(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.
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.
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.
One 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.