More RMH questions (before I start building)

Good Morning Permies Gurus!

I think I (finally) have almost all the materials necessary to build my RMH.  This is going in a 3 season, un-insulated camp on the coast of Maine.  We are really trying to just extend the seasons a little, no plans to spend any frigid time there.  
A few knowns to help filter the answers :
 Firebox will be HD FB lined, surrounded by K26 or K28 IFB.
 Bell will be masonry, not metal barrels.
 I will have a high bypass to flue
 Lowest ambient temp while we are there will be around 40F.  
 Structure is 1770sf, 1200sf of that is open to the central area (ie, no walls); the remainder (bedrooms) doesn't need direct heat.
 Existing 25' tall chimney is dual flue, lined with 8x8 clay tiles.
 The thimble to flue is restricted, so my system size will be 6 1/2".
 I'm lifting the firebox so the bottom will be around knee height for easier loading.
 Fuel will be primarily Eastern Red Spruce.
 I will have a White oven (17x17x14H, inside dimensions)
 Oven walls to be 2" cast refractory with tabs and rabbets to interlock - sealed with ceramic paper gasket
 Air gap around oven is 2 1/2", then K26 insulating Fire Brick to ensure most of heat transfers to cooker.
 There will be a ceramic cooktop (ala Matt Walker designs, Thank you Matt) but only 14" wide by +/- 30" depth.
 A heated bench (size TBD) might be nice to have.
 I am hoping to install some sort of gate(s) to direct the bulk of the heat into either the oven, heater or cooktop.

And now for the questions:
 Is 2" this refractory right for the oven walls?  Should it be thicker?
 Most of the cooktop support will be IFB, but should I use standard FB for the upper part to provide longer heating?
 And will that FB have much effect on heating through the ceramic surface?
 As I plan to surround the oven with IFB, could I use the outer (red)  brick as the heating bell and eliminate the bench?
 Is it feasible/recommended to dry stack the IFB core to facilitate repairs or mods later?
 Is the DSR2 design superior to the BB with vertical riser?  (I have plenty of height to work with)
 If I did add a heated bench, has anyone constructed one with a portion in metal for quicker heating, and the rest in stone/brick for longer use?

Thank you all for your wonderful, informative input. Some of you have been pushing this envelope for a long time and we all appreciate it so very much.  Keep innovating!
Randy

Hi Randy;
It sounds like you have a good plan figured out and your materials as well.
I only have a few responses to your questions.
First, I think dry stack your firebrick in any place exposed to core temperatures.
Matt suggested during his stove chat, that all mortar will ultimately fail in a core. Firebricks flex and move at high core temperatures and the mortar cracks.
Clay mortar can be easily reapplied but a refractory mortar is not so easy.
I have tried both mortar methods, and now all three of my RMHs are dry stacked.

The DRS design vs batch box I think is a personal choice.
Many people are trying DRS #2 and #3. They seem to really like it. I have not yet built one.
I am a big fan of Peter's batch box design, In your case, I would build a 6" batch keeping the box dimensions "stock" other than box length.
A stock 6" batch requires wood in14"  lengths or less... not very common. Batch boxes require space at the rear and front between the wood and walls of the BB
With Peter's Ok I lengthed my 6" batch to easily accept 16" wood.  This allows all three of my wood burners to use standard-length wood.

I see no problem with a half-heavy mass and half-thin-skinned seating bench.  Depending on how well your stove is rocketing the tin skin side might get a bit warm.

Thank you Thomas - I KNEW I asked questions in the right place.

On the size, I was trying to maximize output without asking for problems in a bottleneck.  
And the firewood will be a bit odd - I have neighbors that offer blowdowns on a regular basis.
Oh, I also have a small bandsaw mill, so I get 2 bys and a pile of slabs along with small (but 20 ' long) kindling.
All the orts will get cut to whatever length works best.
That thin skinned portion of the heated bench might just turn out to be tile instead of metal - got lots of options!

I noticed in an older thread that Erica Wisner had "rule of thumb" numbers for bench heater lengths based on system size.  Does anyone have a rough heat loss calculation by foot of (inside) chimney?  

I ask because I think I need to dry build my heater outside to test (I'll hold pretty close to PvdBs dimensions for the heater, but the bell will be an unconventional configuration).  I can easily rig a temp chimney to test the burn and take temperature readings at critical points. But I have about 25 feet of chimney, most of it is inside the heated space.  So if I get temperatures in the 200F range on the test chimney, how will that correlate to the final installation?  Or do I need something hotter (or cooler)?

Thanks,
Randy

Hey Randy;
Yup, with a piped mass as Erni & Erica worked with there are specific numbers to follow.  An 8" J -tube can push apx. 50' horizontally with a 5' deduction for each 90-degree bend until you go vertical.  A 6" is 35'.
Bells were not very common when E&E wrote their book.
As long as your chimney dia. is the same as your batch or bigger, the vertical rise does not count.

Batch box chimneys ideally have an exhaust gas (not pipe) temperature of 150F - 200F   depending on system size and bell size it can go over 300F.
Use a thermometer inserted into the outgoing chimney to give you the real gas temps.

Thank you, Thomas!

Forgive my slightly muddled brain - when you say exhaust gas temp, are you looking at the exhaust as it's entering or exiting the vertical chimney?  
My brief exposure to thermodynamics and heat transfer is ancient history, but what little I do remember is, it's a pretty complex calculation.  
I am concerned that there will be some heat dissipation over such a long flue - and I don't have the option of testing my build with anything close to the height of the existing masonry chimney. So my test system exhaust temps will have to be taken pretty close to the bell exit.

And thinking of taking measurements, does anyone have recommendations of specific (and affordable) instrumentation?  I'd love to add to the knowledgebase out there, and would be happy to keep periodic readings for future builders' reference. And while it would be wonderful to use, I'm not talking about a Testo unit!  Maybe some decent thermocouples that could report to an Arduino system?

Hi Randy;
My temp probe is apx 4'- 5' from where it goes vertical.
I have the temp probe on my shop batch with a bell just above my bypass.
On my studio batch with a piped mass, I have my temp gauge apx 3' above the mass 4' after going vertical, and 2' above my bypass.
Heat rises no matter what.  With your chimney indoors it will stay room temp.  
A bypass makes sure that hot enough air makes it to the vertical portion of your chimney.

I use a standard Taylor candy thermometer inserted thru the single-wall pipe.
This is plenty to know what your gas temperatures are as they head up the vertical pipe.

There are fancy thermocouples available if you like experimenting.

Randy, looks like you have thought this through!

I am by no means an expert on this , but one thing I observed and have kept in mind kind of like an ace in the hole:

At a tour of Wheaton labs I saw that in order to get the exhaust to go ahead and rise after a long horizontal run, they ran the exhaust pipe past the heat again before sending it up and out.

This was in the teepee.  If you’ve been there then you know that they built a bench around the circumference of the teepee.  And if didn’t draw well.  So, they added a little more length to the horizontal run, (maybe a couple 90 turns to go under the ground so as not to leave the tube in the doorway), then turned it vertical, passed it by the stove, which heated the exhaust enough to make it draw.  At the time I visited they were quite happy with it.

I have remembered it because I worry about things like that… what if I get it wrong, and it won’t draw…. now I just figure if all else fails, heat the exhaust enough to make it rise.  A very ingenious modification!  😊

Good luck with your build!

We just finished ours. It’s awesome. Beats the 48” kitchen queen by miles!

Alex;
Superb build!  Looks great!
Have any pictures and tips to share from your build?

This is a fun little informational video:  

I hope somebody can help me with my questions. I have been working on collecting  materials for about 10 years now for Batch Box rocket mass heater.  Actually I can't decide between a J tube or a batchbox. I purchased about 500,  3000ﾟ fire bricks $0.60 each. From someone that used to work at a coal fired power plant.
1. Where should I use the 3000ﾟF fire bricks in a batch Box  Rocket mass heater?
I also was given some unused leftover bags of refactory cement,  I believe it's 2000ﾟF from a crematory. (Maybe  2700ﾟF)
2.  Should I use this refactory cement for the heat riser?
3.  Also is there a step-by-step video of someone putting together a batch Box rocket mass heater?
I like videos better than a picture book. If there is a picture book that would be greatly appreciated too.
 
Greatly appreciated for the answers to my questions. I may have more later I may have more later. Thanks

Hi Tammy.  
Great score on the fire bricks!  Are they light (insulating) or heavy?  That will give a better idea where to use them.  

My intentions are to use standard firebrick for the lining of the firebox, then insulating FB for the remainder of the core.  
Also, insulating FB for the riser - although now I'm not sure if I'll go with the PvdB batchBox or his DSR3!  Decisions, decisions!

Are you planning an oven?  If so, black or white?  I'm going with white so I can cook while the fire is running and plan to use refractory to build the walls.  Can't remember where, but one of the heater threads goes into detail about the forms.

No doubt, with the wealth of knowledge available on this forum, you should get all the information you need.

Happy building!
Randy

Randy Butler wrote:Hi Tammy.  
Great score on the fire bricks!  Are they light (insulating) or heavy?  That will give a better idea where to use them.  

My intentions are to use standard firebrick for the lining of the firebox, then insulating FB for the remainder of the core.  
Also, insulating FB for the riser - although now I'm not sure if I'll go with the PvdB batchBox or his DSR3!  Decisions, decisions!

Are you planning an oven?  If so, black or white?  I'm going with white so I can cook while the fire is running and plan to use refractory to build the walls.  Can't remember where, but one of the heater threads goes into detail about the forms.

No doubt, with the wealth of knowledge available on this forum, you should get all the information you need.

Happy building!
Randy

Thanks for the response Randy. What is a pvdB and DSR3 to talked about?
The fire bricks are the heavy ones.  I just so hope I could use these bricks in the mass or somewhere. If not I plan on making a smoker with them.
I don't think I'm going to make an oven. Mainly using it for heating. Trying to eliminate wood pellet stove. The cost of pellets is insane!
I do have the white insulative firebrick. But I do have the bags of refractory cement to use some where.
Thanks

Hi Tammy;
Great score on heavy firebricks!  Are they in nice smooth condition or do they have mortar on them?
You will want to use those in the firebox and your white insulated ones in the riser (provided the insulated ones have a  high enough rating)
A PVB  is a Peter Van Den Berg batch box.  A DSR #3 is a Peter Berg batch design called a double sidewinder.
I think the DSR might be more than you want to handle as a first build.
A standard Peter Berg Batch box is easy to build but does require some metal fabrication.
I can supply those parts if you cannot build them yourself. https://dragontechrmh.com/
An 8" J-Tube rocket stove might be easier as a first build.

Your refractory cement might end up being surplus.
It has been proven to crack out in the extreme temps of a Rocket.
Clay/sand mortar does better and can be easily reapplied but it also will crack out.
I have three RMHs all of which use no mortar in the firebox/core area at all.

No book written yet on batch box construction.
I'm working on one but it is months away.
They just built a batch box at Wheaton ranch, there may be a video of that build.

Tammy -

I'm sure you will find a use for the bricks - even if you use them for the heat sink in your core and bell.  

PdvB is a reference to one of the very experienced masonry heater developers - that being Peter van den Berg of the Netherlands.  He has been doing empirical research (seemingly) forever!  And we are all very thankful that he has!

His "batch box" heaters were an upgrade from the J-Tube heaters (someone please correct me if I strayed here) and allowed the firebox to be loaded and then closed off to burn down.  The DSR3 is the third in a line of development for a more compact heater built from ceramic board instead of fire brick.  Since it is so much lighter, it may have use in some homes where a heavy heater is impossible.

And the reason I plan on an oven is my location is prone to power outages.  So heat, and cooking are both a concern.

Keep asking questions, but beware, you may get all kinds of answers - 'cuz everybody builds their own heater tailored to their needs. And that is one of the really cool features of the masonry heater.

thomas rubino wrote:Alex;
Superb build!  Looks great!
Have any pictures and tips to share from your build?

We are working on a video. Rest assured it will be posted here and on Matt’s customer build page. Thank you much for your kind words!

Alex -

I have to second Thomas - that is a thing of beauty.  I hope the servers are ready for the bandwidth consumption once your video is posted!

Oh my Alex Klohe!
That is gorgeous!  I just bought plans for the Tiny Cookstove from Matt Walker - I'm eager to see pics of your build and any comments you might have.

Beautiful work!

Action shot! It’s been an amazing heating season. Here we’re just making some tacos with sticks from the yard and it’s 24 degrees outside and we’re wearing t-shirts inside.

The best advice we have is to make sure you have the best materials possible for your core. Don’t skimp! It will keep you from having to tear it apart and rebuild it multiple times.

First mistake was we used second hand ceramic fiber board that turned out not to have fibers so it flaked and crumbled into dust.

Second mistake was we used the crappy pink insulated fire bricks that turned greenish and flaked apart.

Finally we used 2600 degree white firebrick and that was the ticket!

Since the first cooktop cracked because we didn’t give it enough room for expansion the second cooktop now floats on a gasket bed and allows for easy access to the top of the core for cleaning and replacing bricks.

The other piece of advice is to mock up the stove in cardboard so you can have a feel for how it will live with you.

There’s always someone sitting on it and if we had the room, we would have made the bench a twin bed size!