I am trying to select a natural gas water heater to install into my house. It has to service two bathrooms with showers and two kitchen sinks, a dishwasher, and a washing machine.

I've narrowed my selection down to three:

My application is indoor, and my plumber told me to look for a "direct vent" model because we want to vent horizontally to the exterior of the house (distance < 3') instead of going up through a second floor and a metal roof. This leads to the question: how do I evaluate the venting options? It seems that at least two of these models support multiple venting types, and I have no idea how to evaluate them. For example, the T-H3 overview has a bullet that states:

Venting with Schedule 40 PVC, CPVC, PP, Category III stainless steel

I like having options, but there are 4 different things there and I don't know how to evaluate them.

Is there a particular advantage of concentric stainless steel over PVC or vice versa? Is concentric PVC even a thing?

1 Answer 1


Concentric PVC is a thing. I like concentric pipes because they only require a single penetration rather than two. Honestly, any of your options should be fine for a high-efficiency gas appliance and a three foot pipe run. The exhaust temperature of condensing, high-efficiency gas appliances is pretty low, which is why PVC is an option. PVC might be expected to have greater longevity since it can't corrode, while stainless steel can (it's stainLESS steel not STAINless). And it should be cheaper in materials and labor too. Your gas plumber probably prefers it for those reasons, whether he passes the savings onto you or pockets the profit.

  • In a PVC scenario, why do there have to be 2 pipes? Commented Nov 19, 2014 at 18:19
  • 4
    There are always two pipes: one for outside air to be used for combustion, the other for exhaust gasses to escape. A concentric pipe just puts one inside the other to save space and permit you to only cut one hole through whatever it has to pass through.
    – iLikeDirt
    Commented Nov 19, 2014 at 18:51
  • Note that using PVC for the exhaust is something that couldn't be done with older boilers/heaters, since they put too much heat out the stack. Now that they're extracting heat from the combustion so much more efficiently, the exhaust is cool enough that PVC is adequate.
    – keshlam
    Commented Nov 19, 2014 at 19:39
  • 1
    Right, only high-efficiency condensing (85%+ efficiency) gas appliances can use PVC vents. My gas plumber explained to me that condensation starts to happen at about 84% efficiency, so beyond that, the machine has to be engineered to deal with it and have a condensate drain, with the happy side effects of more heat per dollar and simpler venting.
    – iLikeDirt
    Commented Nov 19, 2014 at 19:45

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