I'm working on this gas water heater with 3" hood almost attached to an elbow that reduces from a 4" exhaust flue. This was only attached by a lot of aluminum tape. The problem is that the ends of both sides are exactly the same size. What parts do I need to do this properly?

Here is a picture of the current situation

bad fitting exhaust duct

  • Welcome to Stack Exchange. You might want to take our tour to see how the site operates.
    – HoneyDo
    Mar 7, 2020 at 17:51

2 Answers 2


There are 3" coupling made for this. They are available at home stores and heating ans AC supply stores. enter image description here

You'll probably need a few sheet metal screws or go back to some aluminum tape.


It may not be pretty but there isn't anything wrong with using aluminum tape on this. However, there are other options.
There are a number of reducers and elbows that will fit:
enter image description here
This is a 3" to 4" adaptor that should get you started and can be combined with a flexible elbow:

enter image description here
These are American Metal Products available at HD. You may have to do some adjusting - ductwork is as much art as science. If these particular items don't work for you there are others out there.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.