Adding secondary burn to Fisher Grandpa Bear

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

jrtatonka

Member
Apr 5, 2014
6
PA
I’ve looked through a lot of the threads (that aren’t dead) about secondary burn modifications.

I was thinking about using 3/4“ stainless pipe for the cross tubes, and 1-1/4” for the feeder tubes that go out the back of the stove.

1. is there a consensus on the size of the holes to drill into the pipes?
2. Should the holes be drilled 180 degrees from each other, or angled down at around 120 degrees from each other?
3. would 3 or 4, 3/4” cross tubes be preferable, or would 5 or 6, 1/2” cross tubes be better?
4. How much of a gap is needed between a baffle plate and the front lower part of the stove top / how far front should the baffle extend?
5. I was thinking that I could connect the feeder tubes together on the back side of the stove, and have them share a gate valve to meter the amount of secondary air, and also have a ball valve to shut it down completely, or is there a device/devices that would automatically control the secondary air?
 

ABMax24

Minister of Fire
Sep 18, 2019
1,480
Grande Prairie, Alberta, Canada
It seems like there is a bunch of ways to do it, what's best is hard to quantify.

To you plan on installing a full baffle into the stove? Seems like that should be included in the process as well.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
Go to any store selling stoves and measure holes, distance between them, and number of holes for different models. Give it the same amount for firebox size. (Space between holes the same with longer tubes) Keep the number of holes consistent with cubic inch of firebox size. I think 3/4 feed with 1/2 tubes is fine depending on where the feed gets preheated. Same number of tubes the Certified stoves use for the firebox size. (Space between tubes the same, adding a tube for larger firebox if necessary) I think it’s best to preheat down through exhaust. That doesn’t require any drilling or holes through the stove. Clamp to baffle plate.

I felt like the same people measuring the Fisher Stoves when they were first built to go home and make my own, No problem if you’re not duplicating to sell. Even those that copied to sell got away with it. Just make a spec sheet for each stove you measure with cubic inch and width and depth of firebox, baffle size, number of tubes, space between holes, hole size, and number of holes, and enlarge accordingly.

Schedule 40 stainless should last as well.

Are you using an insulated 6 inch flue? The Lopi Liberty is as close to a Grandpa as I’ve found. Looks like they even copied the earliest ones with 3 piece tops. Very nostalgic of them.
 

Jason721

Member
Nov 4, 2017
95
southern indiana
I just recently got intrested in adding secondary burn tubes in my seria wood stove I use to heat my work shop. There are some YouTube videos where some guys have done it.
 

jrtatonka

Member
Apr 5, 2014
6
PA
got the stove from my aunt. Had a Blaze king Princess, went through 3 combustors before getting rid of it, had to have it going full tilt the entire heating season, and it still wouldn’t keep the house warm (stone farm house) I now heat my house with a anthracite boiler And am never cold. The Fisher is going to be for backup heat.

Yes I plan on installing a full baffle.

that’s and interesting ideas to bring the secondary air pipe down the exhaust
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
got the stove from my aunt. Had a Blaze king Princess, went through 3 combustors before getting rid of it, had to have it going full tilt the entire heating season, and it still wouldn’t keep the house warm (stone farm house) I now heat my house with a anthracite boiler And am never cold. The Fisher is going to be for backup heat.

Yes I plan on installing a full baffle.

that’s and interesting ideas to bring the secondary air pipe down the exhaust
Yes, a hole through the pipe with an elbow pointing down pipes air into stove preheating in the exhaust.
I made a baffle plate with the intake holes drilled through it. Then welded 1/2 X 1/2 stock around the edge and another plate on top. Hole saw the upper plate for a pipe flange. That makes a hollow baffle with 3/4 pipe flange coming in on top. Angled in stove allows a 45* elbow up the stack. Between the exhaust heating intake pipe and baffle heating air, it ignites easily. No tubes, and you have to make a baffle anyway. Nothing drilled or welded to stove. I got the baffle idea from a visit to DS Machine. I only had one intake pipe. Then realized, had I made it with two, I could have added a faucet on each side of the flue pipe to look like hot and cold running water faucets on each side. ;lol
 

jrtatonka

Member
Apr 5, 2014
6
PA
Coaly, i’m starting to like your double baffle plate idea?
did you use 1/4“ or 5/16” for your double baffle plate?
what size holes did you drill in the bottom plate?
do you think a single 3/4” pipe will provide enough air volume for the secondary burn?
how high up did you drill the hole in the stove pipe for the 3/4” secondary air pipe?
how has it been working out for you?
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
Searching for my notes of how many holes and their sizes in the stoves I measured at the time. I know I over thought it trying to duplicate the intake area for the cubic square inch size o sth fireboxes I was measuring.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
Just mild steel A-36. I use 5/16 thick for all my baffles. 1/4 inch warps in the double door width stoves.

Electric conduit / clamp nuts. Or you can use one nut inside, and any female fitting such as a female coupling outside.

I may have posted the size holes I drilled in older threads. I went to the big box stores and measured the holes in what was on the floor at the time. I think they were 3/16? Take a couple drill bits in and do a test fit.

Post #39 of this thread has another link back to a post in 2015 about it.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
Coaly, i’m starting to like your double baffle plate idea?
did you use 1/4“ or 5/16” for your double baffle plate?
what size holes did you drill in the bottom plate?
do you think a single 3/4” pipe will provide enough air volume for the secondary burn?
how high up did you drill the hole in the stove pipe for the 3/4” secondary air pipe?
how has it been working out for you?
Found my notes;
Goldilocks and Grandma size stoves; 3 tubes or 3 rows of holes;
Rear 12
Center 20
Front 24
56 total

95,000 BTU max stoves;
3 tubes with 28 holes each.
84 total

ALL holes are 3/16.
 

jrtatonka

Member
Apr 5, 2014
6
PA
Thanks

ive got another question
i was thinking that instead of putting the baffle on the same plane as the lower top of the stove, that I would drop the baffle down a few inches and have the baffle extend forward a few inches of where the slope starts to the top/outlet. That way the heat/smoke has to go up and around the edge of the baffle. I’m hoping that this would allow the front top of the stove to heat up enough to boiler water/cook on. If that rambling mess made any sense?
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,534
NE PA
Yes, it can come up quite close to the door.