Problem with Takagi & Radiant Heat &Domestic Hot Water

Discussion in 'Tankless Water Heater Forum' started by siammurph, Jun 17, 2008.

  1. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Some of the Bosch low-end units (up to and including the 1600 series) have side-vent forced-draft kits available, but most are installed with natural draft & B-vent, and run around 80-82% combustion efficiencies with modest but not super-low stack temps.

    In the Bosch lineup only the near-condensing forced draft 2400 & 2700 series require stainless venting, due to the low temp undiluted exhaust gas which results in large amounts of slightly acidic flue-condensation. Those series operate at around ~86% combustion efficiencies, ~200F net, for absolute exhaust temps under 300F (figure 250-ish) in normal conditions.

    Next time you install a forced-draft tankless that requires stainless vent, measure the exhaust gas & stack wall temps- you may be surprised. Hot to the touch, yes but well below the kindling temp of most construction & household materials- cooler than the glass surface of a 100W incandescent bulb installed in a floor lamp with a paper lampshade. (How much did you want to put on that bet? ;-) ) A Christmas tree won't light-off even held directly in the exhaust stream (try it!). It may outgas highly flammable volatiles with smoky appearance, but it'll still need an ignition source other than the stack itself. It's the intensity of the condensation, not the intensity of the heat that demands the stainless.

    If it's one o' those, Piggy-B probably won't need to worry about collecting on his fire insurance- he'll be long dead from the carbon monoxide poisoning by the time it catches on fire. The weasel clauses in the LIFE insurance may need attention though... ;-)

    If he installed a power-vented unit in a shared stack configuration with an atmospheric-drafted furnace (or conversely) he's clearly suicidal/homicidal. But if they're both atmospheric drafted it may go forever (or until the miserable thing craps out) without significant hazard unless they actually lean a Christmas tree up against the stack after a decade of neglect raised the stack temp to over 450F (from diminished combustion efficiency), or the main vent was severely undersized for the combined full rated output of HW + furnace exhaust flows at the outset. Backdrafting & CO poising would still be the more likely immediate hazard than fire though.

    There are all kinds o' DIY idiots out there though, which is why some states (including MA) specifically disallow DIY installation of ANY gas fired combustion appliances, no matter how simple.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Took me a bit of search to find it again, but....

    Next time you run into suicidal Piggies looking to end it all via kludgy venting schemes, save your breath (and possibly their lives)- get their email address and send 'em this link to an archived copy of the (only slightly dated) GAMA "Venting Done Right" flash video primer on the subject:

    http://www.slantcom.com/images/gas-appliance-venting.swf

    It's short, takes way less time than trying to explain it over the phone without cheezy graphics to help explain it. If they don't call back, it's on them.

    (A lot of people need help with understanding the orphaned hot water heater flue condensation problem too.)
     
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