Miguel Moreno

Peter van den Berg wrote:The second suspected flaw is the position of the blast gate. It looks like it's above the level of the riser, this is a tricky position. The temperatures might be too high for the materials of the gate, possibly deforming it.

Is your concern the integrity of the blast gate alone, or all of the metal components of the bypass in this position?

I'm wondering what you would suggest as an alternative here? Would lowering the bypass below the top of the riser be sufficient? If so, at what point does lowering the bypass impact its overall effectiveness? I imagine there may be a delicate dance between keeping it high enough to vent effectively, but low enough to maintain its overall integrity in the presence of such heat...

Daniel Ray wrote:This sounded like a fun question to calculate for my 8" batch. Everything is approximate, I didn't take the time to weigh anything but looked online.

60 full bricks = 240#
60 half bricks = 120#

1 tub high temp mortar = 8#

Fireblanket  = 20#

firebox cap ceramic = 20#

secondary air channel = 25#

2 Barrels = 64#

metal door = 10#

So we have approximately 507lbs for an 8" batch without mass. Add in barrels for the bell.

My stove then has about 3 yards of cob at about 1 ton per yard makes my stove 6500 lbs.

Great contribution, Dan!

It's cool to see it all layed out like this.

This really highlights how heavy a system with a cob bench is compared to a hollow brick bell bench.  

thomas rubino wrote:Hi Miguel;
Really, not that much with a batch box/bell.  
Certainly much less than building a traditional piped mass with a solid rock and cob filling.
To start, Your core is built of all insulating brick, plus your metalwork, I doubt you would have #100 total.
The rest is all standard clay bricks.
Weigh a single brick and figure out how many you will need.
Add extra for mortar.
With a bell and a bench, your weight will be spread over a large area, easy to support.

This is helpful and reassuring, Thomas. Thank you.

I am in the process of sourcing materials and refining designs. I have the option of getting both 1.25 x 4.5 x 9 and 2.5 x 4.5 x 9 firebrick.

Which thickness brick would you recommend for the core/riser? From what I can tell it looks like most folks are using the thicker brick for the core. Risers seem to be done a lot of different ways. Thick bricks, thin bricks, 5 minute risers, etc.  I had always envisioned building a straight brick riser, but I'm open to other options so long as they are effective and long lasting.  If you or anyone else has suggestions or ideas, please feel free to throw them my way.

thomas rubino wrote:
Raising the mass on hardy board with bricks is foolproof. Plus you gain a heat radiating surface from the bottom.  
Yes you also created a great pace for dust bunnys to hide and I guess mice might hide there as well.  
No worry's about brick conducting heat to the floor. Your mass will never get very far past 100 degrees especially the further it is from the core.

Just to be clear, are you advocating for an air gap below the hardy board on all sections of the stove, bench bells included?

Just wondering if folks could chime in with some batch rocket weight estimates...

As you may have seen in another thread, my ~idea~ is to build a fairly substantial heater, an 8 or 10 inch batch rocket, sitting on a wood framed floor. The trusses below the floor are all engineered but will certainly need to be reinforced and supported. Estimates of the weight of this beast will be helpful when I speak to the relevant parties to determine the viability of such a project.

I acknowledge there are LOTS of variables at play here. An actual design and materials list would get me a lot closer to a true number, but I am just looking for a starting point, which is why i am simply asking for estimates from those who are familiar with these kinds of builds.

To give you more context, the stove will be your standard 8 or 10 inch batchrocket core, built from the floor up with insulated firebrick, on top of some kind of insulated slab. The core will be surrounded by a masonry bell high enough to get the desired internal clearance. Connected to the main bell will be a secondary masonry bench bell approximately 8 feet long, 2 feet high, 2 feet wide. Maybe another bell too to wrap around the corner! Who knows?!

So, I guess my main query is whats the approximate weight of the core w/ vertical bell combo, and then whats the approximate weight of a hollow bench bell per cubic foot/meter, however ya wanna do it?

Any and all input appreciated!

Peter van den Berg wrote:

Miguel Moreno wrote:Does having a bigger stove, like a 10" mean I have to burn bigger, more frequent fires to get it to work appropriately, even if I don't want want that much heat in the space?  

The thing with a batchrocket is this: it can be run with half a load and burn very clean all the same. Even the duration of the burn is about the same as compared to a full load. I am used to long shoulder seasons so I run my heater once in two days or just once just to get the chill out of the house in such a situation.

Short answer: you could do half loads, once per day. A big fat heater could roughly been run once in two days. See it as a sliding scale, nothing to be worried about. Never run the heater for the heater itself, you are the one to be warmed so you are the commanding factor.

For the rest of the house: I'd second Glenn.

This is all good info, Peter. Thanks.

As cold as my area may be, there is only so much insulation I can put in a 2x6 wall. Sheathing with exterior board insulation is an option, but that comes with a huge cost, and still doesn't boost my insulation factor to the level of a straw bale home.

Seems to me oversizing the heater is the way to go whenever possible, not that that's without its own costs!

Glenn Herbert wrote:A bigger system would mean bigger fires, but not more frequently. As you are in a seriously cold climate with fairly steady heating needs, making the walls of a bell thicker would make the heating more stable and hold heat longer, at the cost of longer lag time before heat makes its way through the walls to your space. If you can fire up the Walker stove for some quicker heat if you really need it, I think I would go with the big fat batch box and expect to usually do fewer fires per day.

Your 317 m3 volume probably translates to around 120 m2 or 1200 square feet, which is not a large house. R-20-25 is fairly modest insulation for a real cold climate, even assuming the roof will be double that. In your situation, I would go with R-60 roof and R-30-40 walls and have less need for heating. Then I expect an 8" batch box plus a Walker stove would take care of you in any weather.

This is great input, Glenn. Thank you.

The heating space is 1400 square feet spread over two floors (28x28 + 22x28).  Time will tell what the insulation factor will be, but I purposefully kept the insulation estimates low to err on the side of caution. I do want to insulate as much as I can reasonably afford and your numbers are a great goal.

But really what this illustrates to me is how unclear the spreadsheets insulation factors are LOL
It would be so much easier if they were clearly mapped to R values instead of the materials listed.

With that being said, I think the best thing for me at this point is to crunch the numbers and determine exactly what my insulation for the house will be.  

thomas rubino wrote:Hey Miguel;
Having a Walker cook stove makes a difference.
I suggest you build an 8" batch.
Foremost is your chimney 8" or 200mm are widespread and readily available sizes, ten inches is not.
An 8" batch is plenty of stove to share space with a Walker.
Plan on two burns to heat up the whole bell system.

Thanks for the insight, Thomas!

I do have access to 10" chimney and all the components for the chimney system, although the cost is certainly greater.

Is your thinking that 10" is simply an unnecessary amount of power when combined with the walker?

I do plan to use them in conjunction at times (the walker is a cook stove after-all and will be used whenever possible to limit propane cooking), but I also like the idea of only needing one or the other, to keep the work and time tending to stoves down. I can certainly foresee there being times in the deep winter where I'd only want to have the batchbox running. In that case shouldn't I have the capacity to meet as much of my heating needs as possible with just the one stove? Especially given how strongly they stress to oversize?

I guess something I am a little confused about is how to balance the output and efficiency of these stoves. I'm not really sure how to describe this, but what i mean is thus: I know I need to burn it enough to heat the bells fully and get the desire output. I know I need to heat the core enough to get a good clean burn. Do these things scale accordingly? Like does one coincide with the other? Does having a bigger stove, like a 10" mean I have to burn bigger, more frequent fires to get it to work appropriately, even if I don't want want that much heat in the space?  

thomas rubino wrote:Hey Miguel;
I consider extreme burning to be back-to-back burns for hours on end.
We burn like that in our uninsulated greenhouse and it remains above freezing all night with no fire.
My shop stove is the same way, a large poorly insulated building.
I burn nonstop when working out there in cold weather.
I used a cast iron roof on the shop stove and although it is still in place and usable it now has a large sag at the hottest part.
In contrast, the factory-made Firebrick tile I have in the greenhouse is as good as the day I installed it.
For longevity, I have found the premade tiles to be the best.

I won't be burning to that degree, but based on your input I definitely think a factory made firebrick tile is the option for me, regardless of cost.

I'm planning a batchbox RMH build per Peter's specifications and am looking for a little insight regarding sizing.

Using the spreadsheet from Peter's site ( https://batchrocket.eu/images/rockets/files/Batchrocket_mass_heater_sizing.xlsx ) I have the following inputs:

Volume of the house - 317m3
Required temperature difference - 40
Insulation factor - 0.8/1.0
-----------------------------------------------------
Heat losses - 10.1/12.7 kW

Now a question about insulation factor...

The European reference points on the spreadsheet are a little unusual for me. It notes 0.8 for a "recent house with 37cm thick insulating clay bricks for example". My house is certainly recent, being a brand-new construction, and will be insulated to at least r20-25. The question then is, what is the approximate r-value of said insulating clay brick? One reference point I found said that they are 1.7/2.2 per inch, which would put it in the r24-32 range. Does this seem accurate?

If so, using 0.8 for my reference seems reasonable. Erring on the side of caution and using 1.0 also makes sense.

So, now to size the batchbox based on the mean power output generated to offset heat losses...

I personally think a 10" system would be best, but I'm curious to hear what other people who have actually used these stoves have to say.

A few things to note:

According to the spreadsheet a 9" system would work with 2 double fires per day, but they explicitly state to oversize when possible and I'd really rather avoid having to run the stove so often on those cold winter days. Keep in mind also that I was somewhat conservative with my required temperature difference. In my region we can have cold snaps as low -40, although they are rare. Small stretches of -20 is more common and that's what I was using a baseline. 20 degrees internal temp is sufficient for me and I certainly don't mind it colder. My current home is wood heated and it's not unusual to be below 15 degrees celsius without issue.

A true worst case temperature differential would be 60 (-40 - +20 / -50 - +10), with an insulation factor of 1 would require 19.0 kW of energy, just shy of the 17.1 output of a 10" system...

But here is the kicker! This won't be the only heatsource in the home. I am also planning to build a Walker cook stove which itself is capable of putting out a significant amount of heat. The general idea is to use the walker to heat and cook in the shoulder seasons and then fire up the batchrocket for supplemental warmth once deep winter sets in. An ambitious plan no doubt, but something I am incredibly eager to tackle.

Curious to hear whatever anyone has to say! Any advice and recommendations welcome and appreciated