready to go. This time of year we wait for the inside temp to drop to about 66 before starting it for night burns. We wake up to about 70* and my 24 hour recording thermometer (in bedroom away from stove) shows a high of 72 overnight. As it gets colder and we burn 24/7 we can keep it within a couple degrees night and day. (I did add a thermostat to the firebox) Adding secondaries to what I've got would certainly ruin the cooktop ability and cause us to use premium wood - take more frequent loading, and end up with more temperature spikes. I guess I'm not a secondary burn kind of guy.

At my second house, my neighbor bought a new Lopi last year much larger than the log home requires. He claims he has the stove figured out, but is adding some electric heat this year. They were cold when it was frigid last winter. I checked it out, secondaries light easily for a couple hours, and it cools with coals until there is room for reloading. Same cycle I experienced with the retrofit Fisher. So I don't think it was me. This is across the street from my log cabin identical to his brought from Finland in kit form in 1972. I put in the same Mama Bear I fitted with the same type Smoke Shelf Baffle as supplied with the later double door stoves (with half the chimney as his) and I don't have electric there for any back up. The reason I didn't go with a Baby Bear for the less than 1000 s.f. area is mine isn't lived in, so it's cold when I get there. Plus it gives me a larger cook top than Baby since it's also my only cook stove and water heater there. I have no use for secondary burn tubes there either.
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I had a heat shield on the back when the stove was in my other house ,but took it off because its not needed with all concrete around it now. I can put it back on , no problem. I agree that it would increase the air temp in the secondary tubes. So there is still some room to experiment and any additional heat will not go to waste. The paint is still giving off some odor but not too offensive.
I put together a new secondary air manifold for the Grandma bear . I cobbled it together from odds and ends laying around . The baffle will sit on top ,
so the tubes will be between the baffle and coals, like on my Mansfield.
After a few days with temps around 40F at night , it is performing like my Mansfield. Hot clean fires and little smoke at start up . After the secondaries kick in and the damper and draft are closed down , the secondary tubes roar as they pull in the heated air from behind the heat shield.The primary air is regulated by a Sotz auto draft that I have had on the stove since the 80's. This draft control has a bimetal spring that opens and closes the draft according to the door temperature. This in combination with a 32' double wall 6" flue straight up through the house , gives phenomenal draft . Colder temps coming will give the retrofit the real test.
Grandma Bear retrofit with secondary air
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Good, it will certainly work better with colder outdoor temps.
The amazing feat is that you made that manifold with only 1 union !
I tried to put another union in but ,I didn't have another close nipple . So I screwed the nipple in then backed it out into the elbow . Only 2 or 3 threads on each end but its solid. I may shorten up the 2 in feed pipes to lift it up higher in the fire box. But it works amazingly good ,little to no smoke and inside the fire box, its just a light brown dust .
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The amazing feat is that you made that manifold with only 1 union !
;lol No kidding!
That would have cost me at least $100 to buy all the pieces/parts to make that work!
I added proper secondary air (their factory system doesn't work well IMO) to my Yukon furnace, the firebox soot went from brown/black to white/tan. It is more finicky now though...:(
Today I modified my secondary air manifold and repositioned the baffle . I had to move the manifold up to increase the fire box . I could only get 5 splits in at one time .I have a 2 piece baffle so I can slide it back if I need more smoke path. My flue is 32 ft so Its draws like crazy. It was 68F today so after I made the changes I couldn't fire it up. I won't be back to the cabin until the second week of Jan . so hopefully it will be colder than today lol.
I have been burning my Grandma Bear retrofit for 2 months now and can't be happier with its performance. The chimney is clean with just a little dust inside . The cap has no creosote sticking to it and is only slightly brown in color. With a full load of mixed maple, ash, cherry or beech , it will cruise at 700 -750F for 4 hrs before it starts to cool down. Maybe some slight modifications come spring time but for now , I'm loving it.
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Any chance this concept could be applied to an All Nighter by drilling holes in the air tubes it already has and placing a baffle on top/above the tubes? Please excuse my question in the Fisher area, but it seems most things that apply to the Fisher's apply to the All Nighters since the All Nighter is a Fisher copy.
I would think the same principles could be incorporated in to any stove . By providing secondary air and a baffle should help to improve any stove. Understanding how secondary air makes a stove more efficient and a bit of tweaking as you go along , you should see favorable results.
Any chance this concept could be applied to an All Nighter by drilling holes in the air tubes it already has and placing a baffle on top/above the tubes? Please excuse my question in the Fisher area, but it seems most things that apply to the Fisher's apply to the All Nighters since the All Nighter is a Fisher copy.
The air tubes would have to be plugged at top and you would not be able to use them for their intended purpose of convection heat with a blower.
Yesterday ,I cleaned the chimney for my Grandma bear retrofit.I only burned about 1 and a 1/2 cord for the winter, but was really surprised at the results.The cap had a little bit of crusty creosote but nothing to be concerned about. The pipe from top down , only had a little granular dust for about 2 ft. but was clean from there down.Overall only collected about a half cup of dust in 32 ft of chimney. So it worked like a charm. I'm thinking of tweaking the secondary to get it up higher in the fire box , for a bit more room.
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Really looking to do this to my stove. Was thinking of a very similar setup with my tubes. Was considering buying weld in socket style fitting instead of threaded. Haven't priced anything yet just a thought. I have no way to thread pipe so I was thinking about the socket style fittings to allow me to cut and size how I want on the go. Without relying on measuring then hunting for nipples. Are you happy with the 3/4"? I was considering going 1" along the side of the stove horizontally , 90 up running up to the top of the back of the stove (angled) , then 90 into the stove reducing to 3/4" as it enters the back of the stove.
I had 2 , 3/4" inlets that supplied enough air . If I were to do it again , I would go with 1" square tube into 3/4" round stainless pipe , for the secondary tubes.This first retrofit was made with what I had on hand.I would not go out and buy all the pipe to make this. Too expensive.
I m thinking about making this new secondary system, if I can find the time .If not, I'll use this setup one more year.
1 " on the back of the stove into 3/4" would work great . my heat shield kept a lot of the hot air at the intakes.
That's a good thought. I think I have some 1X 1 1/2 tubing. I'd like to use stainless but if regular steel will work for a few years I might be able to get by not buying to much. My biggest problem is my door being so stinking small. I'm going to have to assemble a good portion of it inside the stove I'm afraid. Not looking forward to that. Thanks for the thoughts.
My Mansfield has secondary tubes that slide in to the side pieces , so it can be assembled inside the stove .
If I have time (a big IF) this is the way I'm going to redo the secondaries .Along with square tubing down the back like you suggested. If I can make the tubes out of mild steel ( simple and cheap), I'll make up extra tubes to replace as they wear out.
I really like that. Seems like a pretty simple and effective design. That link definitely gives me some visual aid. I'm hoping to get time to do something like that before winter
This past week I made up my new secondary air manifold for my Fisher Grandma Bear. I used 1"x1" tube steel cut 24" across the back and 10" along the 2 sides , the tubes, I used 3/4" black pipe and 3/4" black pipe for the inlet like the old pipe manifold. The plate baffle on top of the manifold is 1/4" plate steel with a gap of 1 1/2" along the front , the baffle ends right where the stove starts to step up. This gap gives me 37.5 sq in. of opening , with a 32 ft 6" stainless flue I get fantastic draw. I have a plate damper in the pipe 2 ft above the stove which I close when the stove comes up to temp. With the stove cruising at 600F to 750F, depending on the type and amount of wood , all I'm seeing is steam a a small amount of smoke after an hour when up to temp. The usable fire box is measured at 24" wide x 16" deep x 14" high to bottom baffle plate which give me a volume of 3.11 cu. ft.
When I was planning this build, I made the manifold out of 1x1 wood so that I could adjust the size and put this into the stove as a finished unit . There is no way that I could build and weld this up inside the stove. After trial and error I came up with these dimensions, knowing that the finished manifold could be put into place. It took a good amount of jockeying to get the manifold in place but with the right combination of moves it fit perfectly.
Grandma Bear retrofit with secondary air
Here is the manifold after all the parts are welded together. The front tube was an after thought when I realized that I could put in 4 tubes instead of 3 . I didn't have enough 3/4" tube for the full width so I salvaged a piece from the old pipe manifold.
Grandma Bear retrofit with secondary air Grandma Bear retrofit with secondary air Grandma Bear retrofit with secondary air
The next pictures show the manifold installed and setting on top of the fire brick . I put in 2 bricks across the back mainly because they fit but not needed. The next picture shows the gap above the baffle plate about 1 1/2" where the step starts up to the upper platform towards the back of the stove. I'll update after a few months , but after 2 days at 32F outside at night , this is working as good or better than can be expected .
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Now that the temps are much lower outside , my Fisher has had a chance to stretch its legs. With the new secondary manifold , 5 splits and its up to temp in about 30 min, after an hour, its cruising at 500-600 . Quick check of the chimney and I see no smoke what so ever . Its working beyond my expectations. I wish it burned like this when I got it in 1980.
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I wish it burned like this when I got it in 1980.

Had you invented secondary burn technology in 1980 it would say Cline on the door. ;lol