Dual Fuel Chubby Stove

To those of you who feel I've bastardized perfectly good stoves, I'm sorry I just can't help myself. But, I've just finished my latest and hopefully last (for this season anyway) modification to my Chubby Stove.

My master plan for burning this season is to burn wood on the fringe months when it's warm during the day, but cold at night. And, to start burning coal later in the season when that day time temps average 45 and below. It's nice to have the option of letting the fire go out during the day, which isn't much of an option with Anthracite. To accomplish this I have to have good wood burning capabilities and the chubby isn't well designed for wood, IMO.

So rather than have two stoves on the hearth, I added an upper chamber to my Chubby and some secondary air piping. The chamber serves two purposes. First, it provides added surface area and baffling to act as a heat exchanger for the hot wood flue gases. Second, it houses a SteelCat catalytic combustor to achieve near complete combustion of materials and make full use of the wood burned. There's also a little black pipe running from one of the secondary air holes up to just underneath the combustor. A catalytic thermometer is mounted on the right, just above the combustor level. The outter wall is 1/8" mild steel (thicker than the steel on the Chubby), and the baffle and shelf the combustor is on is made of 3/16" steel to take the heat (probably overkill). A steel flat rolled into a ring (just like the one in the middle covering up the upper/lower divide) is welded to the inside of the catalytic chamber wall and allows the chamber to sit down inside the chubby, while the chamber wall is exactly the width of the chubby and sits on top of it.

I didn't have to weld anything to the Chubby stove. I did have to widen the secondary air hole by 1/16" with a step bit to accomodate the black pipe, but will still function normally if I remove the pipe. Since the upper chamber is the same dimensions as the Chubby body, the top casting just sits like it normally would on top. I added tabs like the Chubby has to bolt the casting to. This was a rear venting chubby, but I covered the rear vent with a 6" stove pipe cap and put bolts in the damper holes after removing the damper. The hole in the top casting now serves as the flue vent and a piece of 6" stove pipe fits snuggly through the hole down a couple inches into the stove. A steel ring (not yet painted) sits around the stove pipe in that little lip in teh casting and pinches some 3/8" gasket to the stove pipe. It's not really needed, but I 'm paranoid about air tightness.

The couple test burns I've had so far have been successul, achieving 1000 degree output on the catalyst and no smoke from my chimney.

Now for the controversial part: Bio Bricks are the perfect wood fuel for the Chubby Stove. I'm talking about the small ones, Bio Bricks, Hot Bricks, etc. The other compressed fuels like Envilogs and Eco Logs, etc are too big for the firepot. But the small bricks pack/stack in the firepot nicely, are so so dense that they burn for hours, and have almost no creosote. From a cost perspective, $259 for a pallet of bio bricks is equivilent to 1 cord of wood (though I think it's actually more than a cord in BTU and efficiency). Seasoned cordwood in my area is $200/cord give or take. Sometimes you find cheaper, but it's usually pretty green when you see it in person. So while it's more expensive than cordwood, for me, it beats splitting and stacking and of course chopping down to fit the Chubby's small firebox.

Love'em or hate'em, brio bricks kick butt in a Chubby Stove. And, with the SteelCat installed, I can turn the stove down low and get overnight burn times with the cat cranking out 1000 degrees temps. The automatic draft damper I installed helps a lot with that, though with wood I've had to cover up half the opening inside the draft box with a steel plate to get the wood fire down to a low/slow burn. I don't need much intake with wood compared to coal it seems.

Here are the outter pics. I will take some pictures of the inside tonight.

I like it and it looks good too.

WOW, bravo for the .
Waiting for the next photos. Thanks for sharing.

Here is the inside. You can see the bypass on the left, and the Cat on the right. Also, the steel ring (it's springy) pressing the gasket to the wall.

If you look through the bypass, you can see the lower baffle that covers 75% of the area of the 18" cylinder.

Very neat as well as inventive.

..no mailbox post made of welded chain here! I like your creativity!

Props for taking the time to mess around like that...maybe larry will send the unit in for epa certification Its a very large secondary chamber (you can prolly half its height if you wanted).

I too am impressed with your creativity:) The proof is in the pudding as they've said for eon's--burn season, she's a comin & I'll be anxiously waiting results.

So after tinkering a little further, I decided it best to contact Condar and try and talk to someone to get some basic information that I can't seem to find on the internet. What should the distance be between the flame and the combustor? Should there be a baffle? Does it need secondary air? How tight does the bypass damper need to close? What is a normal operating temperature to know if the Cat is engaged and functioning. Is 800 degrees too low? Etc.

So I called Condar and spoke to someone in product information. Their website bosts this small company feel with "excellent customer service." What a hoax! Basically the guy wouldn't answer any questions because I wasn't using a catalytic stove designed for their combustor. They guy wouldn't even tell me what the normal operating temperatures were until I challenged that it was basic information they should know about their product. They were not very helpful AT ALL and acting like that definitely does not help them grow their market. If I were the manufacturer I'd be encouraging people to come up with new uses for my product, to expand my revenue streams in any way possible. The guy literally said that "you're on your own".

Customer service my a*s!

So what do y'all think? Does it matter where the catalyst is relative to the flame, as long as it's not in it? Do you think the bypass needs to close air tight to force the smoke through the combustor? What is a normal operating temperature for a combustor, is 800-1000 too low? Maybe someone who's had a catalytic stove will have some experience. Maybe these are questions better suited for Hearth.com.

I got confused with the preoccupation with the Cat. and it's temperatures.

The main question I have is have you changed the stove from a single fuel to a different single fuel? Or can you use the stove on wood and then go to coal and if so with what modifications? Since you have modified it to burn wood which requires over the fire air just like Bit coal, can you bypass the Cat and burn Bit coal in it?

You may have hit on something here if it really is dual or triple fuel with minimal work for changeover.

I like it. Good luck with it. Nothing wrong to be inventive and experiment.

needs to reach 1000 degree's, yes that bypass damper should close tight upon reaching 1000, preheated secondary air into the chamber would be nice.

When you call these companies next time explain you are developing a dual fuel stove and need one of their cats shipped out to you (you would like maintain less than 3" thickness)... then... after that start getting into your subjects of how to design for their cat! ....your going to be buying them by the pallet!

If I remember how my V.C. worked with a cat:
No flame are supposed to enter the cat (supposed but almost impossible to realise in real life)
There is a baffle to help for diverting flames away of the cat inlet
They all had secondary air in the secondary burn chamber, BTW, the secondary burn chamber is supposed to be insulated to help for a complete gasses combustion
The by-pass must be air tight / yes with rope gasket
On the stove's top you place a T* thermostat, usually you engage the cat around 450*F and burn the stove at a steady 500/600*F. V.C. said burning at over 750*F for long periods could damage the stove.

thank god you got rid of that VC that couldn't take 750 degrees inside the burn chamber without busting at the seams

dcrane wrote:thank god you got rid of that VC that couldn't take 750 degrees inside the burn chamber without busting at the seams

To be honest with you or VC, the 750*F is on the top of the stove, on the center of the griddle, not in the secondary burn chamber where the temp. would be much more than 750*F.
Doug, just remember one thing about V.C. : they have the best ash pan system in the world . And a few other good points...and a FEW bad ones
Not the best stoves, specially the new ones having the Ever Burn systems (I got one of them... ) or the even newer ones having 2 systems in the same stove: the Ever Burn system and the Cat.system in the same stove....