Batch box bell ISA calculation questions

Hello everyone,

I am totally obsessed with the idea of putting a batch box heater in my home. I've wanted a fireplace in my home FOREVER, but I don't have a chimney and don't even have the possibility of installing one (long story). Then I found out about these amazing heaters that I can vent directly out of the side wall and I'm IN LOVE! I'm having a ball trying to design my heater, but I have a few questions about sizing the bell that I must get answered before I can finish my design. I appreciate any help anyone can offer.

Also, I will be putting the heater in a corner and I don't want to "heat" either of the walls (or the bell top, which will be the ceiling in my design). I basically want as much heat as possible to radiate out into the room, so my first question is...

1. What is the best way to insulate walls that we don't want to heat. I immagine that the best way may also be an expensive way, but if there are any economical solutions with a decent cost/efficiency ratio, I'd love to hear about them, as well.

2.  If I'm insulating some of the walls that will make up the bell, do I count their surface area in the ISA calculation, or do I count a particular percentage of their area?

3.  If my chimney opening is about 15 cm from the floor, do I count the first 15 cm of the vertical bell surfaces in the bell's ISA calculation?

Welcome to the forum, Lauren!

One thing you may want to consider is your heating needs - do you want heat right away (metal barrel), or do you want something with longer run time, but delayed start (masonry bell).

I have similar questions about the insulated portion of the bell, I'm sure that the gurus on here have answers.  Perhaps Mr. van den Berg can enlighten us?

Enjoy the design ride - it sure is fascinating!

Hi Loren;
Welcome to Permies!
And congratulations on becoming an apprentice rocket scientist!

The first thing I want to talk about is your comment about venting thru the wall.
I hope you plan to go thru the wall horizontally and then put vertical sections on.
If you are hoping to stay horizontal, you will most likely have draft and wind issues.

Next, if you would like to insulate one wall you would want a double skin. A second "brick wall" outside the main box and then you would insulate between.
Loose perlite is what I used to insulate an outside wall. Rather inexpensive and you just pour it in. Sand would be another insulator you could use.
Morgan super wool would be a top-of-the-line insulator but rather a rather expensive one.
For the ceiling, I would use Morgan Super wool.

Now on ISA questions, I will give you my opinion, however, Peter is the most knowledgeable on this subject.
I believe that any insulated surface such as the roof and most likely insulated walls do not count against the isa.
I would guess that you should count the 15cm at the bottom as they are still absorbing heat and radiating it into your room.
The floor of course is not counted.

Randy Butler wrote:
One thing you may want to consider is your heating needs - do you want heat right away (metal barrel), or do you want something with longer run time, but delayed start (masonry bell).

Actually I was thinking about making my bench with a stone top and a steel base. The base would be that last thing to get the heat, but once it arrived it would radiate it sooner than the stone. So kinda a happy medium. I don't want to suck away too much heat from the mass, but I also don't want to wait hours to feel some either. I just hope that down there it wouldn't get so hot as to be a hazard.

Hi Lauren;
What part of the world are you building in?
How cold will it get and for how long will it remain coldish?
All RMHs of every design with or without a barrel will need time to heat their mass.
Radiant heat can be supplied with a barrel or other metal surfaces near the riser.
If you are thinking of a bench, were you planning on a piped bench? Or a bell bench?

Tell us more about your plans, and what your home is built out of.

thomas rubino wrote:Hi Lauren;
What part of the world are you building in?
How cold will it get and for how long will it remain coldish?
All RMHs of every design with or without a barrel will need time to heat their mass.
Radiant heat can be supplied with a barrel or other metal surfaces near the riser.
If you are thinking of a bench, were you planning on a piped bench? Or a bell bench?

Tell us more about your plans, and what your home is built out of.

I live in the foothills of central Italy. Occasionally gets down below 0 C at night, but not that often. November through Feb/March are the coldest months, but sometimes April's pretty rigid, too. This year, it's been an unseasonably warm October and November. Barely had to use the heat so far.

My husband is vehemently opposed to a barrel. Dang Italians and their designer tastes! So I was looking to basically make a triangular column in the corner with a bench. I was thinking about trying to make the base of the bench out of steel (my husband works in a metalworks factory) to eventually get some radiant heat at a low level. I know that it will take a while before the heat will make it all the way down there, but once the whole thing heats up, maybe it will start to radiate a bit faster than a bench made of bricks would. I also immagine that it won't get quite as hot, which I consider a good thing.  I was planning on having the stone seat extend over the steel base so if it did turn out to get too hot and be dangerous, I could always just cover it up with some bricks. Has anyone ever tried something like this? Putting the metal "down the line" from the heat source?

I'm definitely going bell on the bench. 1. My husband is also anti-cob. 2. The bell benches seem easier to build. 3. I don't have to clean up soot! Woo Hoo!

The area that I'm trying to heat is about 450 m^3. I used the spreadsheet to calculate the size riser I would need and it looked like an 20cm/8in was in order, but I've seen a few people say that it's a beast of a heater, so now I'm undecided and confused.

thomas rubino wrote:
The first thing I want to talk about is your comment about venting thru the wall.
I hope you plan to go thru the wall horizontally and then put vertical sections on.
If you are hoping to stay horizontal, you will most likely have draft and wind issues.

Actually, I was thinking about going directly horizontally.  Oops!  I would be venting just under a terrace. It's a kind of protected area. I don't think that there are many wind issues there, but you definitely have a good point. I'm going to have to figure out if this is feasible.  How much of a vertical section would I need? I guess I could have it exit the house a little lower down and then put a (very) short vertical portion, but it would still be venting under a very long terrace, so there's a definite limit to how vertical I can go. My only other option would be to have a 4 meter long horizontal portion of flue inside my house and I immagine that would also create draft issues. I have a very unfortunate house for putting in a chimney.

Admittedly, this project will be clandestine, or as clandestine as is possible since I'll be venting right next to my front door! hahaha! Am I crazy? I figure if anyone says anything to me about it I'll just say I'm doing my part to protect the environment by reducing my natural gas consumption.

Hi Lauren,
Im from southern Germany, so similar weather pattern, but maybe 5°C colder.
Several years ago I built a tiny 4inch batch box (refractory riser and core) with a 200 liter barrel and a 2,5m long bell-bench made from hard fired "klinker" bricks. Is sits on my patio and is meant to warm your backside at the chilly season . When I fire it up it is always stone cold (sometimes even freezing). Nevertheless, the barrel heats up in less then 5mins and the bench is handwarm at the near side 40mins later. After 1hour the whole thing is warm to the touch and the fire is all embers covered with white ash. Without more wood it will cool down to lukewarm in about 2hours (a little faster, if it's really cold outside).
Remember, it is just a four-incher!

So, in my opinion, skip the barrel, incorporate a 6 inch core into the bell and you will be fine.
As your hubby dislikes cob, you can go with an elaborate brick design or plaster or tiles.

Hi Lauren;
Well, let's see.  To start an 8" batch box is way more than you need, an extended length 6" would do fine.
Have you considered building an 8" J tube rocket rather than a batch box?
They require feeding every 45 minutes or so but are very simple to build.
Batch boxes are also simple to build. They do require some metal parts, but perhaps your husband can build them.

Your idea of using a metal box for the bench/bell will work fine.
As for the bench, were you thinking of a closed metal box and just setting your stones on top of that?
You'll want some mass inside that box. Perhaps an access panel where bricks can be placed alongside the box's inner walls.

The earliest rocket stoves that did successfully vent horizontally were using a piped system not a bell system.
The best know example is Ianto Evans built his at "Cob Cottage" in Oregon.  His location has a very predictable wind direction.

I suspect a bell system would only work if you first exit your metal box with a vertical pipe and then pass horizontally thru the wall.
After you get outside, you can try different ideas for leaving it flat or adding another vertical.

If that does not sound like it would work for you then the alternative would be to use a piped mass inside your metal box.
Many large stones are placed around those pipes and dirt/cob is packed to eliminate air spaces than more rock until full.
With a piped mass you can attempt a horizontal exit, in just the right location with luck it might work.  
I would expect it to need some vertical pipe to encourage the exhaust gas to rise.
Be prepared and warn your husband ahead of time.
If you try a horizontal exit, to have a wind gust blow backward thru your system... lots of smoke in the house.

The semi-clandestine part might be a bit much to hope for, particularly when newly built.

As I don't think you will typically have long very cold spells in Italy, I would suggest putting a metal access panel in the front of your bell to give instant radiation. The metal-sided bench could be good in addition. I have a metal panel about 40 cm x 80 cm in my bell which gives nice fast warmth, in a safe location where nobody will accidentally touch it.

Mass walls or ceiling in the bell will absorb heat even if they can't radiate it directly to the room; they will radiate that heat back to the bell interior as it cools, thus slowing the cooling and using the heat later. So do count mass walls at the exterior where they are insulated on the outside, maybe at a reduced rate since they will not be able to keep absorbing heat for many hours of firing. Thin metal bell walls there would not store a noticeable amount of heat if insulated outside so would not be counted.

Another mass storage method if there is room in the bell would be a brick column inside the cavity; the same storage factor would apply, though I don't think I would count that as ISA as it has a limited total of heat it can hold.

thomas rubino wrote:
Well, let's see.  To start an 8" batch box is way more than you need, an extended length 6" would do fine.
Have you considered building an 8" J tube rocket rather than a batch box?
They require feeding every 45 minutes or so but are very simple to build.
Batch boxes are also simple to build. They do require some metal parts, but perhaps your husband can build them.

I discovered the J tube rocket first, but now I only have eyes for the strat chamber batch box. Easier build, less tending, lower maintenance, visible flames...  what more could a girl want?? I think I've settled on a 150mm (7 inches?) heat riser in a sidewinder configuration.

thomas rubino wrote:
I suspect a bell system would only work if you first exit your metal box with a vertical pipe and then pass horizontally thru the wall.
After you get outside, you can try different ideas for leaving it flat or adding another vertical.

So maybe a visual of my plan could help. Hopefully I can figure out how to load the images correctly. I plan to have the exhaust flue opening about 20cm from the floor and exiting horizontally at about 230/240 cm from the floor. I was thinking of leaving the flue inside the bell, hoping that the exhaust will reheat in the flue and create a secondary draft to keep things flowing in the right direction. I wasn't a whiz a thermodynamics in high school, but the idea seems plausible to me.  I can't actually have it exit horizontally from floor level because, as I stated, this will be right by my front door (bad idea, I know, but I really don't have ANY other choice).

I guess I could put a 30-50cm vertical portion on the flue once it exits my house, but there's the definite limit of my upstairs neighbor's balcony. Hopefully it will work magically and my neighbors won't even notice it, but if they do... as my dad always said, "It's easier to get forgiveness than permission". I figure worst could happen, I'll have to take it down, but that's a gamble I'm willing to take.

Glenn Herbert wrote:Thin metal bell walls there would not store a noticeable amount of heat if insulated outside so would not be counted.

Glenn, you've got me intrigued!!! A couple of quick questions.

1. What thickness of metal panels would be most appropriate?

2. What type of metal? Is steel ok? My husband works in a metalworks factory and has access to many types of sheet metal.

3. Will the metal hold up to the heat over time or will I eventually have to tear the thing down to replace the sheet metal?

4. What can be used for insulation and how thick must the insulation layer be?

5. Would those rolls of insulation (fiberglass, I guess) that come with a thin layer of foil work, or will the foil burn off particularly at the top of the bell where it's the hotest?

I'm tight on space so I really need to insulate the walls in less than 5 cm and I'd rather them not absorb heat for later as my ISA is already just about perfect not counting those walls. This sheet metal with insulation idea seems like the perfect solution for my situation!

If you don't need the ISA of the exterior-adjacent bell walls, I think heavyish sheetmetal would work fine for much of it. People have been using ordinary steel drums around and over the riser for decades, in some cases regularly getting the top to glowing temperatures. Whatever the thickness of steel drums, I think a similar or slightly heavier mild steel would work fine.

For insulation behind this metal enclosure, fiberglass would work in lower temperature areas, but the upper part would need to be more durable. Mineral wool (like fiberglass but made from rock instead of glass) is good to higher temperatures than you will find outside the core of the riser. Ceramic fiber insulation, board or batt, is even better though much more expensive.

Another feature of the steel access panel in the front of the bell beside instant heat would be the ability to inspect and maintain the heat riser without dismantling masonry. After my first heating season, I took the access panel off, took out the heat riser, and got inside the bell for cleaning and inspection. There were a couple inches of fly ash on the floor right around the riser, and soot on the walls from a season's worth of starting fires in a cold system, but nothing else.

Lauren Knickman wrote:
I discovered the J tube rocket first, but now I only have eyes for the strat chamber batch box. Easier build, less tending, lower maintenance, visible flames...  what more could a girl want?? I think I've settled on a 150mm (7 inches?) heat riser in a sidewinder configuration.

I guess I could put a 30-50cm vertical portion on the flue once it exits my house, but there's the definite limit of my upstairs neighbor's balcony. Hopefully it will work magically and my neighbors won't even notice it, but if they do... as my dad always said, "It's easier to get forgiveness than permission". I figure worst could happen, I'll have to take it down, but that's a gamble I'm willing to take.

@Lauren, with just 50 cm vertical at the outside you'll run into trouble when that side is the windward side. In that case, there's a higher pressure on that side of the house and smoke will be blown in. Sometimes back-puffing, most of the time flatly refusing to play ball. In order to avoid this all, your vertical chimney need to be higher than the top of the building and no higher buildings in the direct vicinity.

In case your plans bear fruit, try to avoid sharp corners as the plague. Corners more than 90 degrees are already difficult enough, sharp corners are headache inducing.