New no-cost convection oven on top of steel, air-cooled RMH-style stove saves on electricity

Here's a picture of my new convection oven which didn't cost a penny! Used the body of an old BBQ, cut the bottom out of it and fitted it on top of the bell of my 7" steel-cored RMH where the air-cooling system delivers a steady flow of air at 300F. I figured I may as well use that heat for cooking/baking before it heats the house. So now we don't have to use the electric oven in the kitchen nearly so much!  Heated supper in it last night and hope to bake some cookies in it today

See the thread "Building a 7" J tube steel, air-cooled RMH in a steel bell" for complete details of the construction process.  BTW, the bell is covered in cob rather than sand in a steel enclosure as I'd originally planned.

Also, for those not yet familiar with the design of an air-cooled steel RMH, the air that flows through this convection oven is fresh air from the room drawn near floor level and exhausted near the ceiling level and NOT smoke/exhaust from the combustion!

The first trial run of the convection oven went well the other night. My wife mixed up a batch of raisin oatmeal cookies (my favourite) and I baked them on a cookie sheet in the oven shown above. The first batch baked rather slowly as the rocket was just heating up and the convection temps were in the 250-300F range. But the second tray baked much quicker when the temps were 325-350.

The cookies were great!

The next trial will be a pie.

Now I know that if the power is off we can cook, bake and boil on the rocket without electricity!

I've read your entire thread. Love your ideas. One problem. I guess I'm not imaginative enough to understand your design. I wonder if you would mind posting a scematic of some sort? I have a similar space problem and would love to build a RMH. I think your design might just fit the bill. Thanks for putting in the time and energy, beyond the build itself, to share this.

[quote=Matthew Taimuty]I've read your entire thread. Love your ideas. One problem. I guess I'm not imaginative enough to understand your design. I wonder if you would mind posting a scematic of some sort? I have a similar space problem and would love to build a RMH. I think your design might just fit the bill. Thanks for putting in the time and energy, beyond the build itself, to share this.
[/quote]

Hi Matt,  Attached are a few diagrams which I hope will help. Please note that these were my preliminary drawings and I have changed the design somewhat as I have gone along.
The first is a diagram showing the relation of the core itself to the air-cooling jacket.  The second is a layout of the air-jacket itself before bending. Please note the following changes which I have actually made from this design:
(1) My air jacket dimensions  are now just over 11" square rather than 9".
(2) I have used 2" flat bar for the cooling fins rather than 1" square tubing.
(3) I allowed only 1" of space between the bottom of the core and the bottom of the cooling jacket and thus left 3" between the top of the burn tube and the top of the cooling jacket.
(4) My heat riser is 7" square rather than round.
(5) My exhaust from the cooling jacket is 7" round rather than the 6" shown.

The 3rd diagram is a cross section of my original design which was a mass of brick filled with sand. That has been made of cob instead and the air cooling exhaust is 7" instead of 6 as the diagram shows. I also made my feed tube perpendicular to the burn tube rather than angled slightly forward.

I hope this helps to visualize the concept better. If not, feel free to ask whatever questions come to your mind.

Hope this sort of design may be helpful to your situation. But the 2 major rules you must keep in mind if you plan to build a steel core are these: (1) Never insulate any part of a steel core, rather air-cool it! (2) Make sure that no part of your steel core exceeds 900F where it will begin to glow and corrode with the excessive heat.  Your flame path temps will exceed this by far but you must keep your steel below 900F.

[quote=Bruce Woodford][quote=Matthew Taimuty]I've read your entire thread. Love your ideas. One problem. I guess I'm not imaginative enough to understand your design. I wonder if you would mind posting a scematic of some sort? I have a similar space problem and would love to build a RMH. I think your design might just fit the bill. Thanks for putting in the time and energy, beyond the build itself, to share this.
[/quote]

Hi Matt,  Attached are a few diagrams which I hope will help. Please note that these were my preliminary drawings and I have changed the design somewhat as I have gone along.
The first is a diagram showing the relation of the core itself to the air-cooling jacket.  The second is a layout of the air-jacket itself before bending. Please note the following changes which I have actually made from this design:
(1) My air jacket dimensions  are now just over 11" square rather than 9".
(2) I have used 2" flat bar for the cooling fins rather than 1" square tubing.
(3) I allowed only 1" of space between the bottom of the core and the bottom of the cooling jacket and thus left 3" between the top of the burn tube and the top of the cooling jacket.
(4) My heat riser is 7" square rather than round.
(5) My exhaust from the cooling jacket is 7" round rather than the 6" shown.

The 3rd diagram is a cross section of my original design which was a mass of brick filled with sand. That has been made of cob instead and the air cooling exhaust is 7" instead of 6 as the diagram shows. I also made my feed tube perpendicular to the burn tube rather than angled slightly forward.

I hope this helps to visualize the concept better. If not, feel free to ask whatever questions come to your mind.

Hope this sort of design may be helpful to your situation. But the 2 major rules you must keep in mind if you plan to build a steel core are these: (1) Never insulate any part of a steel core, rather air-cool it! (2) Make sure that no part of your steel core exceeds 900F where it will begin to glow and corrode with the excessive heat.  Your flame path temps will exceed this by far but you must keep your steel below 900F.[/quote]

Thanks, NOW I understand. I thought this is what you were doing but now I know. It will, in fact, work really well for my situation. I just had an inspiration; a built in baking oven in the fresh air exhaust and a hot plate on top. You have really gotten my creative juices flowing. Thank you!!

Matthew, you would do well to remember that a steel core is inherently less efficient than an insulated masonry core; steel can only make sense if you have a source of cheap steel and full metalworking tools and skills. For most people, brick or refractory cement will be much easier and cheaper, and last longer.

Glenn Herbert wrote:Matthew, you would do well to remember that a steel core is inherently less efficient than an insulated masonry core; steel can only make sense if you have a source of cheap steel and full metalworking tools and skills. For most people, brick or refractory cement will be much easier and cheaper, and last lonrger.

I just happen to be a blacksmith and a welder. I also have a 7' section of 8" x 8"
1/2" wall square tube that needs a job, hence my interest in your build.

Glenn Herbert wrote:Matthew, you would do well to remember that a steel core is inherently less efficient than an insulated masonry core; steel can only make sense if you have a source of cheap steel and full metalworking tools and skills. For most people, brick or refractory cement will be much easier and cheaper, and last lonrger.

I just happen to be a blacksmith and a welder. I also have a 7' section of 8" x 8"
1/2" wall square tube that needs a job, hence my interest in your build.

I just happen to be a blacksmith and a welder. I also have a 7' section of 8" x 8"
1/2" wall square tube that needs a job, hence my interest in your build.

Wow! That is one heavy piece of tube! I'd recommend you use that for your burn tube and lower part of your heat riser where the highest temps are. Lighter material would be fine for the feed tube and upper part of heat riser.

Remember too that a good long heat riser 5 feet or better will be the "engine" which drives a good draft.  (Keep in mind your clearance between bell tank and ceiling when planning the lengths of your heat riser  and air-jacket exhaust as they need to be lifted in from the top.)

With your skills this project should be a "piece of cake " for you.  I had to enlist help with all the welding and bending on mine!

Keep us posted as you progress with it. I, for one, will be most interested and (going by the interest in mine), many others will be too.

The fun continues with this convection oven perched atop an air-cooled, steel cored cob bell RMH.... cooked our first chicken in the convection oven last night. Put the bird (about 4 pounds) in at 3:30, it had started to brown nicely by 4:00 and was done and ready to eat at 5:15.  So although I don't know how much, this beast is saving us on electricity.  See the before, during and after pics below.

Bruce, you're making me hungry! Beautiful chicken. Looks like your convection oven is a winner.

Hey Bruce, are you the "Bruce" from JTF that improved upon Mother's waste oil heater?


just curious.

Duane Hylton wrote:Bruce, you're making me hungry! Beautiful chicken. Looks like your convection oven is a winner.

Thanks Duane! Yes, I'm quite pleased with the way it works. Had never thought of this when I first designed it! But when I found how much really hot air it produced, I figured I could harness that for something useful!

Brad Hengen wrote:Hey Bruce, are you the "Bruce" from JTF that improved upon Mother's waste oil heater?


just curious.

Hi Brad, Yes, I'm the culprit! That was a few years back now! But I don't recall what JTF stands for.  I've been intrigued with fire and fireburning appliances since I was a kid so when I heard of RMH's a couple of years ago, it really got me going.

A couple of days ago we roasted a short rib roast with potatoes and onions in the convection oven atop our steel 7" air-cooled RMH in it's steel and cob bell. It was delicious! Sorry, I got no pics this time.

Bruce Woodford wrote:

Brad Hengen wrote:Hey Bruce, are you the "Bruce" from JTF that improved upon Mother's waste oil heater?


just curious.

Hi Brad, Yes, I'm the culprit! That was a few years back now! But I don't recall what JTF stands for.  I've been intrigued with fire and fireburning appliances since I was a kid so when I heard of RMH's a couple of years ago, it really got me going.

JTF = Journey To Forever

That's where I first read about your modification