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venting tankless water heater & weil Mclain Ultra 155

tbart16 Member Posts: 23
i am getting ready to install a Rinnai tankless and a weil mclain ultra 155, I was wondering if I could vent them through the wall through the same pipe. The distance from the units to the wall is less than 10 feet and would use only one elbow per vent. I know they both vent in 3" can I combine them and do it in 4" at the wall?

I am trying to limit the holes I put in the wall. 



  • Tim McElwain
    Tim McElwain Member Posts: 4,556
    NO they have to be vented

    separate and apart per manufacturers instructions. If you can get Rinnai to talk to Weil-Mclain and come up with a special engineering concept that is also approved by your local AHJ then maybe!
  • Slimpickins
    Slimpickins Member Posts: 327

    No you can't do it and why do you want the Rinnai when you have a boiler that can supply your hot water with the addition of an indirect tank??? Just use the boiler for your DHW and you only need one penetration in the wall provided its ok to use a concentric vent on the Ultra. Plus, 9 times out of 10 you have to upsize your gas line from the meter to handle the BTU's of a on demand type water heater. 
  • Jack
    Jack Member Posts: 1,046
    the minimum spread

    between the two vents is 12" inside to inside, as long as they are at the same elevation. The Rinnai vent is concentric 3 & 5". Go to Rinnai.us and pick your water heater model. open details/documents and look at the installation manual for details
  • bob eck
    bob eck Member Posts: 930
    TGWH & WM Ultra

    Use the boiler and a indirect water heater. You will need to make sure your gas line is large enough. Some TGWH have BTU inputs 180,000 to 199,000 plus the boiler that needs 155,000 BTU input. Plus you need to vent both units.
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