Bill Wood Heat Your Water Bill Battagin c. 1993 Bill Battagin Hot water Ä most of our minds can conjure up a variety of images. We've all been in it before in one way, shape, or form. But today, let's look at water heated by a wood stove for residential use. The thermosiphon system discussed here uses no controls, sensors, switches, or pumps. Safety, simplicity, and function are the primary elements in this system's design. Principles Life is full of 'em! In this case we're discussing thermosiphoning. We find different forms of thermosiphoning all around us. From boiling our tea water to the thermals we ride in our daily hang-glider flight, rising and falling currents "drive" convection cycles. In a wood-fired hot water system with no fire in the woodstove, there are no forces to drive the water in the coil, tank, and connecting pipe. Though the system is under pressure, the pressure supplied by the domestic water supply to the tank is exerted equally throughout the system. Add some excitement to your life. On a molecular level this means heat. If you heat molecules of a gas or liquid, they become excited and thus move faster. In this excited state these molecules require more space Ä the volume of gas or liquid expands and becomes less dense. The heated gas or liquid is lighter and rises within the total volume. Those hot excited little molecules rise above their surrounding cooler, more dense cousins. We're movin' now! With boiling tea water, the water in the center of your teapot rises to the top; cooler water on the sides sinks to replace it. We hang glide in thermals generated by hot air lifting off solar heated areas on the ground. Water heated in the coil in your stove becomes less dense and lighter; it rises to your hot water storage tank, which is elevated with respect to the coil. The cooler water in the tank is relatively heavier and thus sinks back to the coil to replace the heated water. As long as you supply heat to the coil, the water will rise (convect) to the tank. You are a thermosiphoning dude! Not just any stove... Most stoves can have a hot water coil installed in them. Generally though, the warranty on a new stove will be voided. The new "EPA-approved" stoves present a variety of challenges to the installation of a coil. Woodstoves are no longer cast iron or steel boxes with little thought devoted to combustion and heat transfer efficiency. Today's controlled combustion woodstoves now employ new high temperature secondary and tertiary burn processes and/or catalytic combustion. If you're in the market for a new stove, beware of the woodstove that costs less than about $700 and is listed as an "EPA-exempt" woodstove. These stoves use older technology, are inefficient (burn more wood for the same amount of heat), are not airtight, and are guaranteed to last a lifetime (of a mosquito). They are exempt from the EPA emissions regulations because they are not airtight. So much air is allowed into the firebox that the fire will burn relatively clean. Because of these lower emissions, the EPA does not require certification of these stoves. In my opinion, installing a coil in the new EPA-approved "High Temp" non-catalytic woodstove can jeopardize the somewhat critical thermal environment found in these stoves. Combustion efficiencies can be dramatically reduced when you place a perpetual "ice cube" in what is designed to be a highly infrared-reflective, high temperature firebox. Lower combustion efficiencies mean more emissions, especially at lower burn rates. Stoves using high temperature combustion techniques usually have smaller fireboxes, thus less space available for a coil. On the other hand, a catalytic stove makes a fine home for a hot water coil. Catalytic stoves are much less affected by the cooling effect of the coil. The efficiency of a catalytic stove is highest during a slow rate of combustion. During normal operating temperatures, a decrease in wood smoke temperature entering the combustor will have little effect on its combustion efficiency. Stoves more than approximately seven years old are all pretty much in the same design boat Ä not airtight nor catalytic. These stoves will be a largely unaffected by the addition of a coil, so go ahead and attack these stoves with your coil installation. The Coil Important guidelines for installing a hot water coil in a woodstove are: ù Use either Schedule 80 steel galvanized pipe (home grown) or stainless steel tubing, pipe, or tank-type premanufactured units. If you use Schedule 80 steel galvanized pipe, I recommend 1 1/4 inch or 1 1/2 inch diameter, rather than 3/4 inch pipe. These larger diameters allow enough surface area in your coil to be able to install a simple "U" (two lengths of pipe) instead of a "W" (four lengths of smaller diameter pipe) in your stove. Copper pipe and copper tubing are not good choices for woodstoves coils. Too many variables exist to address the coil size issue here. Ask your local plumber who is experienced with these systems or the author so all the variables can be discussed. ù If possible, remove firebrick or interior side heat shields and install the coil in its place to save firebox space. You will have to be the judge of the costs and benefits of sacrificing firebox space to generating hot water. The coil will act to protect the side of the stove where the firebrick or heat shield used to live. ù Position the coil as close as possible to the most heat. This usually means about 4 inches off the bottom of the stove, either along one side or the back of the firebox. The hot water coil you install in your woodstove should have pressure-temperature relief valves (PTR valve) connected to it immediately after the coil penetrates the wall of your stove. If not, then your coil can be referred to as a bomb. A safe installation will have a PTR valve inserted in the inlet and outlet of your coil within a foot of the stove (see illustration). In any water heating system, whether electric, gas, oil, nuclear, solar, or wood fired, these valves are installed to prevent explosion in the event of malfunction. Malfunctions may be caused by: 1. mineral build-up on the inside of your coil to the point of complete blockage, 2. accidental closure of one or both of the gate (isolation) valves while the stove is hot, or 3. loss of water pressure to the supply side of your storage tank. Installing the coil in the firebox means cutting two holes (usually with a hole saw) in the side or back of your stove. Carefully plan the location of these holes to minimize the space lost in your firebox. Cut these holes about 1/8 inch larger in diameter than your pipe size. Be sure the center-to-center distance between these holes is the same as that of your coil. In most cases, an extra support should be added to the part of coil furthest from the exit holes. Use a muffler clamp around the coil to hold it against the stove wall. Or weld a tab of steel to the coil so a bolt can be passed through this tab and the wall of your stove. Plumbing Refer to the illustration for some help here. Your coil, either galvanized or stainless steel, should be plumbed to the hot water storage tank with copper pipe. Copper, though not safe material for your coil, is preferred to connect the coil from your stove to your tank. Copper pipe has less resistance to the flow of water. Sometimes, the lower resistance allows you to downsize your pipe diameter. When the storage tank is within 20 feet horizontally and at least 2 1/2 feet above the elevation of the coil, 1/2 inch copper pipe can be used to connect the tank to the coil. Many systems do not fall within this criteria though, so 3/4 inch pipe is necessary. Go with the Flow So, let's be a molecule of water just heated in the coil and head off to the tank, visiting components en route. The first thing we see is the PTR valve. This valve's exhaust port is plumbed to the outside because the inside of your house is a lousy place for wild steam and scalding water. You may need to use a reducer to connect the coil to the plumbing. In my systems I use a "bullhead" tee to make this size change and offer a connection for the PTR valve. Next fitting is a dielectric connector. At this steel/copper connection, use a dielectric union to prevent electrolysis between these two dissimilar metal. This will also act as a place to disconnect your plumbing in the future for changes/maintenance in the future. Now we are in the land of copper and float off through pipe and elbows to the storage tank. As we near the tank, we say hi to an air vent, pass through a gate (isolation) valve, and do a dielectric back to galvanized pipe, before diving into the tank. The gate valve offers a way to isolate the tank from the coil to perform maintenance or repairs. This valve should never be closed while the stove is hot. This would stop the flow of water in the coil, allowing it to get too hot and activate the PTR valve. Most storage tanks used are the existing hot water heaters, so the pipe will have to connect to the top of the tank. This means our pipe will have to go slightly above the tank and then drop down into the tank. We must now discuss air pockets. Places in your plumbing where air could rise and become trapped should be avoided at all costs. If this happens, the flow of water will stop in your system and the water will come to a boil, activating the PTR valve. Out goes wasted hot water. So as you plumb your system, be sure "horizontal" runs of pipe have at least 1/4 inch of rise per foot of run. Install an air vent when a high spot is unavoidable. An air vent will allow air to escape, maintaining water saturation throughout the system. The flow goes on. Let's ignore the tank for a minute, and head back to the coil. As we leave the tank we pass through a dielectric, then another gate (isolation) valve and we're off, falling back to the coil. As before, we need to be aware of high spots and potential air pockets. As we approach the coil, a dip in the pipe of at least 5 inches will prevent the water from reverse thermosiphoning when a new fire is started. This dip precludes the need for a check valve. The last components in our system are a drain hose bib, dielectric connector, bullhead tee ala PTR valve and we're back in the coil. Again, the exhaust port of this PTR valve is connected to the output PTR valve above it, and is vented to the outside. Storage Tanks Many different types of tanks can be used; the most typical is the basic residential hot water heater tank already used in most homes. Whatever you use, figure on 30-40 gallons for one person, 40 gallons for 2-3 people, and 50 gallons for 4-6 people. Your tank should be insulated to at least R-15 Ä the more the merrier. Remember to also insulate the pipes that connect your coil to the tank. The heat loss in transporting hot water can be significant the more pipe you use. My recommendation is to use any of the closed-cell foam type pipe insulation with a wall thickness of at least 3/4 inch. I mentioned raising the tank above the coil in the stove. There is no concrete law regarding tank elevation with respect to the coil. My recommendations are: up to five feet of tank/coil horizontal separation, there need not be any height difference between the coil and the bottom of the tank. For every additional foot of horizontal separation, add two inches to the vertical difference. If you do not have gas or electrical back-up for your wood- heated water and need to purchase a tank, a conventional water heater works well. You can often buy these on sale for less than $175. New water heaters don't leak and are already insulated. If you are willing to gamble, a recycled, but not leaking, water heater tank can be used. These are usually free but run the risk of early replacement. If you need to build a stand for your tank be sure it's strong. A 50 gallon tank full of water weighs about 450 pounds. A strap to secure the tank to the wall is appropriate in some installations. Materials All components in this system are readily accessible from a well supplied hardware/building supply store. Some parts may have to be obtained from a plumbing supplier. Buy high quality gate valves and dielectric connectors. Cheap valves and dielectrics are not worth the headaches they will cause later (and they will). Also PTR valves must be rated 100,00 BTUs or higher, and release at 150 psi or 210øF. My preference on the air vent is Watts model no.___, though other air vents are fine. You will need to purchase a 1/4 inch x 3/4 inch brass bushing to install this (and most) air vent(s). Start looking early for your bullhead tees from your plumbing supplier. They may not have these in stock and will have to order them. If these are difficult to find, use bushings or bell reducers after the tee for the PTR valve. Pressure Test and Light 'er Off With all your plumbing connected, joints soldered, dielectrics tight and drain hose bib closed, open your supply valve to the tank. Check for leaks, then open both the isolation valves and check for leaks again. Are we happy? Good, put the heat to your coil with a fire in your stove. Within 5-10 minutes you should be able to feel a temperature difference of 20-50ø F between the two pipes coming out of your stove. Within another 2 hours you should be able to get into some hot water.... Access Bill Battagin, Feather River Stove Works, 5575 Genesee Rd., Taylorsville, CA 95983 916-284-7849 Questions? Write and please send a SASE, or call between 6-7 am or 8-9:30 pm.