By George Lanthier
Time to do some sizing of multiple-unit fluepipes and wrap this series of articles up. We finished in the last article with the sizing of everything but the common vent pipes and a discussion about the transitions.
What about those transitions, and again, there’s a right way and a wrong way. In Figure 1, courtesy of the fine folks at Tjernlund Products, you can see that there are two types of transitions. The one on the left is wrong and is known as a pancake reducer. The one on the right is correct and is known as a cone or tapered type of reducer. Naturally, both have the ability to transition up or down and so could also be called increasers.
The cone-type is the only one we’ve shown in these articles because it’s the only right one for fluepipes. I don’t know where you would use a pancake type, but I would never put one in a fluepipe and I know they make warm air ductwork whistle.
The cone-type allows the flue gas to easily transition from one size to the next without any additional turbulence added to the flue gas. The pancake reducer will create a lot of problems and can even increase the pressure drop in the flue and create a partial blockage by creating a bullhead-tee effect.
Let’s say that instead of using the cone-type we use a pancake-type and we are decreasing pipe size with the flow headed into the fitting from large to small. The air (flue gas) will strike the sides of the fitting and curl back onto the air trying to get out and a pressure fight begins. This pressure fight will increase your pressure drop and lower your draft flow and there goes the ballgame, just like in a bullhead tee.
Here’s the table I use for building common vents, and although I don’t remember where I found it, it has never let me down. If someone does own it, my apologies and many, many thanks.
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 chimney 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 pigeon droppings to turn into seagull size.
Let’s work an example using Figure 2. This was Figure 5 in the last article. 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 seven inches until we meet the second unit. At this point, the Y-connector, the fluepipe becomes 10 inches based on our 22.5-foot useable chimney height and the input is now 417,000 BTUs. As we pick up the third boiler, the input becomes 625,500 and our common vent from the second Y-connector into the chimney is now 12 inches, based on our 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.
OK, that’s it except for one final thing and this goes back to Mr. Floyd Olmstead, the Handbook of Domestic Oil Burning and my book COMBUSTION & Oil Burning Equipment. 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 oil burners work without problems. In our books, both Mr. Olmstead and myself have a chart that proves without a shadow of a doubt a bit of common sense and engineering. That little fact is something that many take for granted or just don’t know about and is the key to why today’s oilheat equipment just doesn’t work the way it used to. Know what it is? Do you know why fluepipe design is so critical? Because, quite simply, when you cut your stack temperature in half, you cut your thermal draft capability in half, and that’s the rub.
Unless you plan to put a lot of expensive fans all over the place and push and pull those flue gasses outside, you 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.”
Finally, you can now find reprints of all of my articles both in the archives of Fuel Oil News by just typing my name, “Lanthier,” into the search box, or with full-sized drawings at our site in “The Mine” at www.firedragonent.com/Archives.htm. l FON
George Lanthier is the owner of the Firedragon Academy, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax at (781) 641-7099 and his Web site featuring the industry’s only “trade-only” discussion board can be found at www.FiredragonEnt.com.