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Backflow Preventer for Condensing Model install with PVC?

I am installing eight new Takagi tankless water heaters--the condensing models that require 3" PVC. I am installing 8 of them as a gang, with gang ventilation, per the manual. My installer insists that each unit must have a back flow preventer installed on the exhaust. I argue that back flow preventers are for the non-condensing models with the expensive stainless vent pipe. IN fact, I don't see any back flow that is not stainless, so it would be wrong for PVC.

Who is correct?

The whole reason I spent more on condensing models is the cheaper easier PVC exhaust issue. I wanted them installed per the manual, eight max, into the same main exhaust pipe. Nothing he says makes sense to me.

I need this answered urgently, because if I'm right, I need to know if I should lose faith in this guy. I'm not sure he knows what he's talking about, but I need to know the answer to this issue ASAP. I'd appreciate any help I can get. He tried to talk me into Navien brand, which raised my suspicion in the first place. He is my primary contractor's plumber, so I am stuck with him for now.


  • lchmb
    lchmb Member Posts: 2,997 page 26 shows a non return valve and gives the part number.. this is on the condensing unit..not sure if it's what you have
    delta T
  • Ironman
    Ironman Member Posts: 6,555
    The proper term is "back draft", not backflow. A back flow preventer goes on the potable water line feeding a boiler. I'm not trying to be critical - just to avoid confusion.

    Regardless, every common vent condensing boiler that I'm familiar with requires some form of back draft protection.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    delta TRobinInCali
  • Zman
    Zman Member Posts: 6,975
    lchmb said: page 26 shows a non return valve and gives the part number.. this is on the condensing unit..not sure if it's what you have

    The instructions on page 26 shows the venting diagram, which must be followed for a safe warrant-able installation. The "non return" valve is shown on page 10. It is made of PVC so is certainly intended for a condensing application.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • RobinInCali
    RobinInCali Member Posts: 30
    Thank you all for this. I see that my plumber had the wrong term. The only negative about this thing is that we have 8 tankless units, so we need 8 non-return valves. That's about $3,000 I didn't know I had to spend. Insert Sad Face Here.