Rocket-based central boiler

We're in rural WV and have a large doublewide manufactured home, currently all electric.  On 30 acres, 2/3 of which is wooded.

Have 100lb propane bottles for the range and dryer, but furnace and water heater are electric.  We've looked into propane delivery but our road isn't fully developed and the propane company isn't all that enthusiastic about delivery.  We could live off the bottles indefinitely if it were just for the range and the dryer.

Our current home heat is forced air with an electric heating element and a 55,000BTU capacity.  We don't use AC.   Also have a decorative wood-burning fireplace, but it doesn't put out a whole lot.  We can get temps as low as single digits, but generally we don't drop below 20s in the winter.

What I would like to do is build an outdoor rocket-based boiler, preferably batch box, that would at least supplement, if not completely replace, the electric furnace, utilizing the existing blower with a water-to-air heat exhanger.  Ideally, I believe I'd like to build a thermal storage water mass in between the rocket boiler and the house so I could draw from it for both the furnace and for heating water.  I'm in the process of putting in a hot water recirculating system because we waste a lot of water waiting for the faucets to get hot and water is more precious than energy as our well isn't the greatest producer.  I'd like to incorporate a water-based thermal mass because we also have a 24x48 two story barn with a south facing gambrel roof that potentially could support a significant array of solar collectors.

I'm above average competent at building things.  I've replumbed houses in both copper and pex, have run iron pipe, rewired homes for 110 and 220.  Can weld, pour concrete, frame structures and am pretty good at finish carpentry.  Amature machinist and mechanic.  I think I have the skills to build something like this.  I also studied mechanical engineering in college before switching to IT so I have some decent analytical skills and practical experience.  I will be inheriting my Dad's very nice tile saw in a few weeks and I think that will be helpful.

What I don't have is specific experience.  Like how to design and size a system like this, the best materials to use, etc.  

Been reading all I can find and I'm at the point where I'm pretty sure this it's doable, but now I need direction on how to develop some specifics and start acquiring the materials.

I also wouldn't mind being able to intermittently heat the garage/barn with a similar system down the road.  Unsure if it would be best to have completely separate systems for this or combine some elements such as the thermal storage.

My web searches increasingly bring me here so I figured it was time to join.  Looking for advice or links to resources that might be helpful.  Any encouragement (or constructive discouragement) appreciated.

I recommend starting with Matt Walker's new boiler design.  

https://walkerstoves.com/walker-boiler.html

The build is very straightforward and can be improved with a little bit of insulation around around water chamber to keep the hot water temps up longer.

I would say go with a separate build for the barn heat or you would be spreading the heat out across too many elements. The wood fired boiler would be great for servicing your immediate hot water needs, but I don't know the effectiveness on heating a large space using that method.

Thanks.  I had seen the Walker on youtube but passed over it because I didn't realize it was rocket-based.

Looks like it would be a good starting point.

Has anyone purchased the plans that could comment on their value?  $120 is a big chunk of change into my materials budget unless it is a good match for what I need.

I haven't purchased them, but Matt's work is pretty flawless and are most likely worth the money.

That being said, the principles at work in his stove design are found all over the web for free, he has just put in the hours perfecting the designs for getting the best combustion. If you haven't looked at http://batchrocket.eu/en/ then I suggest looking over the designs here.

Ive dreamed of a central boiler too. Ive built several hydronic systems using propane fuel and tried wood in my first rocket stove build last Winter. My copper heat exchanger failed and there it sits still. Also the waste heat is a problem to me. I’d like to place it in a greenhouse so the waste heat is utilized.
How many Btu’s is the walker boiler? Is it really suitable for multiple zones of a hydronic system or wouldit need to be scaled up?
Thanks

Hey Christopher;   One of the extra nice things about buying plans from Matt is his unlimited consultations before / during and after your build.  
Your not just buying a plan you are buying a master builders personal advice while you are building!
I can tell you that Matt is one of the friendliest most helpful people you would ever want to meet!
His plans as they are sold do not have enough water storage.   He has talked about it during stove chat (Wednesdays at noon mountain time) @ broaudio on you tube
He said a larger storage tank could be utilized with his heater.
I suggest you contact Matt directly and ask his opinion.

I agree with Danial that your barn system should be separate.

thomas rubino wrote:Hey Christopher;   One of the extra nice things about buying plans from Matt is his unlimited consultations before / during and after your build.  
Your not just buying a plan you are buying a master builders personal advice while you are building!
I can tell you that Matt is one of the friendliest most helpful people you would ever want to meet!
His plans as they are sold do not have enough water storage.   He has talked about it during stove chat (Wednesdays at noon mountain time) @ broaudio on you tube
He said a larger storage tank could be utilized with his heater.
I suggest you contact Matt directly and ask his opinion.

I agree with Danial that your barn system should be separate.

Thanks for the info Thomas.  I didn't realize what all was included in the plans.  I'm going to read up on batchrocket.eu this weekend (we block non-USA sites at work so I can't even read it on my phone right now) and then probably contact Matt and see what his opinion is and whether or not his plans are a good fit.

Been reading a lot tonight, especially on the dangers of water turning to steam.

Was wondernig if anyone had tried using some sort of synthetic oil to extract heat from the rocket stove and then transfer that hot oil through piping to a water thermal mass.

Seems it would eliminate the issues of water-related corrosion and also the potential for steam explosions.  Presumably, with a large enough thermal mass, the temp changes would be gradual enough that you would have plenty of warning before you got close to boiling it off.

Another benefit is the oil system could be mostly closed (probably needs some capacity for expansion) so air would be kept out to prevent oxidation.

Downside would be a small leak of oil into the stove could be bad.

I believe automobiles are running temps at least in the 250+f range with conventional motor oil and that's already 40F above the boiling point of water without going with any exotica.

Is that something anyone has pursued?

Edit: off-the-shelf Mobil-1 synthetic can supposedly be run at 500F.  Definitely not a cheap product, but it seems to indicate this might be an avenue worth pursuing.

Hi Christopher,     Matt talks a bit about his boiler around the 5:25 mark in Stove Chat 18 and again at  the 15 minute mark on Stove Chat 21
Perhaps there is some information here that will help you answer your questions.

Christopher Patrick wrote:Been reading a lot tonight, especially on the dangers of water turning to steam.

Was wondernig if anyone had tried using some sort of synthetic oil to extract heat from the rocket stove and then transfer that hot oil through piping to a water thermal mass.

Seems it would eliminate the issues of water-related corrosion and also the potential for steam explosions.  Presumably, with a large enough thermal mass, the temp changes would be gradual enough that you would have plenty of warning before you got close to boiling it off.

Another benefit is the oil system could be mostly closed (probably needs some capacity for expansion) so air would be kept out to prevent oxidation.

Downside would be a small leak of oil into the stove could be bad.

I believe automobiles are running temps at least in the 250+f range with conventional motor oil and that's already 40F above the boiling point of water without going with any exotica.

Is that something anyone has pursued?

Edit: off-the-shelf Mobil-1 synthetic can supposedly be run at 500F.  Definitely not a cheap product, but it seems to indicate this might be an avenue worth pursuing.

Neat idea!

Another possible caveat; would seem like a leak, or other oil contaminating event, would result in water in the oil side of things... followed by steamy badness..

I suppose one could heat a non-liquid thermal mass with the oil, which in turn heats the water... if the non-liquid mass is useful for thermal storage/spaceheating this might not be entirely lame, but it obviously would add size, hassle, latency...


It seems to me that a BIG rocket based boiler combined with a foolproof heat transfer method is an extremely useful target.. but I haven't seen a method that ticks all the boxes yet.

Hey Christopher,  welcome to Permies.
Be sure to check out the the forums at https://donkey32.proboards.com/.
It's where Peter and Matt along with other rocketeers hang out.

Can lightweight materials such as insulating firebricks be substituted for the heavy materials in Matt’s Walker boiler?. Or some other lightweight material?. I’d like one on a trailer and I don’t want to heat up lots of mass besides some water.
By the way, Ive seen heat exchange oil advertised for industry.
Thanks.

Spent the weekend on that donkey board reading every thread I could find about heating water.

Right now my concept is either a tank or a coil inside the bell filled with either synthetic motor oil or a specialized heat transfer oil.  This would be piped to a copper coil inside a large water mass with a pump to force the circulation.  Water mass I'm thinking is 8x8x4' tank, probably of cement blocks with an EPDM liner sunk at least partially into the ground surrounded by rigid foam insulation and an insulated top.  The hot oil would heat the water mass and then my copper pickup coil would also sit inside the water mass piped with a pump to the heat exhanger in the furnace in the house.  I'm probably looking at 40' from the rocket and tank to the back of the house where the furnace is.  Have to check what is required of my homeowner's insurance...

This is basically taking the hot water rocket heater on the donkey board, which I believe might be the basis for the Walker version, and replacing the water tank and water coil inside with a tank full of oil and then using that to heat another tank.  Sort of the same concept, but putting oil in between the flame and the water tank.

Rocket heats the oil which just gets hot.  Oil heats the atmosphere-vented water so no boiling explosions.  House water gets heated by that tank.  Would probably need antifreeze in the house circuit but I think the large tank would have enough mass and insulation that straight water could be used.  The advantage, at least in my intent, of the oil, besides it being freeze-proof and non-corrosive, is to extract more heat from the gas stream.  With the current state of the art (open to atmosphere tank inside the gas stream with a pressurized coil inside that) has a bunch of safety built in.  But once the water boils, you stop harvesting useful heat and the extra heat goes to steam production.  Oil would let you collect heat past 212 up to hopefully something around 400-450F and then transfer that to the atmosphere-vented water where you still have the safety, but you can spread that heat over a larger mass of water and maybe get it to 190/200 that will last (hopefully) for hours of extraction.

If I were to put a tank into the bell, I would probably use a 100lb propane tank.  They're about $125 new last time I checked, decently thick and constructed well, easy to obtain, and used are sometimes more affordable, but then you have to deal with the gas residue and rust.

For a coil, I've been wondering if black iron piping would be durable enough over time, but right now I'm leaning towards the tank as it's simpler and involves a lot less fabrication.  It's also easier to swap out if it deteriorates.

Other in-bell idea would be fire tubes running through an oil mass although that is comparatively a fabrication nightmare and would probably have to be stainless and I worry about oil stratification and then cooking.  I can't imagine you can guarantee good circulation inside that sort of assembly as you (hopefully) could with a tank or a coil.

I've also wondered, with the tank, if putting some sort of spiral ducting around it like a corkscrew or Archimedes screw, possibly with "bumps" would help slow the air and give it more time to transfer heat to the tank.  Most likely there'd be a balancing act between that and insuring sufficient draft as the air sinks, but assuming that was effective, then it would be cooling the air more than otherwise, thus driving the draft.  That may be something that could be tweaked after.  I wonder if the centrifugal force would push the colder, denser air to the outside and help concentrate the warm air against the tank, but it's probably negligible.

Also considered maybe using oil casing through the middle of the tank to make a donut-shaped horizontal cross section so bell gases flowed through the center as well as the sides, but that makes maintenance down the road more complicated i would think as well as the fabrication and might even be too efficient at heating the oil, heating it too much.

Couple things I see as issues right off the bat is the longevity of a carbon steel tank inside the bell over time and cooking the oil.  If I could reasonably expect a solid year of service, I could live with replacing the tank with a new one annually.  Oil cooking, I'm not sure about.  Mobile-1 can take 500f and I've seen some specialized oils that will go to 600f.  A 100lb propane tank holds 23 gallons IIRC, but that might be at the 80% fill, and I'm not sure I'd use the entire tank length, but if you figure 25 5-quart jugs of Mobile-1 off the shelf to fill the tank and the coil and piping, which I think is a generous estimate, that comes to a one-time (assuming the oil isnt damaged) cost of around $600.

But what happens if the oil circulation pump stops?  How hot will the oil get?  How can you mitigate that?  Will the resulting temps cook the oil and what does that mean?  Also, what happens to the oil after being exposed to temps close to it's max over time?  How long until that oil needs to be replaced and, if so, what do you do with it (rocket oil burner?  )

I'm unsure about the danger a 20-something gallon tank of oil presents inside the bell.  Leak-wise, it's inside a furnace after all, I'm not sure a little more heat fueld by a leak is that big of a problem especially since the heat coming out of the riser should still be hotter.  Oil doesn't tend to burn well unless it's atomized or in droplets.  Enough flow of oil will probably just run out faster than it can burn unless it sprays.  Maybe an emergency collection basin in the bottom to catch any spills would be enough?  Possibly some sort of blow-out panel in case it flashed off suddenly?  I guess the biggest danger would be a pinhole leak driven by the circulator pump pressure starting a secondary combustion in the bell.  How bad would that be?  Bad enough to have a blow out panel of some sort, or just bad enough to make sure there's nothing flammable in a potential drip path in the oil?

I'm still reading and thinking.  At some point I'll probably open an account on the donkeyboard site and post my ideas to see what others think.  I think the concept has some merit, at least for my needs.

Nice update.

I don't know what availability or cost is like, but it seems that this, or something like it, might be a more suitable oil? https://www.multitherm.com/multitherm-pg-1.html

Here's a place online that carries PG-1

https://theindustriallubricantstore.com/multitherm

$34/gallon by the gallon

About $25/gallon in a 5 gallon pail (plus shipping)

This is an important thread to someone like me,  who wants a rocketmass heater and wants to avoid insurance issues!

Let me weight in on  materials.
I suggest a gas water heater tank, stripped of insulation as your in bell heat exchanger.
They are generally free,  come threaded for standard plumbing fittings,  and they have central chimney.
Concerning longevity of the heat exchanger, maybe wrap it in some stainless steel wire and apply a  thin coat of refractory cement mixed with rockwool fibers,  as a hot face?

On the other end,  in the house, I suggest more derelict water heaters,  electric ones this time.
Connect them with copper,  it's probably the cheapest thing that can take 212 degrees.
From these free, liquid tight, vessels, you can do as you please.
To put a coil in or more of them, I would use the corrugated stainless steel tubing that is used for gas lines.

For your pump,  there is design that the biodiesel types use,  adapted from a car coolant pump.
I'll post a link if I can find it.

As to your working fluid,how about adding a solid to water instead?
Hear me out.
A non-volatile solid added to water,  could increase the boiling point.
If it's dark colored it could become the working fluid n solar collector where the fluid was heated directly, rather than via conduction.
Just a crazy idea.
Here's one more!

I have thought about building an outdoor  boiler as a still,  where the(low pressure!)  steam is conducted into the residence via pipes and condenses there.
The BTUs in a gallon of water turned to steam fat outstrip those in a gallon of water at 212 degrees.
Depending on how precious water is,  you needn't ever return it back to the boiler.
In a greenhouse,  you could cool it to safe temps then deposit it directly into the soil.
In house, you could store it for drinking.
Every firing would displace some old water with newly distilled water.

Here is the link that William was referring to.  https://permies.com/t/33523/world-famous-SBC-pump  made from a small block Chevy oil pump!

I suggest using much cheaper regular hydraulic oil. I think it has a working temp over 400 F .

Many thanks to both of you.

I was about to post that I was considering abandoning this particular project because I was unable to find a pump capable of pumping hot oil at the temps I'm expecting that didn't cost a couple thousand dollars.

Best I could find was a Chinesium pump for a few hundred that could go to about 250F - definitely not enough.

But the SBC pump seems like the perfect solution and cheap enough that I could build in a redundant one, or perhaps even one run by a small gasoline engine should the power fail.

Looks like I'm off on another tangent.

During a power outage it could be run by a cordless drill with a few spare battery's