Rusty Water Heater Vent Pipe in New House

indygalSeptember 10, 2008

We've been in our new home just 4 months. We have a 90% efficient gas water heater in our basement which is vented through the attic. The pipe has two curves between the basement ceiling and the top of the water heater. Both curves have quite a lot of rust on them plus there is rust on the top of the water heater. We're wondering if the problem is that the pipe sticking out of the roof is too short. The plumber said our rust problem is the worst he's seen. He told us that the vent pipe should be above the roof peak, but it's several feet below it. Is the rust likely from the short pipe or could there be another reason?

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What brand, model and type water heater do you have?

What is the venting material?
4 types
Plastic (most often PVC)
Stainless steel (Cat III with gaskets)
Double wall b-vent Alum pipe with galv outer pipe
single wall pipe used for connectors only with combustion clearance

Anything above 90% efficient is usually PVC Vent, anything between 78%-88% is usually stainless steel and both those dont rust

    Bookmark   September 10, 2008 at 5:42PM
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Thanks for the help z1700. As you probably figured the water heater is not a 90% efficient one. That's the furnace. The water heater is an A.O. Smith ProMax. Model is GCV 50 100. The pipe is the double wall b-vent. The pipe has two bends in it. The first is less than a foot above the water heater. The pipe runs uphill for about 4feet and then there's another bend which takes it through the floor above.

    Bookmark   September 10, 2008 at 6:19PM
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Rust can eventually lead to perforation, but until then the stack will work just fine.

    Bookmark   September 10, 2008 at 7:56PM
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First the water heater is supposed to have an 18" vent rise off the top before the first 90, or as much of the 18" you can get while maintaining a minimum 1/8" rise per foot on the horizontal pitch. Where the vent turns up through the house to the roof, there is supposed to be a tee with a cap in the bottom with the vent connected on the side.
Where the vent terminates the roof, it is supposed to be 2' higher vertically than any rooflines or peaks within 10' horizontally.
Make sure it has a cap, a proper roof jack and storm collar. The rust may be rain coming down the outside of the pipe from an improper install.
And lastly, the pipe size must be appropriate for the BTU's vented into it according to its height.

    Bookmark   September 10, 2008 at 9:58PM
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Obviously we have a problem with the installation of the vent pipe. From what you say the turns above the water heater are not done right. From the ground, it looks like the pipe is 5 to 6 feet below the peak of the roof. We don't want another hole in our roof plus we don't want a really long vent pipe extension.
We may have to change out water heaters for a 90% gas or an electric one. The water heater is in an unfinished part of the basement with access to an outside wall. Is there a minimum length that the PVC vent pipe has to be on a 90% gas water heater? Thanks for your input.

    Bookmark   September 11, 2008 at 10:34AM
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Minimum no

Maximum on PVC vented water heaters vary on manufacturer and model, the average is 60'

    Bookmark   September 11, 2008 at 5:26PM
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Thanks for the input z1700. Someone is coming to check out the rusted pipe tomorrow. I'll let you know what their suggested solution is. More than likely it's extending the pipe several feet higher above the roof which is definitely not our first choice.

    Bookmark   September 11, 2008 at 7:47PM
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The rusty pipe in the basement has all been replaced. The plumber connected the wrong type of pipe to the water heater and the vent pipe which the HVAC crew had installed. We now have the correct bends in the pipe and also the new section is double walled like the rest of it. Thanks for the help.

    Bookmark   September 12, 2008 at 12:46PM
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