Have you ever considered the things you’d miss most in a TEOTWAWKI situation? I think a nice hot shower has to be pretty high on my list. It is right up there with coffee. So, once I got my nearly-off-grid little cabin built, I set out to build a prototype rocket water heater.
There are a hundred and one different ways to make a rocket water heater (RWH). But as an introduction, I’ll show you how I built mine. I’ll also mention a few things that I wish I had done differently, just to give you some ideas on how you might construct your own. This is one of those projects that gets better the longer you let it ruminate in your mind.
The beauty of a RWH is that not only can you heat a lot of water with just a small amount of firewood, but for many of us, the fuel is free and abundant and will be available long after the grid goes down. When you think of how hot a campfire is, and then think about how much hotter that same fire gets when you blow on it, that same principle is what makes a rocket stove so efficient. By creating its own constant draft, that higher temperature in my RWH will raise the water temperature 25°F in one pass through the heat exchanger. That’s pretty phenomenal.
Here’s a quick description of how my RWH works: A small pump (12-v, 3 GPM) takes water out of my regular water heater tank, circulates it through a coil of copper tubing which is located inside the chimney of a rocket stove, then returns the heated water back to the water heater tank. Inline thermometers allow me to monitor water temps so I know when to stop adding fuel to the rocket stove. A simple set of valves near my water heater tank lets me heat water with electricity, or turn the valves to either heat water using the rocket stove or my solar water heater.
The Heart of the System: The Copper Coil
The main component of a RWH is a coil of copper tubing. You’ll need a 25-foot coil of type L, ⅜” copper tubing. (Type K has slightly thicker walls, 0.049″ vs. 0.035″, but I have no experience coiling it so can’t speak for it one way or the other.) If you use less than 25’ it will not heat the water as efficiently since it will exit the rocket stove sooner. But that may not necessarily be a detractor in your particular system design.
When you buy the copper tubing, it will come in a coil about 16” in diameter and you’ll need to reduce the coil down to about 4-5” in diameter. It’s very important to do this without producing any kinks. I only know of two ways to accomplish that. Option 1 involves filling the tubing with sand, which I have no experience with so I won’t discuss it. Option 2 is much simpler and involves filling the coil with water and then freezing it.
When you get the coil home from the hardware store, stand it on end and make sure the two open ends are slightly higher than the rest of the coil. Next, tie a rope or strap around the coil at the 12 o’clock position so you can hang it up after you get it full of water, keeping the two ends as the high points. Now, submerge the coil into a rain barrel, washtub, or whatever you have handy so that you can fill the entire coil with water. It’s very important that no air bubbles remain trapped inside. So you’ll need to suck on one end, or force water through it with a hose nozzle to be sure there are no air bubbles anywhere inside the coil. Once you are certain it is air-free and completely full of water, replace the slip-fit plastic end caps and carefully lift it up, making sure the open ends are at the highest point so that no water is running out, and hang it somewhere so it can freeze. I hung mine on the outside of my shop one cold night. If you have a chest freezer where you can stand the coil upright, that works too.
JWR Adds: If using the freezing method, do not tightly cap the pipe. Only use slip-fit plastic caps. If you don’t allow room for expansion, then the copper pipe will split! Alternatively, if you are doing this project in warm weather, you can completely fill a copper pipe with water and tightly cap the ends by tightly crimping a flattened folded end in a vise. Because fluids are incompressible, this provides nearly the same support as frozen water. You’ll just have to manipulate the pipe more carefully, to avoid kinks.
You’ll know you got a good freeze when you can see ice flowing out the ends, as shown below.
Next, to form the smaller coil which is going to fit inside the chimney of your rocket stove, you’ll need some clamps and a piece of 3” pipe as shown below. Gloves will help since this is frozen tubing we’re working with and you don’t want your warm hands thawing it out. The ice inside is what is going to keep the tubing from crimping or knking, so don’t do this if the day is warming up and the ice is melting. Clamp one end of the tubing, leaving the first 12” straight, and then begin slowly working the ice-filled tubing into a smaller, fairly tight coil. Leave enough space (¼”) so the individual coils aren’t touching each other.
Your hands are only bending the piece of tubing that is in contact with the 3” PVC while the larger coil of copper is going to be dancing around, doing its own thing. But don’t worry about that bigger coil. Just keep your hands working very close to the PVC pipe or you’ll end up with kinks. (Note; Schedule 40, 4” PVC is 4½” in diameter, which would make a copper coil a little over 5½” in diameter. That would still fit inside a 6” chimney pipe but would also result in a shorter coil. I went with 3” to have a taller coil inside the chimney and also to be sure my coil would be surrounded by hot air and not be up against the edge of the chimney pipe. If money grew on trees then I would have tried it both ways — just to be able to quantify the differences in a working RWH.)
As you are finishing the smaller coil, you’ll want to leave 12” of straight tubing at the end just as you did at the beginning. I failed to do this and even though it hasn’t caused me any problems I recommend you leave those straight ends. The reason for the extra length is just to keep any soldered fittings 12″ away from direct contact heat of the rocket stove. More about this later.
The last important thing to keep in mind as you finish making the copper coil is the final orientation of each end. Since you drew your plans up ahead of time, you know where to have the two ends of your coil end up in relation to each other. Mine (see photo below) ended up on the same side of the chimney, but then pointing opposite directions to facilitate how they would connect into the outbuilding where my Chlorinated Polyvinyl Chloride (CPVC) hookup was. Using a tee, I was able to get the inlet and the outlet plumbing headed in the same direction as my design required. Also, make sure your two finished ends will fit through your drilled holes in the section of chimney pipe mentioned below. Plan ahead, as it’s a lot less expensive and less frustrating that way.
Other Plumbing Needed
Once your coil is complete, you’ll need some basic plumbing skills to get the rest of the project finished. If you’ve never soldered copper plumbing before, it’s amazingly simple and guaranteed to put a smile on your face when you see it happening before your very eyes. I’ve been doing it for almost 30 years and it still makes me smile every time. Try to keep a straight face when other people are watching you though, it makes you look more professional. Do a web search on the phrase “how to solder copper pipes”. With a little practice, you’ll get the hang of it pretty quickly.
One handy tidbit to know: Type L, ⅜” copper tubing that your coil is made of will just perfectly telescope inside a piece of ½” copper pipe, saving you from buying an adapter fitting or two, while constructing the rest of your heat exchanger.
Your finished coil is going to fit inside of a piece of 24”-long section of uninsulated 6″ diameter metal chimney pipe with a seam. It will look something like this (except that your soldered fittings will be farther away from the actual chimney-mounted heat exchanger because you left that 12” on each end):
The Heat Exchanger
Keep in mind that this type of chimney pipe has a seam and snaps easily apart. You’ll need to disassemble it in order to fit the ends of your copper coil through the two holes you’ve drilled. Once it’s inside the chimney pipe, you’ll solder on whatever other fittings your plan calls for. This is in order to attach your system to your regular CPVC or Pex plumbing lines. As you can see from the accompanying photo, it only takes about two feet of copper before you can safely add a CPVC or Pex adaptor without having to worry about chimney heat melting the plastic pipe. On the left in the photo, the copper fittings are threaded so you can attach threaded unions and begin your CPVC plumbing from there. Using copper-to-CPVC unions will give you a lot of flexibility for any future changes in your RWH configuration. You will also note the copper also ends with a threaded fitting for the drain valve.
Editor’s Note: This annotated photo will also be posted in Part 2 of this article — which will explain the rest of the system in detail.
I have another word of caution: It is best to add drain valves at every low spot in your system. This will save you grief, later. Also, when sweating on 90-degree unions of tees, always leave a couple of inches between unions. That way, if you ever have move or reconfigure anything, you can simply use a tubing cutter to remove pieces, leaving you with ends that can then be re-sleeved in a variety of configurations.
While technically it may be more correct to refer to the copper coil alone as the heat exchanger, I like to refer to this entire unit of chimney pipe, copper coil, and copper fittings as the heat exchanger. With this in your hand as a single unit, you now have a lot of flexibility for making hot water. If you build a system like mine and decide thagt you don’t like it, no problem. You simply remove your heat exchanger and re-install it elsewhere. (I can remove mine in under a minute.) If you are currently renting, then you can hook your heat exchanger up to a portable rocket stove for now and then use it in a permanent system when you buy your own place, later on.
(To be concluded tomorrow. in Part 2.)