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.

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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.

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  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?

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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