Sizing Batchbox RMH

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
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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

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.

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?  

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.

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.  

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.

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!