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www.fueloilnews.com | FUEL OIL NEWS | JULY 2016 45 begins. This pressure fight will increase your pressure drop and lower your draft flow and there goes the ball game, just like in a bullhead tee. Here's the table I use for building com- mon vents, Table 6-5, and although I don't remember where I found it, it has never let me down. Keep a few things in mind when using this chart. First, it is not meant to replace any OEM's specifications and should never be used in place of an engineered system. Second, the useable height is from where the common vent enters the chim- ney to the top and that the chimney size is equal to or larger than the largest size of vent used. If the chimney is too big, the best bet is to reduce it down with a liner or expect condensation stains and debris normally called "pigeon droppings" to turn into seagull size. Let's work an example using Figure 6-9. Each one of these three units has an input capacity of 1.50 gph of fuel oil or 208,500 Btus per hour using 139,000 Btu/gallon fuel oil. Each unit has a 7-inch flue so from the furthest unit on the left the flue remains at 7 inch until we meet the second unit. At this point, the Y-connector, the flue pipe becomes 10 feet based on our 22½ foot useable chimney height and the input now 417,000 Btus. As we pick up the third boiler, the input becomes 625,500 Btus and our common vent from the second Y-connector into the chimney is now 12 feet based on the table. Two things to watch. Did you notice that I used the 20-foot line instead of the 30-foot line? Always work to the worse condition. That's why I also picked a Btu range that was greater than what I had. Although I have fans in my burners, I size as though I just had the chimney, pipe and fittings to work with, because when worse comes to worse, that's about it. In multiple or modular applications, a draft control must be installed in the flue pipe of each unit and in the common breeching, Figure 6-8 and Figure 6-9 due to a higher production of thermal draft. Through the use of multiple draft controls, a very high negative draft can be gradually reduced down to a more manageable rate at the appliance. Don't be afraid to use several draft controls, they are still cheaper than fuel and unnecessary repeat service calls. Okay, that's it except for one final thing and this goes back to Mr. Floyd Olmstead and the Handbook of Domestic Oil Burning. Mr. Olmstead probably knew more about draft than anyone I've ever met or read and he is the written voice of the past that has led me on a quest to make oilburners work without problems. In our books, both Mr. Olmstead and I have a chart that proves without a shadow of a doubt a bit of common sense and engi- neering. That little fact is something that many take for granted or just don't know about, and is the key to why today's equip- ment just doesn't work the way it used to. Know what it is? Do you know why flue pipe design is so critical? Because quite simply, when you cut your stack tempera- ture in half, you cut your thermal draft capability in half, and that's the rub. Unless you plan to put a lot of expen- sive fans all over the place and push and pull those flue gases outside, you had better learn Mr. Olmstead's theories, follow the codes and standards and make sure you also have a balanced draft with as much air coming in as you have going out, or as I call it, "inlet draft". Much of the material on flue pipe design was published as magazine articles in a couple of different magazines and is constantly used in my teachings and the question comes up all of the time, "why hasn't anyone talked about this before?" Well, actually they have. Each manufacturer tells us how to con- struct a flue pipe to their specifications and to follow the guidelines of UL, NFPA and local codes and in addition how to con- struct the flue ways all the way to the top of the chimney. However, with the addition of powerventers, flue pipe design is much more specific as to construction. As a teacher and consultant I found that not as much time or words had been paid to the venting of oil as had been devoted to gas. In researching this and noting that in fact many draft problems that have been with us for years were increasing, the only thing left to explore and document was a better way to design oil-fired appliance flues. Many have used the recommendations we just covered since I first started teach- ing them many years ago, and I'm sure that if you use them you will find many of your draft problems will be diminished or disappear. See ya. l F O N G e o r g e L a n t h i e r i s t h e o w n e r o f Firedragon Academy, a 25-year-old Massachusetts Certified School teaching both gas and oil and other heating subjects. Firedragon Academy has its hands-on train- ing facility in Sturbridge, Massachusetts, at the Beckett Training Center. Firedragon is also a publishing firm publishing George's over 60 books and manuals on HVAC subjects. He is a CETP, NATE, NORA, PMAA and PMEF Proctor and has been a Massachusetts Certified Instructor since 1975. He can be reached at 608 Moose Hill Road, Leicester, MA 01524. His phone is 508-421-3490 and his website can be found at FiredragonEnt.com Figure 6-10