Rocket Water Heater – Part 2, by St. Funogas

(Continued from Part 1. This concludes the article.)

The Finished Project

Since there is a lot of information on building rocket stoves on SurvivalBlog and elsewhere, I have not described any construction details on the stove itself. The following photo shows what my working rocket water heater (RWH) looks like, with an explanation of the number and letter annotations.

Photo 4 Rocket Water Heater-Numbered
Photo 4 Rocket Water Heater-Numbered

This small building was later covered with bat and board siding, with a small door covering the on/off switch and thermometers. Here are all of the key bits and pieces: 1. Inflow and outflow water temperature thermometers. (Thread these into the adapter before you glue the adapter into the tee to you be sure you get the thermometers oriented properly.) 2. On/off switch to the 12-VDC, 3-GPM water pump. 3. Water pressure-relief safety valve is a must when working with hot water. 4. Plumbing unions. 5. Car battery for 12-VDC pump. 6. Solar battery charger. 7. Drain valve. 8. Fuel entrance for rocket stove. 9. Clean out access (closed off when stove in use). 10. Rain lid for fuel entrance during non-use. 11. Ash-removing cleanout stick. “C” is the Heat Exchanger (section of chimney pipe containing the copper coil.) It slides down into the rocket stove chimney and has an additional piece of chimney pipe attached to the top of it.

Something to consider: One thing worked well in my trial but not as well in the actual setup. Obviously that huge copper coil inside a 6” chimney is going to restrict the airflow somewhat. In hindsight, maybe what I should have done was stick with the same size coil, but put the coil inside an 8” chimney pipe and connected it to my 6” rocket stove chimney using adapters. It still works well but smoke tells me I’m not getting quiet the efficiency I could from my rocket stove. I say “maybe” because when I tested out my coil/pump setup ahead of time (see photo 5 below), I got very little smoke. So perhaps in a portable rocket stove setup using a shorter chimney, a 6” chimney pipe is the best way to go. Further experimentation is needed.

But Does It Work?

Yes, it works! The $64,000 question is, how much water will it heat, how quickly, and with how much fuel? I used my RWH exclusively for three months straight when I first built it, just to be able to get some actual long-term, grid-down experience with making hot water and to get a real understanding of what I’d do to make improvements.

Unfortunately, I have a 30-gallon water heater. I will say unabashedly, for nearly everyone living in a 1- or 2-person household, 30 gallons is a huge mistake now and even more so after the grid goes down. A 20-gallon heater is more than adequate, but that’s a discussion for another day. With a 30-gallon water heater as my storage tank, I can heat 30 gallons of 57°F water to 130°F in 45 minutes using less than two 6” round bundles of kindling. My kindling is 16” firewood split into pieces that are approximately 1” x ¾”. (With a ΔT of 25°F which I mentioned earlier, this should come out to 30 minutes, but with ramping up the heat, then ramping back down, the total time is 45 minutes.) With a 20-gallon water heater, or the 10-gallon water heater I wish I had, the time could probably be cut down to 15-to-18 minutes.

Since it runs off a tiny 12-volt, $23, 3 GPM solar hot water pump, a RWH is a very practical TEOTWAWKI option for hot water. I keep a backup pump in my well house and have it on my wish list to get at least four more. Here is a similar pump so you get the idea of size, but this one is only 2.1 GPM. Most of these pumps are rated in the 30,000-hour range and have a wide variety of uses in off-grid, water-heating applications. Just be sure you get one rated for hot water. You cannot run this system without a pump or you risk a steam explosion ruining all your hard work, so get the $23 pump, as a bare minimum.

Dangers and Precautions

There are two things you want to keep in mind at all times when designing and using your system: 1.) rocket stove temperatures will melt plumbing solder, and 2.) water turns to steam at much lower temperatures than your rocket stove will reach.

As long as you always have liquid water in your system, Problem #1 is not an issue and solder cannot melt. Even water at 150°F will keep solder in your copper pipe joints from melting. If you have a fire roaring away in your rocket stove and you drain the lines, then you’ve got a serious problem on your hands. Your copper coil is going to heat up way beyond 150°F and could possibly melt your soldered joints, depending on what type of solder you used and how hot your fire is. The simplest way to avoid this problem is to be sure to always ramp down your fire as the water is getting close to your target temperature and to rake the coals out of your rocket stove before you drain the line.

Problem #2, water converting to steam inside the coil, is only a problem when water is not moving quickly enough through the copper coil. The most likely cause of this would be your pump failing in the middle of making hot water. In that event, the water inside the coil would heat up quickly. For this reason, if you have a built-in, closed-loop system such as mine, you absolutely must have a pressure-relief valve as close to the heat exchanger as possible. It serves the same function as the pressure cap on your vehicle’s radiator. Since my home water pressure never gets above 60 PSI, and my CPVC is rated for 100 PSI at 180°F, my RWH pressure-relief valve which trips at 75 PSI is perfect. If my RWH pump were to ever quit, water in the coil would begin to heat up higher than the 130°F I am normally aiming for.

I’m too rusty on my physics to know whether thermal expansion of the water is going to trip that pressure valve before any steam forms, but at any rate, as pressure builds, the pressure valve will begin burping and emitting water, letting you know there are some issues. For that reason, I always keep a 5-gallon bucket of water standing by for just that emergency. At the first sign of trouble, 5-gallons of water gets immediately dumped down the fuel hole of the rocket stove to extinguish the fire. (Another reason why I prefer “J”-shaped rocket stoves as opposed to “L” shaped.) When I was first getting my system up and running, on two separate occasions I forgot to open one valve or another so that water was not able to flow properly through the coil. As the water heated up and expanded, the pressure relief valve did its job. That let me know I’m getting old and forgetful — that something was amiss.

On a portable RWH, or any kind of open-loop system, a pressure-relief valve is unnecessary.

Conclusion

Probably the best piece of advice I can give anyone who has yet to start construction on their retreat is to plan ahead when it comes to hot water. Even though you don’t know what kind of alternate hot water systems you may be implementing in the future, stub off a few extra hot and cold inline ball valves so that you can add on later. It will cost you a few bucks now but will be much easier than cutting in to pipes, to retrofit, later on. If you are trying to retrofit an existing system, the easiest (and least expensive) way for most people is via the flexible lines coming out of the top of your existing water heater. This is where you would tee in to add a RWH or a solar water heater if you wanted an all-in-one system to store hot water in your current electric/gas/propane water heater.

I hope that I’ve inspired some of you to think about less expensive ways to make hot water now and good ways to make hot water when the grid goes down permanently.

Photo 5 Rocket Water HeaterOne parting thought: Your RWH can be a built-in elaborate system like mine, a small portable unit, or anything in between. Whichever system you build, I recommend you test your heat exchanger and pump together before you get too far along. In the photo to the left, you can see that I built a temporary rocket stove with cinder blocks and sand to test my copper coil/pump combo running directly off a 25-watt solar panel. This very simple setup sucked water out of the blue tub, heated it, and put it back into the blue tub. A 25°F rise in temperature told me this was going to work just fine. Prototyping first, for proof of concept is almost always a good idea.

With this type of easy-to-make, copper-coil heat exchanger, the possibilities are endless. You may run out of your favorite dark roast coffee after the grid goes down, but at least you’ll have all the hot water you’ll ever need.




27 Comments

  1. I see that the 30 gallon tank might be a good way to go, as the system’s temperature can be lower and safer, and easier on the vital pump, yet provide adequate quantity at a useful temperature. Rocket stoves do use fuel efficiently, and perhaps the design goal was to produce hot water quickly as possible. However, if you can make 30 gallons of hot water at 135 degrees, this would keep supply side of ‘worm’ or heat exchanger cooler, and all points in the system’s plumbing cooler, than if one were to heat 10 gallons to 175 degrees, closer to the boiling point that I would rather avoid. And most important to the system’s durability is keeping the coil cooler, so that heat erosion is reduced. I would rather sacrifice efficiency in terms of time, to a have a cooler running system that may last longer, and be less prone to failure, and be safer to operate. Perhaps a happy middle road would be a 20 gallon tank, but you’d have to give a try.

    Looks like you’ve got it, and it might be most durable design. And durability is going to be priceless in the future. We may not have the time and materials to fix stuff. Efficiency comes at a price. Heating 30 gallons to a useful temperature that quickly, is impressive. And 30 gallons of hot water can also heat the home gently through the night. That kind of thermal mass is helpful at stabilizing the temperature, if only heated indirectly via convection by the home’s primary heat source, it’s wood stove.

    This kind of innovation is important. We may have to reinvent the steam engine. This is a step in that direction.

  2. Any consideration given to “batch mode operation”? A larger, ultra-insulated tank (100 or more gallons, R120 or greater) at a slightly higher temperature (150-160F) might allow operating the RWH only every 3-4th day instead of daily.

    You targeted 130F for RWH output and storage; I’d suspect a need for a tempering valve to reduce delivered water temps to 110-120F, at least for the first several gallons from the storage tank (assuming the tank contents are continually replenished with non-heated water, with gravity/pump delivered non-replishment the temp would remain more constant), but didn’t see mention of one. Higher storage temperature coupled with a tempering valve would extend the quantity of stored heated water. (IIRC, tempering is included in modern higher-end shower valves).

    RE: Tank insulation – glass-lined commercial electric water heaters are available in 120 and 150 gallon sizes, closed cell 2-part spray-on polyurethane foam has an R value of 6.5 per inch, so about 18.5 inches would deliver 120. Enclosed in an airtight plywood box (to eliminate air infiltration) it would produce something about 50% larger than a 4X4 fiberglas shower stall. Underground storage (“root cellar” arrangement) reduces the radiant and convection loss deltaT from 130F to X degrees F (whatever air temps are above ground) to a deltaT of 130F against a constant 50-55F (greatest advantage in winter, negative advantage in summer).

    Any consideration given to tempering the RWH input water temperature? My well water is 51F, a few hundred feet of 1 1/4″ black poly pipe on the roof can raise that by 15-25F on a calm sunny day (dependent on flow rate), more in summer. Have you looked at using a RWH as a heat source for structures? Coupled with a large truck radiator and a solar-powered fan a RWH could be used for space heating. A large (1K gallons) well insulated underground tank heated to 120F could store ~450K BTU to be used for space heating.

    1. Hey Nosmo, thanks for the great input.

      I’ll save my thoughts on batch-mode operation for the follow-up article. I don’t want any spoilers. 🙂

      In my situation, I pump water into a 500-gallon storage tank first so the temperature is variable depending on time of year and how long since I pumped. It comes out of the ground at 57°F so I use that as an average for my ΔT computations.

      The only structure I’ve considered using water to heat is my well house. It’s one of the few reasons I’m still connected to the grid. I haven’t been collecting good data the past few years but this year is a go so I’ll know how many nights my milkhouse heater is coming on but it can’t be more than 10. I don’t want to heat with an open flame such as propane and it’s too small for a wood stove so I’m going to experiment with a stripped down bare 30-gallon water heater tank with 135°F water and see if that can radiate enough heat on a cold night to get the job done. I guess I should run the numbers and see what that can handle. Just the 500-gallon water tank does a great job by itself and when the weatherman says that week of really cold weather is coming, I empty the tank and fill it with fresh 57°F warm water. It’s all relative! lol.

  3. Thank you for posting this. I’m starting a project to heat water using a used propane tank and a copper coil 3/8″.i was not going to use a pump, thinking it should thermocyle on its own. Your thoughts appreciated.

    1. Hi Butch, I’m glad to know this article helped some. Good luck on your project. Maybe post an article when you get it finished?

      The rule of thumb on thermo-cycling is that it is basically inefficient. So if it’s being used to capture waste heat, it’s a great way to go. If you’re spending time feeding a fire then it’s not worth the inefficiency. Even in many solar water-heating applications, it’s worth buying a pump. It also works better in larger pipe diameters like the wood stove with the water jacket that Tunnel Rabbit mentioned yesterday with ¾” galvanized pipe, and decreasing well as you get into smaller diameters of pipe and tubing.

      When buying a pump, you just have to be sure it’s rated for hot water. I also check to see how many hours they’re rated for. My system uses so little electricity I use an old worn out car battery I should have turned in for the core charge five years ago, and it’s never let me down on that little pump.

  4. Great how-to article! You might want to consider hard soldering or brazing your joints,rather than soft-soldering them. That would give them much more resistance to melting if your water flow gets too low.

    Tractorguy

  5. @Butch,
    You might want to look at Eccotemp L5 portable outdoor tankless water heater. Used one
    for +6 months during a house remodel, worked very well. Propane powered, ~ $120. If you have plumbing nothing else required, if no plumbing, just water source, will require a small 12v on demand pump. Check it out. Again, excellent performance.

  6. I have used this concept, but not with a rocket stove, for many years. I always used 3/4″ copper and no pump, just used the passive (no pump to wear out, no electricity needed) thermosiphon effect. I have used this on solar hot water systems and on several wood stoves, in the fire box NOT in the stove pipe. I could never bend the 3/4″ tight enough to fit in the stove pipe. The bottom of the storage tank MUST be higher than the top of the heating coil; I have found that a greater height difference works better than a lesser one.
    I used one of the Mexican wood burning hot water heaters for several years to supply hot water to a dairy barn. 3, 6 to 8″ long cut off ends of pine 2×4’s split up would heat the 20 gal of water.
    I am NOT SURE how or how SAFE a stove pipe coil w/o a pump would work, I KNOW it would work in the fire box.

    1. Hi Robert, I’d love to hear a few more details on your copper thermo-siphon.

      Was it copper pipe or copper coil?

      Can you give us a fairly accurate number of years the longest one of your systems has lasted in a wood stove?

      How many loops were inside the stove?

      How big was the storage tank and how far was it from the stove?

  7. Articles such as these are indispensable. Personal growth and intelligence are paramount in our quickly changing (decaying) society.

    In an uncertain world we must all work to become more trained and more capable. Just as it is important to learn to be our own First Responders we should also learn to be our own engineers and builders. The day may well come when you either build it yourself or simply go without. Bad enough at face but far worse if you have others depending upon you as well. Just as it is important to build your Ark before it rains, it is important to learn the skills necessary before the time arises when you will need them. Avoid the shame, humiliation and danger and learn to do things while you still have the convenience and safety of the now. The clock is ticking.

  8. Thank you for the great article with lots of insights and information about the RHW. In addition to all the details about “what works”, we benefit greatly by information about “what MIGHT go awry”. Water is ever present in our thoughts and our planning. While we have made excellent progress in this regard, a major goal for us this year is to expand our water collection and storage strategies. We have a hard time imagining how we could have “too much” water! We have also talked a lot about the question of hot water for personal hygiene and comfort. Timely post, topic coverage, and tips!

    1. Hey Telesilla, glad you found some insights. I’m an expert at finding out what can go awry. Insurance companies won’t get near me. I always enjoy your posts and I’ve got cisterns pretty high on my research list this year too.

      1. St. Funogas! Yes… Cisterns. We’re looking at one of these for water storage with gravity feed to our partially in-ground greenhouse. My husband has been shopping tank-depot.com We’re thinking of one in the 1700 or 1800 gallon range for this project. Our goal is to place the cistern on the high side of the back of the greenhouse, and to supply it with gutters placed for rain catchment. On the subject of hot water (and in addition to water for personal use and comfort), we have a solar hot water heater project on the design board that will feed into barrels placed on the interior of the greenhouse. We’re doing really well so far with winter heating, but want to have more “solution layers” in place as we have concerns about both electromagnetic field changes and the GSM — the effects of which are likely to begin affecting the planet long before earth is once more in a deep freeze, and well within all our lifetimes.

  9. St Funogas:

    Thanks for taking the time to write this and include photos. I understand using the RMS and heat exchanger to heat water, but if you are using your home’s built in water heater, what is the plan to get this hot water to your home’s shower? I use electricity to run my well pump to provide pressure to circulate water to the home’s plumbing fixtures.

    1. Hi DC, I’m glad you enjoyed the article. The only quick short answer to your question doesn’t explain much: use a 12 VDC water pump.

      While I was feeding sticks into my RWH for three months, watching thermometers and checking valves, it gave me a lot of time to think about life, liberty, and the way we make hot water in America. I have a follow-up article called “Further Thoughts on Hot Water for TEOTWAWKI” that should be finished in a few weeks, perhaps JWR will have room to squeeze it in a future Survival Blog.

      I discuss in that article more details on how to tackle some of the more complicated issues of hot water in a grid-down situation. Most of us probably have a 120 VAC 10-amp pressure pump in our well houses to pressure our plumbing systems and that’s a huge energy hog that’s not going to be in operation on my homestead the minute the grid goes down. When I built my little cabin, I engineered the loft floor on the side where my office is at to hold 2-500 gallon water tanks. If the grid goes down, my office becomes the well house and now the water system in the house is gravity feed, not pressure feed. Not ideal but it will beat hauling water with a bucket.

      As I mentioned, there isn’t a short simple answer. One thing to keep in mind when dealing with pumps is they all have a maximum lift. It should be noted in the literature or product details as “lift” or “max discharge head”. Water is heavy and if you are pumping against gravity, the pump can only lift so high. In the pump I hotlinked above in the article, it has a max discharge head of 9.8 feet, meaning that in an open-loop system such as a hose pumping into a bathtub on the second floor, you will not be able to pump water 10 feet higher than the pump. But here’s the cool part: In a closed loop system such as I described with my electric water heater and RWH, there is no lift that needs to be considered. Everything cancels out. For every inch of “up”, there’s an inch of “down” and that heavy down water helps push the up water. It’s the same reason it only takes a very small motor to run the Ferris wheel. So, keep that closed loop in mind when thinking about how to re-engineer your plumbing after the grid goes down (if it needs it) and don’t wait to buy the parts after you get it figured out. And for anyone who hasn’t started building yet, it only cost a few extra bucks to build in a few “what-if” features.

      There’s lots more to add but it will be in that follow-up article.

  10. Great article – thanks for sharing. 1 question – did you experiment with running the water flow co-current vs. counter-current (i.e., from top to bottom thru your chimney) with the heat flow & if so, did you notice any difference?
    Thanks again

    1. Hey SoCal9mm, I did not experiment with that. I was getting such a huge rise in temperature above my expectations and it would have been complicated to reverse the flow it didn’t cross my mind.

      It seems like it should make at least a slight difference since the water would be moving from the waste heat down into the new heat, but, it takes 11 seconds for a given block of water to pass through the 25′ of coil inside the chimney. Since it takes that long, it probably wouldn’t make a noticeable difference on which direction it passes through the coil.

  11. St. F. is correct when he says to be sure to have a pressure relief valve. I worked for the gas company in Baltimore back in the seventies and “sidearm” water heaters were outlawed because of the danger of exploding. I would be very careful when employing this concept.

    1. As Wayne points out, you can’t be too careful when working with something that has the potential to turn into steam. If you design a closed-loop system similar to this that ties into your existing water heater you’ll have a margin of safety with a pressure relief valve very close to the heat exchanger, but you still have the extra safety with the pressure relief valve that’s built into the top of your existing water heater. Pressure relief valves are cheap compared to a trip to the emergency room.

  12. I see that the 30 gallon tank might be a good way to go, as the system’s temperature can be lower and safer, and easier on the vital pump, yet provide adequate quantity at a useful temperature. Rocket stoves do use fuel efficiently, and perhaps the design goal was to produce hot water quickly as possible. However, if you can make 30 gallons of hot water at 135 degrees, this would keep supply side of ‘worm’ or heat exchanger cooler, and all points in the system’s plumbing cooler, than if one were to heat 10 gallons to 175 degrees, closer to the boiling point that I would rather avoid. And most important to the system’s durability is keeping the coil cooler, so that heat erosion is reduced. I would rather sacrifice efficiency in terms of time, to a have a cooler running system that may last longer, and be less prone to failure, and be safer to operate. Perhaps a happy middle road would be a 20 gallon tank, but you’d have to give a try.

    Looks like you’ve got it, and it might be most durable design. And durability is going to be priceless in the future. We may not have the time and materials to fix stuff. Efficiency comes at a price. Heating 30 gallons to a useful temperature that quickly, is impressive. And 30 gallons of hot water can also heat the home gently through the night. That kind of thermal mass is helpful at stabilizing the temperature, if only heated indirectly via convection by the home’s primary heat source, it’s wood stove.

    This kind of innovation is important. We may have to reinvent the steam engine. This is a step in that direction.

  13. Hey Tunnel Rabbit, there are definitely a lot of things that would be fun to try if one had the means. And I agree with you on the steam engine. I have a small one my grandpa gave me when I was about 10, that had an electric boiler on it. It never worked but it would be something to see that thing running again.

    I think if the grid ever goes down we’ll have all the time in the world to fix things, just not the means like you said. While some people will be dreaming of Big Macs and Sunday drives in their Beamers, others of us will be longing after PVC cement and daydreaming about strolling through the hardware store back in the good old days and handing the guy at the counter pieces of green paper in exchange for nuts and bolts, rebar, t-posts, all that stuff we took for granted.

  14. CDC (dot) gov = Make Sure Water Temperatures Are Right
    “Legionella grows best within a certain temperature range (77°F-108°F). To keep water outside the range for Legionella growth, it is important to keep cold water cold and keep hot water hot.’

    ‘A note about cold water: In warm climates, water in pipes that carry cold water may reach a temperature that allows Legionella to grow.
    A note about hot water: It is important to maintain water heaters at appropriate temperatures while following local and state anti-scald regulations. Sometimes maximum temperatures allowed by your state may be too low to limit Legionella growth. Engineering controls that mix hot and cold water together at or near the point of use can reduce the risk of scalding while allowing water in pipes to remain hot enough to limit Legionella growth.’

    Prevent Stagnation
    ‘When water does not flow well, the resulting areas of stagnation encourage biofilm growth, reduce water temperatures to levels that allow Legionella to grow, and reduce levels of disinfectant. It is important to understand the flow of water in your building in order to identify areas of risk where water may become stagnant.’
    **************************************************************************************
    **************************************************************************************
    Legionella is the infamous Legionnaires Disease. There’s a lot of information on the Internet.
    The ‘Rocket Stove Heater’ seems like a good basic design for safe hot water. Heating water on a wood stove is good too.
    Solar Hot Water has to heat the water sufficiently; it has to be checked to determine if year-round solar heating is safely possible.
    Storage of water may cause some problems if not done properly. Check the Internet for information.

  15. Off Grid Water Heater for Tiny Cabin Sled- DIY Heat Exchanger + Snowmobile Camper
    (YouTube Video)
    The Outsider
    Jan 11, 2020
    https://www.youtube.com/watch?v=BREQY1TL7JA
    Duration – 25:28

    “I put together a DIY off grid water heater, using a copper coil, and ammo can. I use the heat exchanger to warm the inside of my snowmobile camper, which I plan to do some winter camping with this year.”

  16. Didn’t read all of the comments, so forgive me if I say the same things as someone else.

    As an FYI water will expand 1600 times when turning to steam. So if you flash the water to steam, then boom. Chernobyl.

    I might point out that the lower flow pump might be something to consider, because you will be moving the water slower through the system, it will have more time to pick up heat. You may get a better Delta T.

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