Common intake vent, separate exhaust, ok?

Discussion in 'Tankless Water Heater Forum' started by crazypipes, Jun 14, 2020.

  1. crazypipes

    crazypipes New Member

    Jun 14, 2020
    New Hampshire
    JUST THE QUESTION: I am needing to run two propane tankless water heaters, but common venting is only allowed by the manufacturer if the units are connected electronically, set to the same temperature, and on the same distribution system. For reasons below, I would like different set temperatures and isolated distribution systems, but I am trying to minimize venting needs. Can these two units share the intake, but have individual exhausts? This would mean 3 vent pipes instead of 4, and I would really like to eliminate that 4th pipe. I cannot find anything about common venting only the intake, but intuitively it seems like it would be ok since these units also allow for room-air intake.

    SKIP THIS, OPTIONAL DETAILS IF YA GOTTA KNOW: This is part of a whole-house down to the studs restoration of an 1880 farmhouse, all walls are currently open and some framing is still in progress. The water supply is from a dug well a 1/4 mile uphill that gravity feeds a cistern in the basement, in the winter the water is barely above freezing. To get to 120 degrees F, we are talking up to an 80 degree temperature rise. My hot water desires include:
    • 2 concurrent showers or 1 shower with concurrent washing machine run
    • 800 sqft of hydronic radiant heat (first floor only, as a luxury not primary heat to home. House has adequate forced hot air heat)
    Given the temperature rise needed, that puts me with needing Rinnai or Rheem's largest 200k BTU models, which are both rated at about 11gpm if you live in the tropics and more like 5gpm in my situation. So this unit will just barely cover my tap hotwater desires and a second smaller unit will be needed for the floors. For reasons of keeping down cycling on the larger unit, because the two units would be sized differently, and to be able to run the floors in a closed loop with simpler plumbing, I do not want them working on the same distribution system. Venting is a PITA because these heaters are in a basement that is pretty much entirely below-grade, and even the couple inches near the ceiling that are above grade still wouldn't get me up the 3ft or so I need to stay above snow accumulation. So we are going through the roof.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    You might need to increase the size of the intake vent. SOme of them use pressure sensors, and having a shared intake may not work. In theory, with one unit running, it could create a vacuum on the other. Don't know if that would create problems or not. It's also possible that it could disrupt the exhaust stream, but the fan would probably keep that from happening, but it might mess up the fuel/air mix. Lots of unknowns. It's always best to follow the manufacturer's instructions. You might be able to get to one of their engineers, but in general, the tech support people just read off of a script, so may not be that useful.
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  4. fitter30

    fitter30 Active Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Wouldn't recommend using a common intake because the pressure in the pipe between 1 running and 2 will change combustion of both units. If your concern is roof or wall penetration look into concentric flue kit then is just one penetration per unit.
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