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I am looking to possibly install concentric vents through the roof for two tankless water heaters I have. But I am concerned about rain coming back down the vent and damaging my water heaters. All the pictures that I see have a direct opening straight up with no elbows on it. Most of the other vents on my home have a 180 degree fitting to point the vent down after penetrating through the roof.

For example,

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My tankless water heater (80% efficient, with stainless steel style vent) does indeed use a condensate drain in its venting system. This was required to comply with local codes and the follow the installation instructions for my particular unit. There is a “condensate drain tee” fitting which has a small hose nipple on the side, to which a condensate drain system is attached.

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The higher efficiency tankless water heaters, e.g., the 95%+ efficiency units, are condensing units and also cool down their exhausts enough to be able to use plastic vent material (such as PVC) will have condensate drains built in to them. I would guess rainwater that falls down into the vent, much like condensate, would safely exit the unit through the condensate drain.

I would recommend reading your water heater installation instructions, and checking with local installation codes to make sure you are installing yours correctly. The instructions and codes may indeed call for condensate drain, which would collect rain water.

I’ve seen elbows installed on the end of some plastic vents - presumably to make it tougher for rain water, birds, or small vermon to get in. Follow whatever the installation instructions and local codes say, and you’ll be fine. No need to reinvent the wheel.

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The caps keep the rain out. It is designed to be open and water will get in and evaporate just like the moisture from your water heaters. This whole system works just fine in a conditioned space. Your only issue would be if you were going to leave your house desolate in the winter... but then you would have bigger issues.

Based on comments would like to add two things:

  1. Excess water will run back to water heater and probably out an output tube (usually going to floor drain).
  2. And if there is a severe instance where your cap is not stopping the water - IT SHOULD - then you can use a flapper valve, pictured below.

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But you should have this (there are different recommendations for different regions and climates) -

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Cap to go out the side:

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And the... it rains sideways in my area cap.

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-----> Only advice to not install a cap will look like this guy!

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  • A three-inch-an-hour rain isn't going to evaporate gracefully away. In the case of a furnace you have a condensate drain system. What about a water heater?
    – isherwood
    Feb 1 at 20:19
  • @isherwood - cap should severely limit and rain coming in. Even in a pouring rain these systems get very very minimal rain. Actually the only problems that I have ever seen is when the air gaps are low where they are sitting in snow - which means you need to extend the stack.
    – DMoore
    Feb 1 at 20:22
  • There is no cap here. The end is wide open to the sky. I've seen many such terminations installed in walls, but not in roofs.
    – isherwood
    Feb 1 at 20:27
  • They are pretty common in roofs as well. There's generally a drain to deal with rain and condensate. I have seen larger ones with a J-bend downwards.
    – Ecnerwal
    Feb 1 at 20:51
  • @isherwood - that is a cap... not sure I have ever used that one, but I can assure you it is not the best/right one for that application. When you do need to vent straight out like that you use a flapper valve to limit rain... This is always a last resort as there should be a better way of dealing with it.
    – DMoore
    Feb 1 at 21:51

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