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RV Doctor ColumnRV Doctor #343 Dear RV Doctor, I have an older 1977 Dodge powered motorhome that I am wanting to power a small 120-Volt AC refrigerator, along with a 10-inch color television, stereo and a gaming system with an inverter. I have hooked up a large second battery for use with the power inverter. My question is, how many watts will it take to power all four of these things? Matthew King, (The Colony, TX)
Matthew, contrary to what others may say, adding an inverter to an existing electrical system is a little more detailed than simply applying basic mathematics to determine what size inverter is required. As a start, however, add up the wattages for each device you want to power. If your appliance or device is rated in amps, multiply that amperage times 120-volts; the result will be the power measured in watts. If the wattage or amperage is not readily evident on the device itself, consult the operating literature that came with each component. Next multiply the wattage for each device times the amount of time (in hours) you'll likely use that component each day. This will give you a more complete picture of your requirements for inverter size. I've included a chart below that lists some AC devices commonly powered by an inverter. Keep in mind, the listed appliance requirements are approximate and the actual running wattage may vary between brands or models. Make sure that the inverter rating you choose exceeds the total wattage requirement based on your needs. There should also be a 30% safety factor added to your total requirement. I must add that inverters designed for RV applications are available in two basic output waveform technologies: quasi-sine wave (sometimes referred to as modified sine wave), produced by low frequency transformers, and pure sine wave output inverters produced by microprocessor-controlled, high frequency circuits. Although low frequency inverters are less expensive and considered efficient devices, they are, however, prone to voltage fluctuations which can cause disturbances in some DC circuits in the RV. Pure sine wave inverters, on the other hand, are extremely sophisticated and can produce AC power at or above the quality levels of shoreline power grids. Comparisons can be made by looking at the total harmonic distortion (THD), produced by each type. The harmonic difference between the low frequency RV inverter wave form and true sinusoidal wave shape is a percentage figure that affects the operation of induction type loads such as motors, compressors, or other capacitor-started devices. The higher the percentage, the higher the heat factor; the higher the heat factor, the more damaging to the device. THD, (purity of waveform), produced by low frequency inverters can exceed 40% with some units purported to approach 47%. When the output AC is not being called for, most inverters will go into a simmer mode until the demand for power is evident again. During this downtime certain monitoring functions must continue. This results in a continuous draining of current from the battery bank. Obviously, the inverter that draws the lowest amount is preferred. Once you know your total daily requirement for power, you'll know what size inverter to purchase. Once you know the size of the inverter, then you can effectively approximate the size of the battery bank needed to power that inverter. You may need more than just that one extra battery. You'll need a battery bank large enough to produce the daily wattage requirement for as many days that you want to remain free from a charging source. So as you can see, it takes some math, some homework and some forethought into choosing the most effective inverter for your motorhome.
Dear Gary, the head of the nylon drain plug on my water heater broke off while removing it to drain my Atwood water heater. Is there are trick to removing the remainder of the plug without damaging the threads? Hopefully I'm not the first person to ever experience this. Any ideas? Les Williams, (White House, TN)
I have seen this problem before Les, so you're not the only one! And it's basically caused by someone tightening that plastic plug a little too tight and possibly using an incorrect sealant on the threads. Over time, exposure to the heated water has caused it to be nigh impossible to easily remove. Another factor is that exposure to UV radiation degrades all plastics making them brittle and more prone to breakage. If the head is totally broken off, you'll have to drill a hole through the center of the plug and use a tool commonly called an "easy-out" to remove the plug (see photo). Also called a screw extractor, easy-outs are available at any hardware store and come in various diameters. You'll want one about half the diameter of the plug; the larger the easy-out, the less force is required to back the broken plug out. The flutes on the easy-out are "left hand" in that when tapped into the hole in the plug, they will grip and embed themselves into the edges of the drilled hole. You can then easily use an adjustable wrench or socket to remove the plug. After removing the remnant of the plug, turning the easy-out in the opposite direction while holding the plug will cause it to release its grasp. Using an easy-out will also keep you clear of the soft aluminum female threads on the inner tank of the water heater. Clean any left-over sealant from the drain opening threads and install a new brass plug. Be sure to use a fresh water approved sealant on the male threads of the new plug...and don't over-tighten!
Dear Doc, in one of your articles on holding tank probes you offered the idea of using "well nuts" by drilling a 3/8-inch hole. Great article, by the way. What size well nuts would you recommend buying and does this move the actual probe far enough off of the tank wall to insure it will take awhile for the buildup, etc., to really be a factor? Bill Barnett, (Trussville, AL)
Thanks Bill, for the compliment. I use 3/8-inch well nuts; that is, the hole size is 3/8" in diameter. Be very careful drilling into polyethylene, ABS or any plastic holding tank. The drill bit has a tendency to grab the soft plastic and create an off-round hole. I recommend using a Forstner-style drill bit. You'll want the holes for each probe to be perfectly round to avoid any chance of a leak. Simply push the well nut through the hole all the way up to its flange. There is no need for any sealant behind the rubber flange of the well nut itself; however, it will be necessary to apply sealants to the threads of the machine screws. The screw passes through the well nut and is threaded into and through the embedded nut and into the tank. Thus the machine screw becomes the actual probe. If no sealant is used on the screw, liquid will "follow the threads" and leak down the outside of the tank. I also recommend using a washer with an outside diameter a little larger than 3/8-inch so solid pressure remains against the outside wall of the tank. Tightening the screw bulges the nut section inside the tank and creates a good seal, (see photo). Do not over-tighten the well nut. If the monitor panel wires have ring connectors attached, be sure to insert the machine screw through the ring connector first before threading it into the well nut. Be sure also to use stainless steel machine screws and washers. Unfortunately, paper and waste can still hang onto these probes because they stick into the tank a little further. But if you clean the inside of the holding tanks once a year with very high water pressure and fill the tank before evacuating, such happenings will be minimized.
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