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5 ton RMH on basement floor

 
Posts: 21
Location: Vermont, USA
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Hello, I've been planning an RMH in my basement for years and now is the time to start! (Before anyone asks, yes, the basement a convenient place for my RMH to be)

I don't have any experience engineering tons of weight... if you do, could you please give your opinion (based on math or first-hand experience) on whether my floor is strong enough to hold a 5-ton monstrosity?

The weight of the bench/thermal mass is 220 pounds per square foot.

The floor is made of 4'x8' sheets of 1/2" plywood, supported by pressure-treated 2x4's every 16", resting on a concrete foundation. I assume the concrete is 6" thick... hopefully.

The ground is very solid underneath. Let me know if I'm being overconfident about the ground and concrete, but I think the only question is: will the plywood distribute the weight safely onto the 2x4's?

If not, I could pry up the necessary sheets of plywood and reinforce beneath them with more 2x4's. Best wishes, and thanks for your time,
-Richard
IMG_3003.JPG
Old wood stove to be replaced with RMH
Old wood stove to be replaced with RMH
IMG_3004.JPG
Can floor support 220lb/sq ft?
Can floor support 220lb/sq ft?
 
gardener
Posts: 3666
Location: latitude 47 N.W. montana zone 6A
994
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Hi Richard;  Welcome to the wonderful world of rocket science!
Tell us more of your design plan.  
I assume your planning a J tube?  6" or 8"?
A piped mass?   Using what as the fill?  I'm curious how you came up with a weight?
Were you thinking an all cob bench? Or a brick faced bench filled with rocks and cob?
Or were thinking of building a brick bell?

Were you planning on sitting the core and mass directly on the floor?  
If so that will work under your mass but Will not work under your core, it must be raised  up with bricks and cement board or built on a 4" minimum of perlite clay or other insulators.

Will your 1/2" plywood support the weight.   Probably as long as your mass is crossing many floor joists, rather than running with them and only using two or three.
To be safe , I would lay 1/2" cement board down directly on the plywood or use clay bricks laid flat with air spaces and place cement board on top of that.

 
gardener
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I would do something like what Thomas suggests.

Since your floor is only ~3.5" deep,  I would remove the plywood and 2x4s under the footprint of the RMH.

I would replace the 2x4s with bricks on edge, adding bricks around the perimeter of the footprint,and filling empty space with perlite concrete.

Top that with cement board and then build your mass.
 
pollinator
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I recommend getting the RMH book from Ernie and Erica. You are going to need airgap, metal foil and insulation among other things.

But the short answer to your question is that a regular plywood floor can support a regular 6inch or 8inch Rocket Mass Heater.
https://donkey32.proboards.com/thread/99/insulation-rocket-wood-floor
 
Richard Kniffin
Posts: 21
Location: Vermont, USA
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Thank you all for the ideas. Yes I'm planning to sit the bench on the wooden floor (perhaps with more plywood or concrete board to distribute weight), and then put insulation under the combustion unit to prevent unwanted combustion.

Thomas, here is more info on my plan. My house and heating situation are most similar to the "Phoenix 8" Rocket Mass Heater" on page 53 of Ernie and Erica's book, "The Rocket Mass Heater Builder’s Guide". If you want background info, pretend that I am in that house, working on that project.

I'll be using a basic 8" J-tube, piped through a bench of thermal mass, with heat exiting through an existing interior chimney with good draft.

I want to use this tried and true J-tube design because the unit is for my living space (it had sure better work!), and because I've seen/worked on a couple RMH's like it in workshops. The only unusual things I'm planning are

1) making the bench 2' tall, a little taller than usual, in order to fit more mass into a convenient footprint, and to maximize heat storage with a full 5 tons of mass
2) using brick for nearly all of the mass, with cob in between bricks and ducting to conduct heat and hold things together, then a cob exterior

Does anyone have experience using all brick for the mass? I intend to keep the mass as solid as I can, using cob to plug air leaks between bricks as mentioned above.

Here are the weight numbers:

common red brick is supposedly 120 lbs/cubic foot
my mass is designed to be about 82 cubic feet of solid material (not counting the hollow space of the ducting)

120 X 82= 9,840 lbs., then add a few incidentals here and there to increase the weight, and that's close enough to 10,000 lbs.

The footprint of the mass is 46 sq ft, so 10,000 divided by 46 equals just under 220 lbs/ sq ft.

A question just struck me: when calculating thermal mass, people aren't counting the burn box, barrel, and the rest of the combustion unit, are they? I'm only counting my bench in these numbers.

Cheers,
-Richard
 
master pollinator
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If you don't mind my asking, I would be very interested in your approach to navigating the maze of permits and insurance (or not).
 
Richard Kniffin
Posts: 21
Location: Vermont, USA
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Hi Douglas, there are no building codes in my town, or most of my state. Insurance I don't know much about, and I suppose I'll look into it. As of now, I'm considering an RMH to be a definite safety upgrade from the old woodstove that we've been using for decades.
 
thomas rubino
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Posts: 3666
Location: latitude 47 N.W. montana zone 6A
994
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Hi Richard;  
I predicted your reply to Douglas about Vermont's wood stove regulations. I spent many happy years in the green mountains.
I have not heard of anyone using almost all brick but it sure could work.
I will mention that stones have more thermal mass than clay  but you can go to home depot and buy a pallet of brick cheap and easy.
Stones might be free but you have to pick each one up by hand....
With a brick surround your cob can be any quality no need for super nice .

I have an 8" J tube with piped mass in our greenhouse/studio. We keep a greenhouse warm all winter long with no fire all night. We do burn it constantly from morning until night.
I built it in 2013.  29" high  28" wide and 13' long. Brick encased cob and rock filled.  I'll include a photo
The one thing that I will suggest, that is Not in the book. Is not to make the 180 degree turn using the pipes. Instead put a small brick box.  Run in straight from your core and then straight back towards the outlet. Add a cleanout into the box. This will help your flow greatly, that 180 is a ten foot deduction due to drag.
Another tip would be to build a five minute riser rather than a perlite clay riser.
A candy thermometer inserted in your stove pipe as it leaves the  mass (known as a dragon breath monitor) will give you true readings of gas temp., much better than external pipe temps.
I would put 1/2" cement board on top of your plywood .


RMH-rebuild_70.JPG
Early version
Early version
20200810_185217.jpg
Newest version
Newest version
20200810_185240.jpg
Dragon breath monitor
Dragon breath monitor
20200810_185309.jpg
[Thumbnail for 20200810_185309.jpg]
 
Richard Kniffin
Posts: 21
Location: Vermont, USA
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Thanks for the photos and info, Thomas!

Yes, I may try to incorporate stone wherever possible to increase the mass compared to brick. I think I have access to free brick though, so it's very tempting, as long as I can get enough unbroken pieces! It looks like earth fill could work instead of cob in many places, too, reducing labor requirements on that end. With the Phoenix 8" RMH that I mentioned, they used earth fill.
 
thomas rubino
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Posts: 3666
Location: latitude 47 N.W. montana zone 6A
994
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Yes your fill can be about anything as long as it is contained.
Muddy muck from any backwoods road .  Just leave no air space.
 
gardener
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Location: Upstate NY, zone 5
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Earth fill (depending on the particular earth) would likely be a bit lighter than cob, and be more insulative since it might be looser, especially if it is sandy. Rammed/compacted earth may be fine. Stone, especially granite or similar, would be more dense than common brick and hold more heat in a smaller volume. You have plenty of good stone in Vermont Brick for internal fill has no need to be nice square pieces, as long as you have cob to fill irregularities.
 
gardener
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Richard Kniffin wrote:
The floor is made of 4'x8' sheets of 1/2" plywood, supported by pressure-treated 2x4's every 16", resting on a concrete foundation. I assume the concrete is 6" thick... hopefully.




Well Richard. I'm not too keen on "i assume"  If your 2x4 are on piers instead of laying directly on concrete. It will not be very strong. Pretty sure your plywood will sag under the weight. I think you'd need to lay 2 or 3 cement boards on top of that to avoid creep.

My best advice, cut the plywood, and pour a proper insulative slab. Something like clay balls concrete, or crunched pumice. Cut your joists, link these with rebar. And you will have a sound situation. The first thing to do, i think, is to check how your floor is really built.

A slab, as described, is .5 cubic meter, weights in the 800 kilos range, and you'll need 80 kilos cement approximately. If you go in the 40sqft footprint range.

That's 4 or 5 concrete mixer loads.


Hth.
 
Richard Kniffin
Posts: 21
Location: Vermont, USA
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Hi Satamax, the 2x4's are definitely supported directly by a concrete slab. I assumed it was 6" thick, but the original builder tells me it's actually 4". Knowing for certain that it's a concrete slab, do you still make the same recommendations?

Realistically, I will be working quickly to finish the project before winter and I'm not likely to pour new concrete. Best wishes and thanks for your thoughts!
 
thomas rubino
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Location: latitude 47 N.W. montana zone 6A
994
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Hey Richard;
Your 2 x 4's are sitting on the 4" slab . Your good.
Build away but do put some concrete board down over the plywood.
 
Satamax Antone
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Richard Kniffin wrote:Hi Satamax, the 2x4's are definitely supported directly by a concrete slab. I assumed it was 6" thick, but the original builder tells me it's actually 4". Knowing for certain that it's a concrete slab, do you still make the same recommendations?

Realistically, I will be working quickly to finish the project before winter and I'm not likely to pour new concrete. Best wishes and thanks for your thoughts!



Hi Richard.  Well the builders over in the US make me mad with those building techniques. Over here nowadays, it's all poured concrete. Floor and walls, even the gable ends. Foundation had to go 90 cm bellow ground level. And this is a concrete apron slab.  A first floor slab will usually be 8 inches.

When i hear 4 inches of concrete, i can just wonder. At least, is there rebar in it?

Anyway. Now, you have to address the plywood side of the problem.

A 16 inch wide section of 1/2 plywood between joists spaced at 16, (256 square inches)  if i follow my wood charts, can support 275 pounds of evenly spaced load. This is under your  expected load. So you definitely need  a reinforcement on top. Cement board seems to be the usual material of choice.  I like things to be strong. Myself, i would go for  two layers, if not three. For another reason as well, fire spacing.

If i follow my local code. No fire, nor smoke path should be any closer to wood, than 18cm, That's about 7-1/8 inches. I would go further.  There is also an advantage in raising the feed tube. You don't have to kneel so much.


Hth.
 
Satamax Antone
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Richard, on a side note, i build a lot with wood, by trade; And i hate concrete, or at least making it. But making and pouring half a cubic meter of concrete is nothing. 3 to 4 concrete mixers full, depending on what mixer you have. That's a half day job big maximum, if you have all the materials on hand. I know, most of the us citizens don't have a concrete mixer. Over here, there is one in nearly every garden.
 
Richard Kniffin
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Location: Vermont, USA
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Hey Satamax, very interesting to hear about concrete in your area- it's true, in the US people generally don't have their own small mixers. When you said "concrete mixer", my first thought was a huge mixing truck! On the smaller scale, we sometimes use plastic buckets with handheld drill-powered mixers.

The floor of my concrete foundation is 4 inches thick, but I believe the walls are 8 inches wide, and I'm sure they have rebar embedded inside. In my area a foundation has to be even deeper than yours, 4 feet deep, to avoid frost damage, but luckily we don't have to worry about earthquakes.

Thanks for the information from your wood charts, that's very helpful, and I appreciate the metric/english conversion in all your numbers! Hopefully I can become fluent in both, as you seem to be. Regards,
-Richard
 
Richard Kniffin
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Follow-up:

Douglas Alpenstock asked about codes and insurance... there are no building codes here, and my insurance agency says that the insurance company, Traveler's, didn't know what a rocket mass heater was, but eventually they found someone who does, and Traveler's said no problem, they'll insure that. I'll be asking for more details to make certain that I can build it my way, and if they need anyone to inspect it, but so far I've been told to just go for it!
 
Richard Kniffin
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I've laid out the footprint of my build and started on the combustion unit, but realized I should double-check with some experienced people about the air channels underneath...

I read today in Ianto Evans' book that air channels over a wood floor should be 4" high, meaning a regular brick on edge. I already laid my bricks on the flat for greater stability, so the air channels are only 2.25" high. Have I made a big mistake? Does everyone use 4" as the standard for air channels?

One option is that I can finish the combustion chamber and burn it carefully, checking how hot it gets underneath and seeing if it's an issue.
Another option is jacking up the whole thing above my air channels and making them taller.

Here's a full description of my insulation and protection under the combustion unit so far, working from floor level upwards-

.5 inch durock to distribute weight onto the plywood
2.25 inch high brick air channels exiting front and back, as well as side to side. Channels are all 4" wide.
.5 inch durock
2.75 inch clay perlite. I was aiming for 3 inches but after leveling the firebrick on top, it was lower. Ianto recommends 4", other people recommend 3"
2.25 inches of firebrick for the floor of the burn tunnel

I omitted the aluminum foil that some people recommend, because I think it will end up getting nicked, broken, and shredded when I vacuum the air channels, not providing any protection years own the road.

As I type this all out, it feels like I should lift the unit up enough to create taller air channels... before it gets heavier. Please weigh in if you have experience or opinions!
IMG_3104.JPG
2.25"H X 4"W air channels
2.25"H X 4"W air channels
IMG_3105.JPG
burn chamber in progress
burn chamber in progress
 
thomas rubino
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994
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Hi Richard;      
If this was my build I would say you are good to go.
In my opinion the 2.25" air gap between 1/2" concrete board is plenty of safety factor.
 
pollinator
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I know I am a 'johnny come lately', but why not remove all the timber under the RMH and start from the concrete base.
As a Civil Engineer I dont see any advantage of leaving the wood on place.
 
Satamax Antone
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John C Daley wrote:I know I am a 'johnny come lately', but why not remove all the timber under the RMH and start from the concrete base.
As a Civil Engineer I dont see any advantage of leaving the wood on place.

I tried to advocate for this, to no avail!

Richard, I think i would leave it this way.  But you have to be aware that the most  problematic thing with a rocket on a wooden floor, is conduction, heat in that case, is transferred in all directions, "it doesn't rise" As you usually hear.  And bricks laid flat, have a greater area , so more conduction occurs. Mind you, they also give more strength, as more surface area, means more load spread. Than vertical bricks. My best advice would be , throw a thermocouple or two just in contact with the wood, under the burn unit and plenum. So you really know what temps you have there. More than 60C°, and you get wood charring after a while, which is a fire hasard.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.553.9372&rep=rep1&type=pdf

https://www.google.fr/search?source=hp&ei=oSlnX7nPF8Wmab2Qt4gN&iflsig=AINFCbYAAAAAX2c3sSZAp7t5kob2J-7pg8ILAg8-QWoz&q=low+temperature+wood+charring&oq=low+temperature+wood+charring&gs_lcp=CgZwc3ktYWIQAzIHCCEQChCgATIHCCEQChCgAToICAAQsQMQgwE6BQgAELEDOgIIADoECAAQAzoKCAAQsQMQRhD_AToECAAQEzoICAAQFhAeEBM6BggAEBYQHjoICCEQFhAdEB5Q5QlY6z5gy0FoAHAAeACAAYUBiAHWEpIBBDI1LjSYAQCgAQGqAQdnd3Mtd2l6&sclient=psy-ab&ved=0ahUKEwj5o-uXvvfrAhVFUxoKHT3IDdEQ4dUDCAY&uact=5
 
Richard Kniffin
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Yes, I agree that removing the wood and starting from a solid fireproof base is the most foolproof, intelligent thing to do. However, I'm building on top of the wood because my father owns the house and doesn't want me cutting holes in it just yet! With this minimal-impact method, the construction is reversible, if the RMH is not satisfactory.

Satamax, I'll definitely monitor the temperatures near the wood. Either way, it could turn into a good experiment to provide safety info for others.
 
Satamax Antone
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Richard, have you factored the cost of replacing 4 or 5 sheets of ply, and a few joists,  if it proves to be a failure? Better or worse than the risk of the house burning down. of being able to use the proper materials, not to worry about sagging wood under heavy mass. ability to add mass.
 
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