Dialing in my new (to me) Grampa Bear

[johnstra]

Hello Fisher fans. I've installed a rear-vent Grampa Bear in my shop and I'd like some guidance getting my burns dialed in. I have a straight up 20' 8" chimney (9' of single-wall and 11' of double-wall class A).

I installed a baffle last night.

My biggest concern right now is flue temperature. I loaded the stove last night with 7 splits on top of a bed of coals. I have a probe thermometer in the flue and it only topped out at 580* and it dropped down to about 300* within 30-45 minutes. The lower stove top was 550* and stayed there for a couple of hours. That seems like a low stack temperature to me - I'm measuring about 18" up. I do have a key damper in the pipe but I left it wide open.

I did not measure my stack temperature before installing the baffle.

I'm thinking the gap between my baffle and the stove top might be a little too tight. Do those stack temps seem ok, or should I widen the gap a bit - it's about 2" right now.

The other thing is smoke. After the fire settled in and my stove-top temp was sitting at 550*, I had lots of smoke from the chimney. After about 2 hours, it cleaned up dramatically with almost no visible smoke. I have my draft vents open about 1/2 turn. What can I do to clean up the burn earlier in the cycle?

thanks,
-john

 

[coaly]

More air - are you running about a full turn until up to temp? Smaller splits on the bottom and load in all directions to get air through the splits?

8 feet of single wall pipe is technically the maximum on top vent stoves, (I have to look up where I saw that -their heat rating is also based on 8 foot ceiling) so you have a lot of heat extraction going on.
Since double wall pipe inside is required on rear vented models that are intended for through the wall installation, I'd use double wall over 8 feet of single wall pipe as well. (Page 7; A-2 of the III Manual) Page 7; (E) Adapter required on rear vent model Dura-Vent Part No. 8872 is required to connect double wall pipe to flue outlet on stove. The first manual doesn't address this type installation. This will raise your flue temp and the stronger draft will bring you up to temp quicker.

Dropping the baffle front a bit will allow more heat up possibly getting you closer to 350*+ stack temp. Decreasing efficiency until you get it set up right, or stay with the inefficient connector pipe set up without baffle.
Plus you have an elbow or T off the back, so it's not straight up like a top vent. Is it a solid elbow that can't leak any air in? Make sure there's no gaps in the pipe joints too. 3 screws minimum at each joint don't always seal up the joints. Don't be afraid to add more short 1/2" screws if there are gaps in the pipe joints. Every little bit may help!

 

[johnstra]

Thanks Coaly.

Yes, a little more an a full turn to get started. I did pack the wood pretty tight last night when loading, so I'll change that and make sure more air can move through the wood.

I'll look into going with double-walled pipe down the road The T I'm using is old and beat up. I used stove cement to seal it up as much as I can. I'll replace this with an elbow asap. Other than that, the joints are tight.

 

[johnstra]

Here's how my last burn progressed:

Hour Flue Lower Upper
1 486 635 435
2 462 725 500
3 313 550 400
4 257 450 300
5 252 425 300
open draft caps a smidge
6 282 450 300
7 248 375 275

How does this look? This was a full load of oak. After the 7 hours, there is a healthy and very hot coal bed, so the stove will hold heat for several more hours. I'm convinced I could let it sit for 4-5 hours and be able to reload it.

I feel pretty good about this performance. The key damper was closed about 3/4 and the draft caps were 1/2 turn open.

This is with the baffle in.

-john

 

[johnstra]

Coaly? Anybody?

 

[coaly]

Good, just make sure after 4 hours you have little to no smoke.
It's difficult to compare burns with outdoor temperature and pressures changing, so trying the same thing with damper 1/2 closed on another day may or may not show differences.

I think we could have a good stove nerd conversation !
Instead of opening air intakes, it may be more beneficial opening the damper more. That does the same thing as giving it more air, but at a higher velocity.
Without getting way technical, higher flue temp, higher air pressure, taller chimney, (to an extent) and LESS Resistance increases draft. The last variable, resistance is the flue shape and size, elbows, reducers, cap, and the variable damper. So that changes the velocity (slowing by adding resistance). The lowest pressure in the flue system is at the stove collar. The faster the flow, the lower the pressure creating a higher pressure differential at the intakes. The higher velocity coming in, increases turbulence created by baffle. Also forcing the burning gas mixture to make abrupt turns as it travels through the stove (the direction of loading) and air space between wood mixes the air better with the combustible gasses. This is what gives a better oxygen mix.
The baffle gives a cleaner burn due to higher temperatures in the combustion region. Temperature is the most important factor for complete combustion. Tests were done with electric heating elements on the stove sides so any temperature could be maintained irrespective of other conditions. Measuring the temperature in combustion region, total combustion air flow, ratio of secondary to primary air, moisture content of fuel, type and size of fuel, and pre-heating of secondary air (this is not secondary air admitted with tubes, only the air admitted above the fire). Of these variables the single most important factor was combustion region temperature. Completeness of combustion was assessed by measurements of carbon monoxide, total hydrocarbons, particulates, and smoke density. As long as the temperature of the combustion zone was 1100* f. or higher nearly complete combustion was achieved. So the baffle impedes outward heat flow affecting the flue temp slightly, not the resistance in the system.

 

[coaly]

Coaly? Anybody?

I had to sleep on those numbers
You didn't expect a simple answer did you?

 

[Mo Par]

I've never measured my Grandma that precisely but those numbers sound close to what I'm seeing. I usually get 10 hours more or less overnight, dampers closed slightly less than half a turn. There's more than enough coals in the morning to fire it up again.

John