Originally Posted by ads67
Terminology used in my post was what was quoted to me by HVAC professional. So, if it was an incorrect term (which i don't believe it to be) pls. don't shoot the messenger.
I did understand when I asked about it that you were merely repeating what an installer told you. Exactly why a picture is so useful in this instance - it allows everyone to know exactly what you are talking about.
And, FYI, hvaclover posted a picture of a sheetmetal plenum and a canvas flex connector so I'm still a little bit unclear what your original question is...
You descibed a sheetmetal plenum connecting your new high eff furnace to the existing return duct. Your question APPEARS to be, is the "boot" necessary, and you APPEAR to be using the term "boot" to describe a canvas flex duct between the sheetmetal plenum and the furnace.
I would suggest the answer to your question is "no, the boot is not required" however one of the reasons it is there is to reduce vibrational noise from the furnace to the return duct. If you are not currently annoyed by any noise from your furnace right now, then you should be fine without the canvas flex duct. You do have a filter on that furnace, right?