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In a 4 story multi-owner house in Rotterdam, the Netherlands there is an 80mm semi-rigid aluminum duct like the one shown in the image below which contains the water heater exhaust of my neighbors who live below. Since I have made structural changes to the roof this duct needs to be extended. Ideally, it can connect with a rigid pipe that can then connect with the roof exhaust.

several aluminium / aluminum ducts, one coming in horizontally through the wall in the back, making a 90° turn downwards and leaving the image at the bottom; one marked "new 90 mm rigid aluminum duct" coming in horizontally from the left, open-ended; a flexible one coming in vertically from the bottom right, marked "existing 80mm semi rigid aluminum duct", open-ended

I cannot find a piece to provide a mechanical connection from this semi-rigid pipe to a rigid pipe. Do you have any suggestions?

Is there any reason why the following hack wouldn’t work?

The 80m semi-rigid pipe is placed inside a minimum of 50mm distance inside a 90mm rigid aluminum pipe. Griffon HBS-200 (a universal, durable, water and airtight and protective sealant) is used to seal the connection. Vaster Aluminium Tape then wraps the entire connection. This detail is sketched in the image below.

a schematic drawing (cross-section) of the proposed solution, showing existing 80mm semi rigid aluminum duct (marked as such and in red) coming in from the bottom, inserted into the new 90mm rigid aluminum duct (marked as such and in dark blue) for a length that's marked as "min 50mm", showing liquid rubber (marked as such and in light blue) at the end of the new duct and aluminium tape (marked as such and in grey) covering the joint from beyond the ends of the ducts

Questions

  1. What connection should I use (if you can share products from other countries I can look for their equivalent in the Netherlands)?
  2. Is my hack a stupid idea and why?

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A couple of potential problems:

  1. Sooner or later the duct will have to deal with liquid water running down the interior wall. It'll be condensate from the exhaust gases themselves, or condensate from warm moist outdoor air falling onto a cool pipe, or rain water entering in a storm.. Anyway, the water will pool inside that connection.
  2. Liquid water will turn acidic because of other compounds in the exhaust gases. Are the proposed sealants appropriate for the usage?

Perhaps you could make your own adapter. Crimp some pleats or tucks into the end of the 90 mm duct so that its diameter is reduced. Continue until it shrinks enough to fit inside the 80 mm duct. Slide the 80 mm over the crimped 90 mm, secure with sheet metal screws, and then tape over the joint.

The tucks can be made one at a time with long-nose pliers by grabbing the metal and twisting, but there are pliers made for crimping ducts in this way. (Thanks for the photo, Amazon.) duct crimp pliers

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