5 ton RMH on basement floor

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

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.

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.

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

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

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).

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.

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 .

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.

Yes your fill can be about anything as long as it is contained.
Muddy muck from any backwoods road .  Just leave no air space.

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.

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.

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!

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.

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.

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.

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

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!

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!

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.

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.

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

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.

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.

Certainly, building strong from the beginning is the best option. As I said, not every decision was up to me. You'll be pleased to hear, Satamax, that I reinforced the flooring with an extra layer of plywood and 2 (sometimes 3) layers of cement board. When I'm running the heater consistently, I'll update this thread with safety data on heat reaching toward the floor.

I'm about to build my cleanout areas, using 8 inch cast iron doors. How do you folks attach metal to brick? Does the same old recipe of clay-sand work?

I know that expansions joints can be helpful, but I haven't seen clear instructions about the basics of sticking a metal door to brick securely without typical mortar.

Thanks for your time!

I appreciate you sharing the build, Richard, and everyone who is advising/weighing in/giving opinions. It's a wonderful RMH learning thread and I hope you keep on sharing the build, Richard, right to the very end and beyond, after it's been lit and going for a while.

Metal doors are usually attached to metal frames that are shaped to fit the brick work and maybe even have bolts through the steel and into the bricks hemselves.

Another trick to attach metal parts to  bricks, a glass fiber, or fireproof joint behind the lip. And holding it to the backside of the
bricks with restraint strap you know, the coiled, pierced metallic straps? Held with the smallest type of rawlplug you can find.

You can also use some type of L bracket pinching between front and back of the bricks.

Hi Richard; Lots of options.
I just used clay sand mortar on my cleanout door. Its been 2 years and nary a problem with it.
Also you can drill and attach with "Tapcon" masonry screws, Home depot sells them and drill bits. This is how I attach my batchbox doors.
Last choice is concrete mortar. That far from the core your temps will be low enough that concrete will last.  

Thanks everyone for the ideas, and thanks Annie for the encouragement :) Here are some pictures of different aspects of the project so far. I've burned the unit 3 times- 1st time it worked like a charm, 2nd time it got much hotter and burned off lots of coconut oil from the barrel! 3rd time it took awhile to start, but ran smoothly and didn't put out any more coconut oil. Next time I fire it, I'll have the horizontal piping all hooked up.

Now that I've hooked up the complete length of pipe, I've had one good test burn, and then run into problems with other tests:

1. Normal trouble trying to make a cold start with a new RMH (it had no mass attached to the pipes, so it was cold even after my test firings). Bleh, lots of smoke! I've had success on some days, failure on others, so the weather and temperature differential from house to environment is the first obvious culprit. But also...

2. Leaks developed around most seams (barrel, pipes, etc.). When the fire fails at its worst, then smoke backs up through every gap it can find. I probably used a cob mortar mix that had too much silty soil, too much clay, or both. Once I cover everything with extra inches of cob, I hope the system tightens up permanently.

3. My barrel lid with fiberglass gasket and ring clamp also leaked smoke! The first gasket I tried came loose after a few firings, so I probably didn't cement and cure it properly. The second gasket with better cement did not help at all though, so perhaps one problem is that I was only hand tightening the clamp around the lid. Maybe it will be truly airtight now that I've used a wrench to tighten it.

I'd love to hear other people's experiences or advice if you've run into these same problems! I've lost plenty of sleep trying to problem-solve, stressing about the coming winter and wondering if the effort is worth it, wondering how safe I can keep my family with this smoking contraption!

Trying to remain optimistic that all will be well when the outside temps fall- then I'll pre-heat the chimney to start a good draft, and try again (with some mass in place to hopefully catch the heat and keep things warm for future firings).

Hi Richard,  You are certainly not alone in your plight to get your dragon to flow in the right direction. This can be a very distressing time, but doesn't need to be if you know that this is just the teenager years of rebellion, smoking, and "life sucks" comments we all do growing up.
A fellow rocketeer Mark Dumont is going through the same woes as you are. Catch his thread here: Build-Rocket-Mass-Heater-Rocketingand pay attention to the suggestions made.
If you still have questions, come on back and we'll do our best to help out.

Thanks for the words Gerry, they're very helpful.

Now don't tell anyone, but I may have placed two bricks inside my 180 degree bend days ago to block it off when I was using the electric heater to prime the chimney pipe... and then forgot about them when I went to fire the system up! With those bricks removed, it sure works a lot better!

This is certainly a humbling initiatory process :P

On the serious side, I probably forgot those bricks in there due to stress and lack of sleep. My wife says it's like a surgeon leaving a scalpel in their patient's leg, and I agree :)

Best wishes and safe heating to all of you other rocketeers out there,
-Richard

Don't worry Richard, your secret is safe with me. Everyone has a story to tell and its nice when it has a happy ending!  

With colder temperatures this week, and thinking I had removed a critical blockage in the pipes, I had high hopes that my in-progress RMH would run better... but I've only had only moderate success. I'm working on covering up the pipes so they're not leaking heat and taking strength away from the flow of air, but there are other factors causing big problems. Now it's time to reevaluate everything that I'm aware of and hopefully fix it-

1. I made some small mistakes in the J-tube, leaving the brick section of the riser 1/8" narrower than I intended. Along with a feed tube that's 1/8" larger than intended, this is surely a problem, since the feed tube will be sucking in more air than it can logically push through the smaller areas.

2. My wood is not short enough, or else I'm feeding it wrong, because it stands on top of coals from the previous batch and ends up being above the rim of the feed tube. How do you feed your J-tube without piling on top of a growing mountain of coals? I tried pushing coals into the burn tunnel, out of the way, but this often causes the air to come back out of the feed tube with smoke and flame.

3. Condensed water leaks back down my chimney. This will rust holes in my chimney eventually. Has anyone else had this problem and solved it? Weeks ago I had put a piece of window screen across the top of my chimney to keep birds out- now I'm going to remove that screen and see if helps. Of course bone-dry wood will create less steam, but I can't believe that everyone else with an RMH is using perfect wood.

4. When everything is going wrong and I close down the feed tube to keep smoke from coming back into the house, that smoke still swirls out of the band of the barrel lid. This is even with a second attempt at air-proofing my barrel lid. The rim of the barrel has a couple small imperfections, but I thought the fiberglass gasket would easily cover them up, and tightening the band around the lid would squish it all tightly closed. I guess not. Has anyone had this problem? Can barrel lids truly become airtight, so they never leak even in the presence of smoke being forced up against them? You'd think this would be the easiest part of the whole build! Need a new barrel? The one I'm using is basically brand new, only used once to transport a huge bag of orange juice concentrate.

5. Because I'm building on a wooden floor, my burn chamber is raised higher than usual, then the manifold has to push hot air down to get it to the opening of my horizontal flue. I don't know if that's asking too much of it. Some days it all works fine.

6. I have a 180 degree turnaround in the horizontal flue, and I built a box out of brick rather than using pipes for this section. The box might be too big, slowing down the flow of gases... I haven't seen specific directions on how to make one so I just built it intuitively. Would you build this turnaround by carefully maintaining the dimensions of the rest of your pipe?

I've spent 20 years using at least 7 different woodstoves, and this RMH is really killing me. It burned perfectly when I had a short, naked pipe going straight to the chimney, but now it feels like I'm wrestling with a dragon, and each day, I know I'll have to wrestle with it again. Best wishes to all of you, please comment if you see any solutions or possibilities I haven't thought of. Hope your heaters are running better than mine!

Hi Richard,  

1) I wouldn't worry about it. Think about it this way, every time you put a piece of wood in the feed tube it changes the csa (cross sectional area) dramatically so its never going to stay the same anyways.

2) I'd let the coals burn down before adding more wood. Best not to push the coals down the feed tube either as you've experienced. They don't have quite as much of the time element to help combust the gasses when pushed further down either. However, if you need to step out for a while, having some short pieces on standby are always a good option to top up the feed but without sticking out the top.

3) Window screen is too fine. It will plug up very quickly. If you do have to use a mesh of some sort, a very coarse one will still do the job. Strongly doubt it has anything to do with condensation.
Can't remember if you have an insulated chimney where it exits the building on the roof? An un-insulated chimney will condense moisture very quickly. As long as your above the dew point, meaning your exhaust temps are high enough not to condense and form moisture, you should be alright.    
I used to have a problem with condensation too and a small hole drilled into the pipe at a bend in the pipe allowed a weep hole for it to escape.

4) It sounds like your barrels are firmly enough attached and sealed. Leaking should only happen on a very cold stove only at startup. Once you increase the delta T of the exhaust, all those cracks will be sucking inwards. A T&G Dragon breath monitor (aka candy thermometer) inserted into the vertical pipe about head level will allow you to instantly see what your temps are.
Surface pipe temps will be lower than the central flow of gasses in pipe so will be inaccurate.

5) The extra distance the gases need to travel downwards shouldn't be a problem for a properly running stove. A bypass would solve this issue for sure during any finicky times.

6) This box can be made any size. However, if your flue gases are already too low, a bigger box may suck out more heat and not allow enough to get to the vertical pipe and create enough draft.

window screen is heresy!

Basically, every little square is creating it's own drag. Through the boundary layer effect. No wonder it doesn't work. Remove it. Birds don't nest in chimneys.

Richard;  
How tall is the box at the 180 ?
I had made mine 15-16" tall and deeper ... It did not draw well, just like yours.
I had to dig it back up and shrink that bell down to barely larger than than the 180 took up.
Now it is working great.