Locating a rocket mass heater over a hydronic heat system in cement slab

Hello, I am building new construction with a radiant floor system in a large shop building. I want to install a RMH as a back up to the radiant floor system. Are there any special consideration in placing the RMH directly on the slab? Should the hydronic piping avoid the area directly under the RMH. I am signed up for the RMH workshop in Sept, but I will likely be laying out the pex tubing and pouring the floor before then. Thanks for any help.
Colin

Hi Colin, How did that heated slab project go? The Europeans have been doing heated slabs for 40+ years now. Back in 1984 I was working in HVAC and on several occasions specified hydronic slab or perimeter heating. The 'on-the-wall' gas fired pass-thru boiler was all the rage at the time, and still is. I do not see why you cannot adapt a coil to scavenge heat from your RMH. Good luck with it, Cheers, Mik

Colin,
Better avoid piping of whatever kind under the RMH. Heat can accumulate there beyond 212 F running the risk of failure of the pipes when it's made of LDPE. Concerning the weight, you'd better investigate the lbs/sq ft you need for the mass heater.

Not much point heating under the rocket heater anyway, is there?

The use of a RMH for water heating is like if you would put a heater loop on your automobile, so that when you got home from a drive, you could hook the engine block up to your water heater. The Primary function of the engine is to move the car, the heat generated is a 'byproduct' of that use. This analogy is such that the RMH is firstly a radiant heating device, and secondarily a convection heating device. Now, when you put your derriere on that heated bench, you have conduction heating. You need to make a decision whether this unit is Primarily used for heating water....then, you MUST consider the need for limiting the heat density, to less than 212F/boiling point (unless the water in the piping is pressurized...and THAT's another ball of wax completely..). You DO NOT want, under ANY circumstances that the water in the stainless steel water jacket, or, boiler piping EVER exceeds boiling point...as then you have a steam boiler...DON'T go there, that can be very dangerous. There have been a few who made a masonry heater (or RMH in your case..) and after the main firing chamber ran the flue so it was surrounded by sand , and in the sand was embedded a copper water coil. The trick here is to either make sure the flow is such that it does not get above 180F, which calls for temperature differential control/s, by pass valving that electro-mechanically controlled, or some such arrangement. One way to do something of the sort is to have a large (1,000gal+) holding tank, that its purpose is to 'hold' and store the heat during firing. In that tank you have a separate exchanger coil submerged, so that in the shop at large, when a thermostat calls for heat, a simple Grundfos circulator pump moves the water thru the slab piping. In this way you segregate the two loops, the perimeter heating loop, and the loop that transfers the heat from the RMH to the holding tank. Either way, or whatever you do, you WILL need to have a source of 120vac handy, or, a dependable solar cycle unit...as you can find 12vdc controls, and circ pumps for such an application. OH, what size is this building, what is the R factor of the walls, how many windows, etc, etc....WHAT is the heating load and loss factor, or have you not figured that out yet..?? THAT is where you should start first. Any reputable HVAC contractor should be able to do a Heat loss survey for you (I used to...30+ years ago do them by hand..., it is much simpler now with computer programs..), Good luck, Cheers, Mik

PS....heated slab.!.., what is your heating degree days, climatic conditions, and are you planning on perimeter insulation and thermal breaks betwixt the perimeter footer and wall and the actual heated slab..?? THIS is very important, when I did do heat loss surveys, for the average 'town home' on an 'on-grade' slab, the perimeter that is exposed to the outside is considered to make up 10 -15% of the heat loss of the entire structure. I hope you have figured that in, more on that later, look at US Concrete specs for info, easily found on the 'net. later, Mik

Thanks all for your comments. I am putting r19 insulation in the walls and r 30 in the ceiling and perimeter. getting the heating load and loss factor is a great idea. I am having a contractor build the shell of the structure and I will be finishing the interior so getting those figures will help me implement that in a more informed way.

The building won't be started untill mid August so I still have some time to figure out the details. Again i really appreciate all the advice! I've been meaning to start a thread on our whole homestead project. Its raw land with power and water at the perimeter so we have a lot of work ahead of us.

Thanks again
Colin

Colin S. : Lots of good advice ! Definitely ask the contractor that you plan to use, about "Bridging'' in general, and specifically about heat loss from bridging at the area
where the Slab foundation intersects with the outside walls, and how water proof the insulation(s) will be below grade.

For the latter potential problem I want to hear about an ''Umbrella'' or other water barrier that keeps the soil next to my house dry -of course this can not work unless
the area around the structure is landscaped to drain water AWAY from the structure, ''to daylight'' 360 degrees !

After careful Re-reading of your post I see that you are scheduled For the RMH workshop in September, good luck enjoy it and be prepared to be amazed ! Ask Ernie
and Erica What they think of using an RMH for water heating - I am sure that they will tell you that this is even above the level of training of the Workshop !

There is much good information to be learned about building housing type structures available to you From the good forks at Cold Climate housing - THis Organization is
an Arm of the University of Alaska - Fairbanks ! You and i know Cold - - - - They Know COLD ! Just google the topic Cold climate housing they will pop up 1st ! !!!

Mike Ross raised an interesting point ! I can take a different look at the question of Energy Storage in the personal private vehicle that you park in your drive at night !
If you are connected to "The Grid" the power that you buy between 4:30 -7:30 PM each evening is the power that cost your utility the most to make!

Now, the local pizza place may offer a 3 P.M. Special, or a weekender 11 P.M. special, and they never raise the rates for a Pizza during their prime time 6 P.M. to 8 P.M.,
But both they, and the utility wish they could !

As your car stops in the drive, it arrives with a boat load of excess energy that instead of being Planned for, captured, and used ( Heat energy trapped in the engine
block or stored in the engine cooling equipment- or full banks of electrical energy- from off peak charging ) Is actually reused in less than 1 % of North American homes!

Re-using the stored energy whether on-grid or off-grid should be a goal ! However -

Any plan to use the heat storage and heat transfer characteristics of liquid water must deal with a couple of issues related to these plusses. Any use of water to rapidly
absorb Heat Energy from the Combustion Zone of the Rocket Mass Heater RMH is doomed to retard the operating Temperatures of the RMH s Combustion Zone,re-
sulting in an inefficient dirty smoky fire.

Or Commonly the explosive and potentially Catastrophic release of energy as the Water and the Heating coils designed to transport/circulate water at a point well below
212 degrees F fail as the water Flashes to Steam ! Think Boston Marathon Bombing with more 2nd and 3rd degree full thickness whole-body burns !

While Mike Ross did cover the need for a circulating pump to keep a continuous circulation of water flowing through the heating coils in a well designed system, he did
not mention the increased danger of catastrophic failure of ANY Solid Fuel ( Wood or Coal ) System when a power failure occurs after the internal temperature of your
Rocket Stove Stabilizes somewhere above 1800 Degrees F !

If you could find a heating contractor to install such a system for you, you would still need the equivalence of a 2 year heating/cooling Degree to be able to understand
what to do to deal with the multiple potential emergencies possible in this situation ! I would never trust MY brother-in-Law to house-sit such a system !

for the good of the Crafts ! Big AL !

We are looking at a similar project.

I too have been researching the rocket mass heater idea for a few weeks, to find a more efficient way to heat with wood instead of the old stove we find our selves mostly using, since our house is surrounded by woods.

We ordered "the book" and think we have a generally good grasp of the concepts. However, we already have a house with propane powered in floor radiant heat water filled pex, so building yet another mass storage device wouldn't really take advantage of the resources we have available.

We are trying to find those with experience using water as the heat storage with wood and are finding the examples to be a bit thin.

I found a discussion on this forum about Geoff Lawton's design, but that is an outdoor unit and not really suited for indoor use. I have seen nothing mentioned of the videos by Van Powell which heat a coil of water on top of the rocket barrel and use thermo siphoning to move the water, preventing pressure problems. His experimentation and observations seem rather compelling, though I am not sure I would feel comfortable leaving a fire open to a room to burn unattended as he states is a goal in the last rocket fireplace video. https://www.youtube.com/watch?v=EVpseNZI9vY

Having lived with a water heated heating system for years, it doesn't seem it would be that difficult to figure out how much water at a time a particular rocket stove could heat with even a full load and make sure you have that much available at all times. Either a copper coil with thermo siphoning as he has in the video, or a larger tank on top that heats a larger amount of water at a time to be sent to the exchanger in larger quantities. Since we have many sqft of floor to store the water in as well, there surely is a way to figure out how to get it to work. We want the water to go at least as hot as 140 occasionally, but no more than 180 on a regular basis. It would seem a simple matter of math, then add a bit more water (mass) for a safety's sake.

I have read a few cautions about Legionella bacteria and from inadvertent experience (open systems are no longer best practice) it does seem that as long as the water is heated up to over 140 right before it is released into a tap, the bacteria is killed. In 15 years, no one has ever gotten sick in spite of the fact that the water sits stagnant all summer and eventually gets cycled back when the floor heat is turned on for the winter.

However, now that I know there is a possible concern, I am inclined to separate the heating water from the potable water. I would like to find the best way to do that heating it with wood inside the house.

Having tried many heating methods, steam radiators, wood stove, in floor radiant, forced air furnace and heat pump, we prefer them in the order I listed them in. We were disappointed to find we do not like the slow constant release of heat with a whole house radiant floor as the sole heat source. It isn't able to respond fast enough to the varying temperature changes a typical day holds here in Virginia. This seems to be an issue noted with masonry heaters as well. This how we ended up using mostly our wood stove and leaving the in floor system off in all but a few distant rooms. We find we prefer having colder bedrooms, medium working areas and warm lounging areas, rather than one uniform temperature throughout the house. This was hard to achieve even with separate zones. The temperature does vary throughout the day, but it is so slow to respond it often can't cool/heat fast enough.

The rocket stove mass heater seems like it offers both the immediate heat factor from the barrel and the storage factor as well. If we can only figure a way to attach it to the mass storage we already have I think we would have a winner. We still plan to leave some zones off altogether.

We are adding a solar water heating component that we picked up from someone removing it from their house. (Does make you wonder if it worked so well, why they were removing it.) The previous owner said it did well at pre-heating the water, but I am sure we will need to augment it. (Separate post in solar forum) After talking to someone with this system we are now leaning towards hooking it up to the potable water heater instead.

So any suggestions on how we can best utilize what we have most efficiently and comfortably?

While the idea of just "trying" a tried and true design sounds like a good start, I know from the many experimental building ideas already in the house, a thing that takes as much time and energy to build as a well designed rocket mass heater would not be something easily "tweaked" once it is in place. After hearing it takes up to a year for the cob to really dry, one can expect to be looking at even the experiment as being a long term project. I want to be reasonably sure we are pretty close to what we want.

Adding a cooking component, as is seen in the masonry heaters, appeals to me as well, so for now I am thinking of creating something like the Van Powells rocket fire place with an oven between the top of the barrel and the water coil. The oven would also absorb some of the heat before it got to the water coils which should reduce the worry of overheating the water further.

I am envisioning building the oven out of cob on the outside with metal inner walls. The heat would flow through an empty channel between the walls up and exit out of a top hole to heat the water coils on top of that. Each piece could be "modular" and removed if not used or some part needed replacement. Looking like the oven in this video but with copper coils on top to heat water like the Van Powell design. Also with no hole in the top of the drum, so no smoke in the house.

Since we already have the pex in the floor, I figure we need to insulate underneath and/ or use mostly insulating cob, with a thin structural finish coat until we get to the oven bit, to reduce the heat on the floor.

What do those of you with experience think of combining those ideas?

I have an in floor system and have been heating the water with an old Kentucky wood gobbler. I've been looking for a way to heat the water with a rocket stove. So far I haven't found anything I like. Everybody throws up their hands and screams "boom squish" when you raise the subject. The stove I have has a 600 gallon water jacket that goes around the firebox and some flueing that acts as a heat exchanger. I'm looking at building a J burner in front of that furnace and exhausting it into the firebox to use that unit as a heat exchanger in place of the clay/sand thermal mass. I'll open the door and fit an insulated panel in the opening with a hole for the tube from the burner. If it doesn't work I can just pull the tube and panel out and go back to using it the way I have been.

I'll be using a 55 gal. drum over the riser and I want to clamp another drum on top of that riser drum to heat water there too. I'll position it so it has the same water level as the water jacket tank. It will be non pressurized, like the water jacket tank, and I'll pump water through it, like the water jacket tank. If it gets too hot it can boil off the heat, like the water jacket tank does now. Again, if I can't make it work I'll take it off.

The firebox on this furnace is almost a cubic yard. I put a couple of wheelbarrow loads of wood in it and it will go for 12 hours. It smokes a lot when I first light it and it burns for a while and then smolders for a while. so it's not very efficient. I expect to spend more time feeding a J burner and having more fluctuation in the building temperature. But, I have been putting an awful lot of time, and effort, into loading, hauling, cutting, splitting and stacking firewood. I'd rather spend my time nursing a little fire than working the wood for a big one. And, firewood is getting harder and harder to come by.

I have another wood stove (a Fisher) inside the building that I'm also modifying for more efficiency. I'll be using it more for the quick response... and having something to back up to.

I'm in a big metal building that was intended to be a workshop, not a living space. The proper fix is to build the house that I had intended before the economy went south. But, like the rest of the 99% I saw my wealth pissed away in Iraq, or looted by the bankers, and now I have to get by as best I can with what I've got.

I tried doing a heat loss survey and that's a joke. Everything that you can find a R rating on has a bunch of weasel words in the fine print that negates whatever claim that was made in the large print. Or, the claim was a flat out lie. The building manufacturer assured us that the building would be airtight. Yeah, Right! I spent big bucks on a spray on epoxy/ceramic coating, that did zilch. Then got far better results from a glue on foil foam foil (for 1/3rd the price), that I'm sure did more by blocking air infiltration than from the R factor it added. It doesn't do much good to crunch numbers you can't trust.

I'm sure that if I insulated the footer, with a landscaped umbrella, it would cut my fuel bill, but, taping bubble wrap over the windows, or adding more dogs to the bed at night, are the kind of projects that can happen. I can modify what I've got in place a lot faster, easier, and cheaper than building a RMH from scratch (not to mention that I don't have a good place to put it). I know that the modifications won't be as efficient as a pure RMH, but I'm hoping they'll be enough more efficient that they'll pay for the effort.

Carrie and Tom S. : When in trouble, or in doubt, run in circles, scream and shout BOOM - SQUISH !

However, If we are Not willing to embrace change we will see ZERO Improvement! These two lines are inherently opposing views! But, It truly comes
down to a certain level of Aptitude, Skills, and Knowledge! These specific Attributes can only come from experience ( including Book Learning ) !

Given that you can have a water system malfunction at any time, and that over time all water systems Will fail, using any solid fuel heating system
combined with a water system has inherent risks. It is MY Understanding that pressurized solid fuel systems are banned in the Entire British Isles.
A good start on enhancing your learning in a safe way, would do an online search for a Regular domestic hot water heating system supplemented
with a Solar hot water heating assist ! Pay to get it professionally checked for correctness, and then learn to sketch the whole system in its entirety
and do it perfectly correct 10 Xs out of 10 Then learn what happens when A valve is closed, one valve at a time, then figure out what happens if a
pump fails! Then come back here to these pages and tell me you want to know that much more that you will need to know to KNOW that you are
operating a solid fueled water heating system correctly and safely!

Hint, if I am going to leave town i am not going to ask my brother-in-lay to watch it for me !

I will not personally propose a system here in these pages knowing that the improper closing of a few valves Will create a pressurized system !

Anyone questioning the inherent massive danger possible from being shown how to operate a fully charged solid fuel heating system can look no
farther than the situation today at U-Tube. Today U-Tube is so hungry for video content they will accept ANY Video for public showing with no system
of safety checks themselves or through appeals from knowledgeable concerned citizens !

I can recommend One video from U-Tube It shows a 'myth buster episode' where they blow up a house with a 'faulty hot water heater' !

W.O.W. this turned out much more negative than I planned to make it ! However There is currently a workshop scheduled soon that promises to
show A system for producing safe domestic hot water heating, which I will share here if you understand that I am not personally vouching for a
system I have not seen ! For the good of the Crafts Big AL

Late note : As promised, the information you want is posted at Rocket Mass Heater Workshop, with emphasis on heating water! and is located in the
rocket stoves forum here at Permies.com A.L.

I don't know where "pressurized" came into the discussion. The tank on the system I have is the equivalent of an open pot, there is no valve you can close to create pressure in it. The same with the modification I was proposing. Half of my brothers in laws I would trust to boil water in an open pot. The others? Well, lets not even go there. Anyway, There are other options I'm debating.

They quit making this model of furnace over 15 years ago and the guy who had it said it has been around a lot longer than that. He replaced it because it sprang a leak and the experts at the company that built it said that it was impossible to patch. Right! The leak was on the bottom, heat rises, and, epoxy is rated to 190 degrees. I smeared epoxy on a metal plate and slapped it over the hole, then ran a couple of sheet metal screws in, to hold it while it cured. It's been 4 years and it's still holding. Okay? Stainless steel rusted through, and I'm sure that spot isn't the only place effected, the whole bottom has to be pretty thin. That furnace has been running a long time without exploding, and with that patch I don't think it would be possible to pressurize it even if it had a valve for that. It might start leaking some hot water out the bottom... but nobody has been under it since I put that patch on.

It just seems like a rocket heated water heater should be possible... and there is certainly a demand.