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MBTU appliance between 2 FAN MIN-MAX columns

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prepping for Red Seal exam sample test question: vent connector size
2 appliances B common vent/ c connector vents
75mbtu fan assisted furnace
30ft total vent height
3ft connector rise (C vent)
Table C4 column 3" = fan min-max 54mbtu-64mbtu
column 4" = fan min-max 83mbtu-136mbtu
75mbtu falls over the 3" Max AND under the 4" Min.
none of the GVRs seem to address this scenario

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,852
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    Let you competitor take that job. He can argue with the inspector while you are making money on a customer with no venting issue. LOL

    Actually this is not an exact science. You default to the larger. But @Bob Harper is the expert on gas appliance venting. Hope he chimes in with the right answer.

    But I still say. let the other guy deal with it. LOL

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Diamonddave
    Diamonddave Member Posts: 10
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    hi, thanks for your insight, however this isn't a field question, rather Red Seal exam studying question. the odd Fan min-max columns fall between 2 btu ranges & are above the max of one column but under the minimum of the next column
  • Diamonddave
    Diamonddave Member Posts: 10
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    Here is table C4 from B149.20 note 75mbtu fan min-max at 30ft vent height/3ft connector rise has no spot between 3” & 4” connector sizing 
  • Bob Harper
    Bob Harper Member Posts: 1,036
    edited February 25
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    Hi Dave. This is the table for two or more appliances. You only listed one. I think you're in the wrong table but, yes, this will come up. You could claim to interpolate between values, but you cannot extrapolate. You must also derate the connector for length of run and elbows so by going with the 4" you can get it down to where you need it to be. Just understand that in the real world, these tables are BS generated by a computer. These are not actual measurements taken on actual equipment firing. They are theoretical with limitations. For instance, they only apply when the chimney or vent runs up inside the thermal envelope and not exposed below the roofline. There, you are instructed to rely on 'engineering principles'. One last thing- cross-check against single walled connectors. Hope this helps!
    Bob
    Diamonddave
  • Bob Harper
    Bob Harper Member Posts: 1,036
    edited February 25
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    Just an observation on the magic of B-vent when used as a 'vent connector'- there is none. Yes, it is very smoothwalled. Yes, the elbows have a reasonable radius. Yes, the inner liner has a male-down orientation to contain condensate. Yes, it carries a 1" clearance to combustibles vs. 6" for unlisted single wall connector (aka, 'stovepipe'). You don't have to search for increasing wall thicknesses in larger diameters. It can legally pass through combustible walls to get to the vertical vent (through floors and ceilings only as 'venting'). Lots of pluses.
    Now, for reality: the aluminum inner liner absorbs heat very quickly and conducts it to the dead air space and outer jacket 3 times faster than stainless steel. In other words, it COOLS stack gases compared to single walled connector or, my favorite, type L vent. L vent is built exactly like B-vent only using a stainless-steel inner liner. The UL 641 listing for oil also encompasses UL 441 for CAT I gas so it can do both---and pellet venting. And woodstove connector when powder coated black (no galvy. to form toxic zinc fumes above about 500F). L vent is insulated, carries the same 1" clearance for CAT I gas as B-vent, can be used in the same GAMA sizing tables but......performs MUCH better! If you want to reduce stack losses in the connector, and have a clearance issue, use type L vent.
    BTW, the same goes for aluminum liners. They COOL stack gases once they've heated up. The mfrs. will tout how quickly aluminum heats up for a quick draft and that's true.....for a minute. Then, instead of 'turning off' meaning not conducting any more heat from the stack gases, when would tend to hinder draft pressure, it continues cooling the stack. Oops.
    Aluminum was ok'd back when everything was low efficiency CAT I with tons of dilution air (which cools stack temps) so condensation wasn't a big deal and CAZ negative pressure wasn't much of a thing. Now we have 80% furnaces without dilution air vs. water heaters with 600F+ exhaust temps plus dilution air. BTW, B-vent is rated for 550F while L vent is rated 570F and endures a 1,400F flash fire test.
  • Diamonddave
    Diamonddave Member Posts: 10
    edited February 26
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    this is the sample question & answer options, usually the min-max tables overlap to accommodate, however this problem falls right into a hole on the min-max tables

  • Diamonddave
    Diamonddave Member Posts: 10
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    this problem considers the connector length (3ft) and 2 elbows allowed, nonetheless there's no GVR or clause to address this odd scenario, thus on my Red Seal exam, how do i answer it correctly?
  • yellowdog
    yellowdog Member Posts: 157
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    I was always taught that you cannot put an induced draft appliance and a natural draft appliance on the same flue. What happens when the chimney plugs above the connection point and the draft inducer comes on for the furnace?