Stainless Coil In a recent article, Steve Heckeroth mentions a stainless steel coil in his fire box. I have been trying to locate a source for a coil for some time. The only company I could locate was out of business. Could you help me out? Scott A. Wilson, POB 2773, Big Bear City, CA 92314 I cannot tell people how to find a hot water coil for a woodstove (to heat domestic water for household use) without first discussing a couple important items for installing this device properly. In my mind it's analogous to giving someone a gun without the proper knowledge of how to use it Ä mixing fire and water can be a dangerous proposition. There are some laws that must be followed to have a safe wood-heated hot water system. Even a competent Do-It-Yourselfer may be biting off more than he/she can safely chew tackling a project like this. I'm not trying to discourage anyone, but beware of the "fudge-factor." We are talking about thermosiphon systems Ä pumped systems are significantly different and require other techniques for installation. ù Always install a pressure-temperature relief valve on both the inlet and the outlet of the coil in the stove. These must not be any further than two feet from the stove, less than one foot is better. Plumb the exhausts of these valves to the outside where you can see the pipes' end. ù Use either Stainless steel or Schedule 80 steel galvanized pipe inside the stove. If you use anything else, you'll be taking an unnecessary risk. ù From the coil in the stove to the tank you're using to store your hot water, run the "hot pipe" (higher outlet from coil) such that there is never a "high spot" where an air pocket could collect (heed these words!). Even if you have to travel across, say, some ceiling joists, shim the pipes so they will rise at least 1/4 inch per 12 inch run. If you have an air pocket in your system, you won't have any hot water. The "cool" pipe (lower outlet) should, after the pressure-temp. relief valve, drop at least 6 inches, and then follow the same criteria as the "hot" pipe. This is far from a complete description of a woodheated hot water system. But at least if you do these things you won't blow up the stove and the cat under it. I'll describe a complete system in the next issue. Go to your woodstove dealer or plumber experienced in woodstove coils. Many people in this field do not know how to do this kind of installation. Find someone who has experience to help you. If you send away for a coil, ask if you can call those people for help during installation. Or hang on till next issue and I'll be back. Ä Bill Battagin Common Ground Home Power: I finally did it. I made myself a drafting/drawing table and I'm in the continuing cycle of designing my futuristic PV system. As I draw a line from point A to point B, and after I have referenced numerous back issues these are the questions I have. 1.) In an ungrounded Low Voltage system (under 50 Volts) do the negative and positive both have to be disconnected in lines coming from PV panels before controller? 2.) In ac-DC system, with ac side having system ground: From what I understand you connect all grounds together. My initial instincts say ac will flow into DC side of ground in case of mishap. If you could shed some light here. 3.) What are methods of making connections with large 4/0 wire to say #2 etc.? 4.) How does Oregon's Low Voltage License affect small PV users and Installers? A little info on shunts would be helpful. And small wires sensing large current draws. Example: an Amp-hour meter sensing current off negative Battery lead. Is this true or did I read something wrong? P.S. I graduated from an Energy Center in Red Wing, MN in '86. Any of you Red Wingers keeping up with the latest, drop a line. The school is now closed. John Viner, 1362 W. 8th St., Eugene, OR 97402 Hi John. Whew! That's a bunch of good questions. Some of them will take a full scale article to treat effectively, but we'll try to give you a thumbnail sketch of each. 1) Section 690-13 of the NEC states that "Means shall be provided to disconnect all current-carrying conductors of a photovoltaic power source from all other conductors in a building or other structure." That's clear enough until we read on to Section 690-15. It states that "Means shall be provided to disconnect equipment, such as a power conditioning unit, filter assembly, and the like, from all ungrounded conductors of all sources." Confused? Join the Club. This would seem to say that both the negative and positive wires from the PV array AND the battery must have a disconnecting means from the controller in an ungrounded DC system. That's FOUR switches! OK, that's the way it's written Ä here's the reality check. Nobody I know does it that way, none of the currently available power distribution panels for AE systems do it that way, and I've never heard of an inspector requiring it Ä yet. If you ground the DC side, you only need have a disconnect in the positive legs. 2) I assume we're now talking about a grounded DC side? Your ac overcurrent or ground fault protection should open the faulted circuit before anything comes to grief. No ac fusing? Uh-oh. The reason that NEC gives for bonding both the ac and DC grounding electrodes is that it's possible for currents from an uninterrupted ac fault to go through the ground to the DC electrode and impose a higher ac voltage on the PV array frame or whatnot. Not bloody likely, but possible. 3) Twisting and soldering the wires, using split bolt connectors, or using crimped and soldered lugs bolted together, all of which are covered with copious amounts of tape and/or heat-shrink tubing work fine. See HP #2, p33; #7, p.36; #14, p.36; and #18, p.35 for more details. Take your pick, but remember that the splice has to be in an approved enclosure to be legal. 4) I'm a little fuzzy on the Oregon low voltage license, but my understanding is that it is designed for radio and alarm systems installers and is generally limited to power outputs of 100 watts or less. Might work for a system with just a few PVs and no inverter, but no good beyond that. 5) A shunt is simply a hunk of metal or length of wire with a known resistance between point A and B. Passing a current through a shunt causes a voltage drop (caused by the resistance) between point A and B. Using the Ohms Law formula I = E/R, we can calculate the current flow without having to break the wire and insert an ammeter. Ampere-hour meter manufacturers put the shunt in the negative leg because it's much easier to be accurate at the negative battery terminal. However, an instantaneous amperage measurement could be made with a shunt anywhere in the circuit. Ä Bob-O Steam Dream Our home is on 45 remote acres heavy with brush and black oak. We are gradually clearing out pockets of this and planting Ponderosa pine, Douglas fir and other trees. The clearing activity leaves us with large supplies of burnable material Ä more than we need for the house but not good enough quality to transport or sell. Normally, our only alternative is to burn this in brush piles which is wasteful. Is it at all possible to use this fuel source to generate steam; drive a turbine, and generate small amounts of electricity? Some preliminary ideas and calculations would be appreciated. We have asked this before and do not mean to belabor the point, perhaps the idea is far too impractical for "home power". We very much appreciate your publication. No Name or Address on Hard Copy To answer your reader re: steam turbine electricity generation, I believe the problem with turbines such as those used by electric power plants or ocean liners is that they are not efficient in small sizes. However, there are many reciprocating steam engines available (both new [yes, castings & plans are available] and used) that would run an electric generator just fine. And there are also plenty of boilers available to power engine Ä and, of course, one could easily be built new by a welder. Home-based steam-generated electricity is not all that common since the boiler needs to to monitored and (especially in the case of wood or coal firing) serviced with considerable attention. Those who do use steam (such as Terry Williams, Steamboating Issue #44) generally use it to recharge batteries and to run power equipment (saws, washing machines, etc.) for a few hours at a time, maybe a couple of times a week. Of course the rpm of a steam system could easily be adjusted by varying the sizes of the pulleys of the engine shaft and the generator shaft. The horsepower rating of a steam engine cannot easily be compared with the hp of an electric motor or an internal combustion engine since full torque of a steam engine is available at all speeds. In other words, a steam engine can handle a much higher load than a typical internal combustion gas engine of equal horsepower. By the way, I saw a classified ad (I think in TMEN) for info on home steam engined electricity. I hope the above is helpful to you. On another subject: I imagine that you folks too have realized that Bill Clinton has figured out a way to tax our air and sunshine. His proposed tax on BTUs would include BTUs produced by electricity, some of which is created by windmills and solar panels. We have, indeed. arrived. Bill Warren Mueller, Editor & Publisher, Steamboating, Rt. 1 Box 262 Middlebourne, WV 26149-9748 ù 304-386-4434 ù FAX 304-386-4868