Original post from http://sustainablepreparedness.com
One of the most multi-purpose tools on the homestead is a wood cook stove. Ours not only cooks the food and keeps the house toasty warm; it also heats our hot water!
The two main components, aside from the wood cook stove, are a water coil (#6 & #7 on the pictures below) which is a pipe that runs through the fire box to heat the water, and a range boiler (picture on left) which is a large tank that holds the hot water before and after it circulates through the wood cook stove.
Active vs Passive
There are a couple of variations on the “hot water from your wood stove” scene. One involves the use of an inline electric circulating pump to force water through the water coil; the other uses the simple principle of heat rising to accomplish the same thing. It is called a thermosiphon system. “Active” systems (using an electric pump) have some advantages, but in the opinion of this writer, not enough to offset their negatives for most people. An active system can produce as much as 50% more hot water than a passive (thermosiphon) system, and since more water movement takes place, there is less chance of water overheating and creating dangerous pressure levels. But anytime you unnecessarily involve a mechanical or electric device in essential systems, you are asking for trouble. For instance, if electricity is lost during winter, you would have to potentially shut the wood stove down or dismantle the hot water system to prevent dangerously high temperatures and pressures. And some inline pumps have a poor reputation for reliability. Even if you are on a renewable energy system with a very efficient DC inline pump, it still uses electricity throughout the day while the stove is running, and that can add up. Bottom line? Whenever possible, keep it simple and go with a thermosiphon system! And that is what we are going to focus on in this post.
How Thermosiphon Works
The range boiler is placed higher than the water coil in order to take advantage of an important principle. Just as hot air naturally rises, so does water. When connected properly, the water that heats up in the water coil will begin rising toward the elevated ranger boiler. As this hot water begins moving, it naturally pulls cold water in behind to replace it, and before you know it, a “thermosiphon” has begun–circulating water from the range boiler through the water coil without the aid of any pump! Our range boiler holds 40 gallons and only needs 1 to 1.5 hours of stove run time before it is nice and hot. When the tank is insulated, this hot water can last all day. This makes summer time wood cook stove use inside the house a possibility in some drier northern climates (where a flash fire in the morning is bearable). We open all the windows that have screens, open the front door (which is next to the stove), and place a strong fan in the doorway that blows hot air out the door and pulls cooler air in the windows. In warmer climates, it might be advisable to hook up a solar water heater for use during warmer times of the year.
1-Hot water in from water coil;
2-pressure release valve; 3-Hot water out to house;
4-Cold water in; 5-Cold water out to water coil.
Choosing the Right Components
First, the stove. While many different types of stoves could be used for this system, the easiest and best choice is an Amish made wood cook stove. Not only is it so versatile, it comes all set up for the job with knockouts in the back to fit a water coil. Often you can order the stove with water coil already installed. Most other stoves must be manually retrofitted to work with a water coil or water jacket, which involves drilling holes in the back–not exactly something I fancy doing on an expensive stove. The other nice feature is that the Amish made wood cook stoves are generally quite air tight, which means a longer burn time and more efficient use of wood. My stove can be cranked down so far that is almost puts a strong fire out! The same cannot be said of many older and more ornate wood cook stoves.
Second, the water coil. This should be made out of stainless steel. Your only options are single coil or double coil. Double coil, as the name implies, makes an extra loop around the firebox and heats up more water more quickly. That sounds great, but think it through first. While you want your water to get hot, you don’t want it so hot that the pressures rises to dangerous levels and blows your system’s pressure release valve frequently. So you are doing a balancing act. Hot water usage and the size of your storage tank (range boiler) are factors that will affect your decision here. If you have a large family and are using a larger range boiler (50-80+ gal.), you might consider a double coil. If, on the other hand, you will not be using a lot of hot water and your range boiler is smaller (30-40 gal.), you would be better off with a single coil. Water jackets are also available for regular wood stoves and come in a variety of sizes for low, medium and high output. Choose accordingly.
Third would be the storage tank or range boiler. It certainly is possible to use a standard water heater as it is insulated and already has an appropriate number of ports for some systems; but I chose to go with a range boiler that is made for the job. It is a super heavy duty tank that is lined with a masonry type lining inside to prevent rust-out. It also has plenty of ports for fittings in just the right places. Mine is made by Vaughn Corp. but I purchased it through Stoves and More (who also sells excellent Amish made wood cook stoves). I really like mine, but have a friend who had to re-tap the thread on some of the ports because there was welding splatter present. Hopefully that was a fluke.
Fourth would be the pipes used to connect the water coil to the range boiler. Ideally it would be 1″ diameter and made out of stainless steel. While 1″ is best, it is possible to use 3/4″ for systems with a large tank and plenty of rise from the water coil to the range boiler. But we are after the least amount of restriction so the water can circulate freely and not overheat. The ideal material would be stainless steel threaded pipe, but that can be quite costly if your range boiler sits more than a few feet from the stove. In situations with longer runs, one could use stainless steel for the first several feet away from the water coil (both inlet and outlet) and then switch to copper or even galvanized steel. Copper is NOT recommended for use close to the stove as solder could melt from the joints if water flow stops or pipes are dry while the stove is running hot. Galvanized steel pipe may be used for the entire loop between water coil and range boiler, but it will not last as long as stainless steel or copper. Try to avoid 90° elbows as much as possible–they create restrictions that reduces circulation. It is better to use 45° elbows if possible. But if sufficient rise is present and the range boiler is close to the stove, one or two 90° elbows probably won’t hurt anything. I have some on my system, but I used 1″ pipe and placed the range boiler very close to the stove and a sufficient distance above it. The type of pipe that leaves the outlet of the range boiler is not as critical, but I decided to stay with galvanized steel for several feet before switching to pex, due to the high water temperatures present in the range boiler. One important point…if you plan on connecting copper to steel, be sure to use a brass (preferred) or dielectric fitting in between to prevent electrolysis. The reason brass is preferred is that a dielectric fitting could possible melt from the very high temperatures present. Also, be aware that stainless steel is a different animal than regular steel; it can connect to copper directly without any problems.
The engine that drives a thermosiphon system is vertical rise from the water coil to the range boiler. Within reasonable limits, the more rise present, the stronger your circulation will be (all other things being equal). While placing the range boiler on the second floor is certainly a good option, here is a rule of thumb for vertical placement:
- For every 2 horizontal feet of run away from the stove, there should be at least 1 foot of vertical fall from the cold water outlet on ranger boiler (#5) to the cold water inlet on the water coil (#7). Example: Range boiler is 10 feet away from stove. Cold water outlet on range boiler (#5) should be at least 5 vertical feet above cold water inlet on water coil (#7).
My range boiler is less than 4 feet from the wood cook stove and has around 2 feet of vertical fall from the cold water outlet of the range boiler to the cold water inlet of the water coil. I have found it to work quite well.
While some may choose to do as we currently are and use the wood cook stove all summer, your climate or other reasons may make that an unthinkable option. And there is no need to sweat it out with all the potentials we have for hybrid systems. One option is to use the wood cook stove during winter and hook up a good quality solar water heater for use during spring, summer and fall. Often this may be accomplished using the same range boiler for both systems to lower overall costs. That is our ultimate plan for next summer. Another option is to run the main outlet from the range boiler (#3) through an on-demand (tankless) propane water heater before sending the hot water to any fixtures. If the water is sufficiently hot, the water heater will not fire up. If the water is too cool, it gets heated up as its being delivered to your faucet. This, of course, is not a completely independent option as it uses propane, but it is very convenient. There are many other possibilities–the sky is the limit!
Water not hot enough? Here are some possibilities:
- Fire not hot enough?
- Water coil or jacket placed too far from heat?
- Try a double coil water coil or larger output water jacket
- Range boiler too large? Try a smaller tank
- Pipes connecting range boiler to water coil not large enough (creating a restriction)? Try larger diameter pipes
- Try insulating pipes and range boiler
- Try moving the range boiler closer to stove and/or higher to improve circulation
- Last resort, install an electric circulation pump
Water overheating and blowing pressure release valve regularly?
- Try a slower fire, especially when initially starting a fire? It can take a little while for some systems to start circulating after a fire is started. During that time, water in the water coil can potentially overheat.
- Try a larger range boiler
- Try larger diameter pipe between range boiler and water coil/jacket
- If using a double coil water coil or high output water jacket, try installing a single coil or lower output model
- If pipes or range boiler are insulated, try removing insulation
- Last resort, install an electric circulation pump
As you can see, improving circulation can help when dealing with either extreme. Much like the human body, proper circulation is vital!
In addition to personal experience and second-hand testimony from friends, I am much indebted to Lehmans for their helpful booklet Hot Water From Your Woodstove.