Feedback Request: 6" BBR w/ Copper Pipes in the Bell

[jonnoh]

Hi Everyone,

I would really appreciate feedback on a stove I'm in the process of designing.  This stove is intended to serve as the primary heat source for a 2,000 sqft insulated shop.  The shop has ~1,000 linear feet of 1/2" PEX in the floor (5" concrete slab w/ 2" XPS insulation under and around), so I'd really like to use the slab as my heat storage mass.  Tubing is split into two zones, so I'll be able to direct the heat somewhat to the side of the shop i'm using.

The shop will also have a 20 gallon electric water heater used as a 'diversion load' from my PV system.  That system alone won't produce nearly enough hot water to heat the floor, but it will give the stove a head start.

My initial design concept is based around a 6" BatchBox Rocket core spec'd per the batchbox.eu guidelines.  Here's a few details:

  -Core: The core will be primarily constructed from 1" ceramic fiber board, with firebricks in the base and back of the box. 
  -Riser: I'm thinking of using the '5 min riser' approach (1" of ceramic fiber blanket lining the interior of a 8" stove pipe). 
  -Bell: For the bell, I'm thinking of using square clay flue pipes (13" interior width), as I think they may provide a bit of insulation, thus raising the internal temp of the bell a bit.  I also think they look nice...
  -Heat exchanger: Still a lot of thinking to do on this, but my current design uses a series of 1/2" copper pipes run vertically, each linked to a 3/4" 'manifold' at the top and bottom.  I'm thinking of including two exchangers inside of the bell on oposite sides of the riser.  Exchangers will be plumbed to the 20 gal tank.  Will include several 30PSI relief values, as well as an expansion tank.

Here's a few pictures of the design.  
photos.app.goo.gl/SQLLjDFJWT5ktYiy5
photos.app.goo.gl/8w3YbNaHkKq84xvY6
photos.app.goo.gl/9q1aiTdTArMrjXbh7

So, at this point, I'm interested in feedback on 1) effectiveness of the design at transferring heat from the wood to the water, 2) any glaring safety concerns, and 3) design improvement ideas.

Thanks in advance

[Dan (Upstate NY, USA)]

What temperature is your solder going to melt at and rupture a joint inside the stove? With 29psi behind it?

I think you are going to put out your fire in a spectacular way with the copper/solder joints in the stove.

Safer to use steel with relief valves & mechanical joints or heat up a tub of water and have a copper coil in that...

[jonnoh]

Jan 18, 2021 16:56:17 GMT -8 Dan (Upstate NY, USA) said:

What temperature is your solder going to melt at and rupture a joint inside the stove? With 29psi behind it?

I think you are going to put out your fire in a spectacular way with the copper/solder joints in the stove.

Safer to use steel with relief valves & mechanical joints or heat up a tub of water and have a copper coil in that...

Thanks for the quick response Dan.  I have considered black pipe for this.  Wouldn't be too much more expensive, but I'm not sure it would exchange heat nearly as well.

Regarding the solder joints, there would be no joints inside of the bell.  The lower manifold would sit under the bell (right below a sheet of 1" ceramic fiber board), and the top would be about 4" above the bell 'cap' (also made from 1" ceramic fiber board).  This way, the only components of the heat exchanger exposed directly to flue gasses would be the vertical pipes.  Since they're running vertically, i would expect it to have an excellent thermosyphon,  keeping all components (especially the solder joints) well below the danger zone.

Edit: Dan, when you suggested 'Steel', what exactly were you thinking?  Black pipe or something more 'exotic'?  I'm realizing that with black pipe (which is rated for steam), I could probably go quite a bit larger on the diameter, resulting in more water in the bell, and more surface area.  That could overcome the drop in thermal conductivity (relative to copper).  

Thanks again for the suggestion Dan, I'll definitely give steel (or iron) pipe another look.

[pianomark]

Jonnoh,

I don't think clay flue pipes will hold up under the extreme rapid thermal cycles in a rocket stove, especially near the top of the riser. Maybe if it is enclosed and held in place with some kind of outer shell, it might crack and then remain in place, but even then I am skeptical. And then of course then you also lose the aesthetic value.

I would also have safety concerns; to have an effective/safe thermosyphon, you might need larger transfer piping and adequate height differential. Even in an open (unpressurized) system you should have an easy exit for any steam flash. I agree with Dan, you might want to consider heating an open tank of water with coils to transfer heat to the floor. More complex, but more safe.

Matt Walker has plans on his website for a tank-type rocket water heater. Maybe that could be adapted to your needs.

Just my 2 cents worth, other here have more experience, I'm sure.

BTW, Matt has a really cool live stove chat every Wednesday on youTube at 2 PM Eastern standard time USA. (11 AM PST)

[neil]

Instead of a clay bell, what about the traditional steel barrel. Wrap the copper tubing around the exterior of the barrel and instead of solder use compression fittings, wrap the whole mess with insulated blanket.

[Solomon]

For safety sake, I would definitely recommend a non-pressurized system.

I have often toyed with ideas about using a large (as possible) partially or uninsulated unpressurized tank as a heat storage mass and radiator. But the key is putting the heat exchanger in some area where it won't get too hot. I was thinking a water jacket on the lower part of the bell combined with a piping run inside the exhaust end of the mass. You want to design your system for good thermosiphon. Can't have any high spots other than the tank.

Another issue you may run into is the temperature being much too high to offer good heat transfer and instead having issues with the water flashing to steam in the pipes because it's not circulating fast enough. This can be solved, like I said by placing the piping in a cooler area of the system or an area with higher mass, also by increasing the size of the piping and surface area. In engineering, often optimal systems contain infinitely sized components.

You want a system that still works or at least won't kill you, should things get out of hand, power goes out, water runs low, over fire, over fuel, etc.

You also don't want condensation, which shouldn't be a problem in your design.

[josephcrawley]

Jan 18, 2021 17:12:23 GMT -8 jonnoh said:

Thanks for the quick response Dan.  I have considered black pipe for this.  Wouldn't be too much more expensive, but I'm not sure it would exchange heat nearly as well.

Regarding the solder joints, there would be no joints inside of the bell.  The lower manifold would sit under the bell (right below a sheet of 1" ceramic fiber board), and the top would be about 4" above the bell 'cap' (also made from 1" ceramic fiber board).  This way, the only components of the heat exchanger exposed directly to flue gasses would be the vertical pipes.  Since they're running vertically, i would expect it to have an excellent thermosyphon,  keeping all components (especially the solder joints) well below the danger zone.

According to a 70's era passive solar book I have 1/2 is to small to thermosiphon because of the drag. This bears out with my own experimentation. The author recommends a minimum of 3/4. The book is "At home with alternative energy". A fun read!

[Dan (Upstate NY, USA)]

Thicker that copper black steel pipe would work. But...

Safer (and cheaper/ more cost effective in the long run) is to heat up a tank of water at the top and then run coils through the water.
I used to work at a nuclear power plant as an operator so I know the fundamentals of heat exchangers and design.

If it was my garage I'd just put a 55 gallon drum full of water on top of the stove, have a fill pipe at the top with a toilet valve up there to maintain full.
Then I'd run pex to copper coil in the the 55 gallon drum back to the pex. (less efficient heat transfer definitely but more resilient/rugged and fool proof)

You'll find that radiant heat alone will heat the floor 3 times better with a steel bell then monkeying around with two stage water exchangers... but if the garage is large and you want more even heat it's worth the effort to put that hot water through you previous investment in the floor pex and concrete.

[gadget]

Jan 19, 2021 9:01:39 GMT -8 Solomon said:

For safety sake, I would definitely recommend a non-pressurized system.

I can tell you from personal experience, even wide open 1/2 copper pipe is not 100% safe. A dry super heated open copper pipe will explode with steam if water is introduced suddenly. The steam cant escape out the hole fast enough. It happened to me. It blew off a rubber hose and that saved the copper pipe. Sounded like a gun shot. The pipe was 100% open on one end.  The gases expand so much and so fast, the 1/2" hole on the end of the pipe will only let out a small percentage of the pressure. I'm estimating your copper pipe will add about 5% heat to your radiant slab needs.

If you want to get serious about heating water, you want to build a heater that is "inside" the water tank. You will need an induction blower due to loss of heat into the water and heat exchanger flow resistance. Research modern high efficiency wood boilers. Its the only way if you want any useful water heat. A batch rocket or similar stove is completely the wrong design for radiant heat. Maybe good for a shower but not much more then that. Not enough BTU's. You would have to feed it for hours and hours then you would have to much heat from the rest of the heater.

Radiant slabs are the best!!

[jonnoh]

Hi Everyone,

Thanks so much for all the constructive feedback.  I clearly need to go back to the drawing board on this design.  Key takeaways are 1) non-pressurized system is the only way to keep it truly safe, 2) Clay for the bell is bad idea, 3) Rocket core may not be ideal for a hydronic setup.

Regarding the last point, I'll need to think on that a bit more.  It's not clear to me why a rocket mass stove produces enough heat mass of cob, but not enough to heat a a concrete slab.  Seems like its just a challenge of capturing and transferring efficiency.  How many BTU's can a 6" BBR produce?  My heat loss calculations tell me I need somewhere in the neighborhood of 30,000 - 40,000 BTU/hr to keep it comfortable (minus the heat generated by my PV diversion electricity).  Ideally, I could supply a day's worth of heat (~600,000 BTU/day) in < 8hrs of burning.   


Am I dreaming?  Seems like a double barrel stove could do it, why not a rocket stove?  Note that I'm planning to keep this setup inside the garage, so any radiant heat on top of what I can transfer to the slab counts!

Thanks again.

[gadget]

Jan 20, 2021 9:30:41 GMT -8 jonnoh said:

Hi Everyone,

Thanks so much for all the constructive feedback.  I clearly need to go back to the drawing board on this design.  Key takeaways are 1) non-pressurized system is the only way to keep it truly safe, 2) Clay for the bell is bad idea, 3) Rocket core may not be ideal for a hydronic setup.

Regarding the last point, I'll need to think on that a bit more.  It's not clear to me why a rocket mass stove produces enough heat mass of cob, but not enough to heat a a concrete slab.  Seems like its just a challenge of capturing and transferring efficiency.  How many BTU's can a 6" BBR produce?  My heat loss calculations tell me I need somewhere in the neighborhood of 30,000 - 40,000 BTU/hr to keep it comfortable (minus the heat generated by my PV diversion electricity).  Ideally, I could supply a day's worth of heat (~600,000 BTU/day) in < 8hrs of burning.   


Am I dreaming?  Seems like a double barrel stove could do it, why not a rocket stove?  Note that I'm planning to keep this setup inside the garage, so any radiant heat on top of what I can transfer to the slab counts!

Thanks again.

You already have the hard part done, the radiant slab. You are miles ahead. You can also use multiple sources for heat! Heat pumps, solar thermal, wood, etc....

Now if you could have that solar divert to a geothermal heat pump setup that would be nice. You can get way more output by relocating heat vs creating it with heating coils.

[Solomon]

Have you seen the rocket boiler on Peter Van Den Berg's site?

[jonnoh]

Jan 20, 2021 21:29:35 GMT -8 Solomon said:

Have you seen the rocket boiler on Peter Van Den Berg's site?

Absolutely.  That thing is a amazing.  Obviously WAY out of my league when it comes to the design/building process, but it does have some interesting design elements to be considered (ie, flue gas flowing through tubes inside of exchanger tanks, or fire-in-tube, rather than the other way around).

One idea I've been kicking around (probably terrible) is the possibility of diverting flue gasses from the bell of the stove to the flue of a gas water heater.  This would employ a blower which some water heaters already have (typically called "power vent", I think).  3" insulated diversion tube would come out of the top of the bell (where gasses are hottest) and get 'sucked' through the water heater flue, then 'pushed' back to the main flue pipe.  

The big advantages to me are 1) no risk of overheating the water (blower controls how much heat goes through the water), 2) rocket stove can be run completely independently from the water heater, 3) heat exchanger 'flue' will never see smoke from a cold fire.  The big challenge would seem to be diverting enough 'clean' flue gas through the water heater.  On that subject, anyone have an estimate of the typical air-in / flue-gas out flow rates of a 6" Rocket stove.  I'd like to try to find a high-temp blower that is reasonably well matched to those flow rates.

I'm sure there are a hundred reasons why this wouldn't work, but it does seem worth working through.  

[jonnoh]

Jan 20, 2021 21:16:11 GMT -8 gadget said:

Jan 20, 2021 9:30:41 GMT -8 jonnoh said:

You already have the hard part done, the radiant slab. You are miles ahead. You can also use multiple sources for heat! Heat pumps, solar thermal, wood, etc....

Now if you could have that solar divert to a geothermal heat pump setup that would be nice. You can get way more output by relocating heat vs creating it with heating coils.

Multiple sources is definitely the idea, and I'm trying my best to stick to sources that I have in abundance (sun and wood).  The heat pump approach would be a way to increase the efficiency from the PV source (by up to 3x or 4x, relative to resistive heat), but the problem is price.  One of the super low temp air-to-water heat pumps (eg, Arctic Heat Pumps) would be perfect, but the price is out of reach for a shop heater.  

I think a PV diversion to resistive electric heat supplemented by a wood stove of some sort is probably my best bet.  Just need to figure out the 'wood-to-water' peice of the equation.  There are certainly great off-the-shelf options for that (eg, Econoburn wood boiler), but again, the prices are way out of my budget.  I should add that a more traditional outdoor wood boiler is not an option for me, as the shop is not located near my primary residence, and generally only gets used on weekends in the winter.  Definitely don't want to wait for 200 gallons freezing cold water to heat up before I start feeling the heat.

[Solomon]

Jan 21, 2021 6:07:24 GMT -8 jonnoh said:

The big advantages to me are 1) no risk of overheating the water (blower controls how much heat goes through the water), 2) rocket stove can be run completely independently from the water heater, 3) heat exchanger 'flue' will never see smoke from a cold fire.  The big challenge would seem to be diverting enough 'clean' flue gas through the water heater.  On that subject, anyone have an estimate of the typical air-in / flue-gas out flow rates of a 6" Rocket stove.  I'd like to try to find a high-temp blower that is reasonably well matched to those flow rates.

I'm sure there are a hundred reasons why this wouldn't work, but it does seem worth working through.  

I absolutely love this idea. It follows my rules of utilitarian and idiot safe. My father in law in his last house had a propane water heater that had a flue fan because it had to be piped like 40 feet out to the side of the house to vent.

There are lots of used water heaters around and the fan kit is an off the shelf part. You could wire up a simple thermostat that runs the fan. Plug that into a second thermostat that allows the system to run only when the rocket stove reaches a certain temperature and you've got a water heater. Just duct from the the bell or barrel to provide hot gases to heat the water.

Here's the problem though, standard gas water heaters are designed not to condense the flue gases. A gas water heater that spends much time in condensing conditions is going to rust out very quickly. Condensing water heaters are constructed of stainless steel and cost several times as much as the normal cheap ones.

Seems to me such a system would be great to use in parallel with another full time water heater (I have a heat pump Rheem), to keep it from condensing if you use a cheap water heater. You could set the water temperature to be much higher to collect the heat and use a tempering valve to avoid killing your partner and kids.

You could avoid thermosiphon draft issues by creating high or low spots in the system so it won't want to thermosiphon, since it's fan powered.

You'll need well insulated high temp ducting, depending on where you're drawing gas from.

Now I'm kicking myself for selling my old water heater that I just replaced with the heat pump.

Another option would be if you could find some sort of large piped heat exchanger. I played around as a kid trying to build a steam engine with this heat exchanger, was about 6" nominal with about two dozen half inch holes. Could never get it to draft. Later I figured out that steam engines use a venturi forced draft.

With a water heater you might want reverse draft for better efficiency. Regular water heaters are designed not to be that efficient. Since we're burning wood, maybe it doesn't matter.