Clay/Perlite for limited lifetime or Morgan Wool for longer but unknown lifetime

What would you do?

I've been researching RMHs for a couple years on and off and now that I'm retired I'd really like to get a start, but...

Whenever I come back to the topic things have changed.  That change is good.  I just don't know if the "Five Minute Riser" is right for me.  First, I'm weighing the cost factor.  If I go with Morgan Wool the cost is maybe four or five times the clay/Perlite option.  While I am OK with that cost I'm still confused about the expected lifetime of the Wool version.  I see videos of early versions being rebuilt.  Either way, I am also confused about how to make the barrel easily maintained (for a rebuild) as I see both perlite and wool options being rebuilt at some time.  This is an area that has surprised me (though it shouldn't be a surprise).  Early versions of RMHs seemed to not address this at all.  I see the rebuild videos of those early versions that almost totally disassemble the burn chamber/riser/barrel.

I am also changing my mind about a J-tube vs a batch RMH.  I'm leaning toward batch, so, I have more research to conduct.  I hope by the time I return for more information that design hasn't been changed too drastically by the forum.

I have the Wisner's early edition of their RMH book.  Is there a newer edition that addresses some of these issues, specifically details of a batch version?

I love how people are tinkering with the designs.  I just don't know how to keep up : )

Tim

EDIT:  I now see the ebook about risers being offered.  Just purchased and I'll start there...  Thanks

Hi Tim;
The best riser is 2600F+  insulated fire brick, also the most expensive.
Second, best is a five-minute riser, depending on size  the cost is less than $90
A perlite /clay riser is perfectly good, the cheapest by far, but the most labor-intensive.

Rebuilds on RMHs look scary, in reality, they are not.  It is actually fast and easy.
Clay mortar pops apart, and scapes off into a bucket. You add water and walla new mortar!  Try that with concrete!
A removable lid on your barrel for easy inspections.
Gently lifting your riser off and setting it aside is no problem.
These are hand built stoves, once you build your first one your whole perception of how hard they are to build will change.
Rebuilds are fun, not scary.

As far as batches.   Peter Bergs website has the most complete information. https://batchrocket.eu/en/
Indeed Peter continues to develop new design batch boxes, The sidewinder, and the Double shoe box stove.
I am writing a book on batch boxes for beginners, Perhaps it will be available

My teacher of masonry heater building told me that at beginning he was using insulated raisers built from high density firebricks, but later he realized that at initial stage firebricks are insulative and by the time they start conducting heat they are already heated and help the combustion process. He never offered to his clients a "Five Minute Riser", nor IFB riser. He never offers cast ceiling in the bell either - does not work.

In industrial applications, insulating refractory materials are never used where the contact with fire will take place - they are too porous and will get eroded so would need to be changed to often which would be costly. They are being used BEHIND high density firebricks.

I designed my rocket heater around robustly built chamber made of high quality firebricks -  Alsey Pilot DP, placed on shiner not stretcher. If someone constructs heaters for other people, mistakes will be costly with future repairs. Some people claim that having high thermal resistance - high alumina bricks is not necessary, but at the same time the truth is that high alumina bricks have higher number of survived heating/cooling cycles (spalling loss) that makes them more resistant to constant heating and cooling in home application. In stove design, materials are very important - same like in properly designed buildings. Cheap Muddox extruded bricks will never perform like precisely pressed quality product used by metal/chemical industry. Of course it has its price - like everything in life.

Thomas, I would be interested in your book.  Is it in a draft form?  Would you like someone with a beginner's perspective to critique it?  Would that be of any value to you?

To be honest the research I have done so far seems to create more questions than answers.  It would be nice to have a recipe to follow from someone that can describe the process, the materials and the sources in plain language.

Let me know if that is of interest to you.

Tim

Hi Tim;
Currently, that book is in my head... it's a busy place in there so no telling how soon it will materialize.
It is pushing me to get out... so we shall see.
There is already a list of standing offers of help and even offers to purchase early editions already!  

Get that dang thing down on paper!  : )

JK

I'm rethinking the batch box design of a rocket heater.  After researching building a fire box core and riser system out of firebrick I realized that I'm looking at close to a hundred firebricks, about half that being full standard size.  That's over $400 just in firebrick.  Even with a five minute riser I don't see how to reduce that cost much so I am going to revisit the J-tube versions built with cast perlite and clay with a small amount of fire brick to make the burn chamber cleaning more durable.  I am picturing a J-tube that is cast in a size that allows for the burn chamber to be lined with replaceable half size fire brick.

I don't have a big problem feeding the J-tube a couple times per hour and like the idea of small batches instead of wood stove quantities of fuel.  I'm guessing that if a J-tube fire goes out due to failure to feed for an hour or two that restarting the fire would be quick as long as the system is still warm enough to continue drafting.

I already have the clay and perlite is only $12 for 4 cu ft.  I am going to give thought to a J-tube and possibly a five minute riser combined with a batch type bell instead of a bench.

Hi Tim;
A J-Tube is certainly simpler and cheaper to build than a Batch box.
With an 8" expect to feed fuel every 45 -60 minutes.
You are 100% correct in that a warmed-up J-Tube practically lights itself!

You slightly overestimated your firebrick needs when building a batch.
I buy 35 full-size bricks for just over $2.20 each.   Half bricks tend to easily crack, and cost more than a full brick!
I also buy a bunch of plain clay bricks @ .50 each to build surrounds and bells.

Do yourself a favor and build an 8" J-Tube, six inchers are very small.

On sizing a J-tube, my experience is that if you have decent natural draft, an 8" J-tube can run well with a 6" chimney. Mine is only about 16' from top of feed tube to top of chimney, and goes through the wall and up outside.

I would just build the feed tube and burn tunnel from full sized firebrick, as far back as the end of the burn tunnel roof. The rest will hardly ever see abrasion and can be cast refractory or perlite-clay for insulating value.

I built my core from castable refractory, and in the seventh heating season the surfaces are starting to noticeably deteriorate. I think it will be fine for several more years, but may need repair or a rebuild after that. The cracks that developed soon after building are not a problem, just stress relief in all the places that need it.

Thanks again Thomas.

I originally roughly guesstimated the 100 bricks while looking at the Wisner's book, The Rocket Mass Heater Builder's Guide (pg 98, 2019 printing) where they show the courses for a 6" j-tube build.  So after sitting down and actually counting the bricks per course the total bricks (a handful of partial ones) is 156.  If I assume I'll go with a five minute riser and stop with bricks after course 6a and 6b, which are the base of the riser and drum, it is still 120 bricks.  That might explain my jaw drop.  I realize fire bricks are probably the most costly item but there are many other items that I assume will drive the cost up near $1k.  

That isn't necessarily a deal breaker.  I said in an earlier post that I'm prepared to spend some money on this project.  It was just eye opening.  Early in my interest in these heaters I read post after post and article after article of people building cob heaters and repurposed materials and their total claimed cost was a couple hundred dollars.   Yes I know, that's an apples to bananas comparison to what I hope to do.

Years ago I purchased a used pellet stove which required three, yes three, auger drive motors before I could even get it running.  Then to add the cost of pellets.  Well that's a no go.  We live in a very well insulated home on the WI/MI border and our natural gas bills in the winter rarely go over $60/month.  That's a third of what 30 bags of pellets would cost me today.  Not to mention the electricity to run the pellet stove.

I'll start to look at the 8" version to add a little fuel capacity to the heater.  I have a number of paving bricks (not concrete).  Maybe half of what a bell would need (if I decide to go the masonry bell route vs a drum).

Glenn, I'm hoping to use a masonry chimney that already is installed and goes through the poured basement wall up over my roof peak about two feet.  I think it should work well.  I do need to inspect it thoroughly though.