Proposed Batch Box RMH Floor Designed for Outside Air.

Outside air for a BBR?

I know it's been explored some, but after some internet search, I couldn't find much about it other than Paul's video where he came out unfavorably about it.  And that's fine.  No one likes stale air and it is undoubtedly a far superior thing to burn your farts as fuel.  But what if your dwelling is sealed up too tightly and an outside air channel was built right into your chimney, just begging to be used?

That's the situation I find myself in.  After learning I could no longer use my living room fireplace due to having buttoned the house up so tightly, I resorted to using the ash door for make up air and the results were astounding.  Then I began asking myself if I couldn't use the make-up air flue in the basement the same way for a BBR.  Now I understand a RMH is not a candidate for make-up air because it relies on its fuel opening for air.  But a BBR is a different animal.

So, with this new inspiration I propose to build a BBR with a provision for feeding it outside air, fed to the fuel load in much the same way as current practice, as well as feed pre-heated air to a P-Channel.
In addition, once the fire is mature, the entire air supply will be subject to pre-heating.

This is imagined for a 7" Sidewinder BBR.

Three level floor.
To do this I propose a three level floor.  

The floors will be standard hard firebrick.
The first floor laid flat. East-West
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Like so.
The first bricks will need a gap cut from between them so that air can pass through from underneath.

The second layer will run the other direction, North-South, with the bricks running along the lengthwise center line of the first floor getting material removed from their sides.  This will open a channel down the center of the burn chamber from front to back.  I'll take about 1.5" off mine so the channel will provide the appropriate cross section for my intended 7" CSA.

The middle third layer of firebrick will run perpendicular to the second layer, East-West, with a full length brick spanning the gap.  The rest of the floor will have the bricks oriented the same as the second floor. North South.

Now we have an air channel from front to back running through the center of the burn chamber floor.

Manifold.

A manifold will be made from a length of rectangular steel tubing.  3" x 6" (guessing here, not exactly sure what might need to be fabricated) and cut to the interior width of the firebox.  The manifold should rest on the first layer, have an opening cut to accept air from the channel, and sit proud of the third layer by at least an inch.  Air holes will be drilled along the length of the manifold to direct airflow across the floor towards the fuel load.

An optional side channel may be added to the third floor to accommodate a P-Channel.  

A damper is installed near the makeup air supply to control the amount of air to the stove.

So, in short,  outside air enters the BBR bell via a dedicated stove pipe, galvanized, or other type vent and enters the raised, hollow pedestal that supports the core and is separate from the rest of the Bell.  
A damper is used to prevent gasses from entering the bench.

When a fire is started in the BBR, the updraft created by the small riser works in conjunction with the 35' chimney to pull hot gasses through the system while fresh air is pulled up through the pedestal and into the bottom of the burn chamber where it travels under the top layer of firebrick and into the manifold where it is redirected across the burn chamber floor.  As the fire matures, the burn chamber floor gets hot and pre-heats the supply air.  If a P-channel is used, the extra trip back down the burn chamber floor allows additional time for even more pre-heating.

Remember.  This is only for those who desire a BBR that can utilize outside air.  A lot of us have building codes that demand it.
Let me know if you see any obvious oversights on my  part, or if this has already been tried and failed.
Creative criticism is welcome and expected.  
Help me make this idea better.

Hi Thomas,

"The Rocket Mass Heater Builders Guide" by Erica and Ernie Wisner also explains that outside air brings more problems than benefits. In case you're still looking for information.
It's a good read before building a RMH as it condenses a lot of info into an accessible form.

I would not alter a BBR and expect a good result. Years of testing went into those designs. Fiddle with them and you can expect to fiddle for months and best case get the same result as from a proven design.

And let me try to understand this: you made your house too airtight. So now your wood stove doesn't run correctly. So you open another door of the wood stove and it works?
Couldn't you just make an opening in your house's hull? (Window cracked open, some vent slightly open, a new opening that was invented exactly for that reason)

Or lower the exhaust fan setting while maintaining the same influx of air through the other fan? (in case you have controlled air)

Hi Benjamin.  Thank you for replying to my post.  

As a certified Permies Rocket Scientist, you must be aware that many of us live in jurisdictions that have building codes that require outside air for solid fuel burning appliances and I have seen many masonry stoves and steel woodstoves that are designed to utilize outside air.  

That said, I fail to understand why a Batch Box cannot be adapted to work properly with outside air.  

If you re-read my post, you'll see this proposal would only be of interest to persons who wish to participate in BBR tech but:
1.  Have a house with too tight an envelope ( and have no intention of just cutting a hole in a wall to let air through).
2. Have a building code enforcer and/or insurance policy that requires outside air supply.
3. Already have provision for outside air readily available and just need to know how to use it with a BBR.

So rather than just dismiss the idea outright, let's do what engineers and innovators do and talk about what works, and why, what doesn't, and why, and how it might be made to work in a foolproof manner.

J-Tube rockets are not part of this conversation.

It can be done and is being done.
Let me start with the heater door.
In Europe there exists a great selection of oven/stove/heater doors but none of them is designed for BBR (narrow, tall with an adjustable opening for primary/secondary air). Nobody would bother to make custom door, because it usually would be time consuming and not aesthetical enough (unless quite expensive), which is important, because in Central Europe masonry heaters are mainstream product, purchased by people that require aesthetical look and long time problem free operation.
Because of that a solution was invented:
-firebox has a door that more or less fits the firebox size
-ashbox is built below with a slit in its ceiling close to the door
-ashbox has a small door that lets in primary and secondary air
-ashbox can have an opening in the side to provide outside air; it would be controlled by a damper to give the choice: closed damper, open door or reverse

I saw this solution (outside air through the ashbox) numerous times.
The only potential problem that I would see would be bringing air with too much moisture, especially if the inlet on the building wall is located close to the ground. Water having very high specific heat could affect the combustion process much more than cold air. The inlet pipe would have to be positioned at a slight angle, so condensed water would drip out out of the building. In case of extreme humidity, the operator would have a choice of switching to inside air.

Thank you for that, Cristobal.  I had heard of attempting airflow through the traditional grate over the ashbox but that configuration did not feed the air to the load in the same way as Peter's Batch Box designs.  

I'm considering a pedestal bell upon which the core will sit.  It will have the outside air run through the main bell low and enter the pedestal bell low.  The pedestal will act as a warm air reservoir and feed the supply air into the core floor channel as I described earlier.  It would assist in  early season starts as it would already be near room temp.  Subsequent firings should be easier as the pedestal has been bathed in radiant heat from the main bell.  I would also make my top burn chamber floor from splits to get heat to the air channel faster.

Thomas. I would recommend that you include the inlet door (like the one for the ashbox). This way you will have an option to let the interior air (with cracked window or entry door) in case of extreme outside humidity. It could help to start the fire quicker. Also from time to time, you would be able to use it as "vaccum" to replace the house air with no mechanical help or when fans break or electricity stops. It's good to have fully passive system, at least as a backup.