Is there any reason why I can not use large ( 8" ) pvc pipe as ducting? It certainly makes for easier joints and it would seem that its' smooth surface would result in quiet and effecient movement of forced air. Any thoughts advice?

No particular reason any longer, as flex duct is a plastic liner like a shopping bag. There might have been some worry in the days of 140F temperatures, as that is about what PVC gets soft at; but most furnace run about 120F now.

There is always the thought that ducts are holes from room to room that fire and smoke can pass through, so that is one consideration for metal duct.

www.heatpro.us FREE heat/cool right-sizing

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

Did you use the pvc for ducting? I am about to begin a project and am wondering the same thing?

While you are wondering, wonder whether or not to tell people if you are going to put 140F air through the PVC, which could cause it to melt or sag. The answer might depend on giving such information up front, subjects of interest to home insurance companies.

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Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.