Reply by Greg The Cajun Box Sculptor

If you want to use metal pipe you should use spiral pipe that is made for dust collection. HVAC pipe …as you found out…is no good for dust collection. It is for Heating Ventilation Air Conditioning, which does not have anywhere near the pressure rating as dust collection requires. You can also use schedule 40 PVC pipe which is far better than HVAC. Several woodworking supply companies sell adapters that allow you to connect it to standard 4” dust port tool fittings.
There is a lot of info out there about the numerous options of dust collection options.

-- Every step of any project should be considered your masterpiece if you want the finished product to reflect the quality of your work. http://www.FineArtBoxes.com