HVAC duct planning is a difficult enough of a science, but some installations are designed to fail as
HVAC ducts should not have holes inside the walls.
Why do I say planning duct work is a difficult science?
Because their job is to balance air flow and the indoor temperature, and that ain't easy! Many, many factors go into designing the HVAC system for a house - sizes of rooms, open spaces, numbers of windows and doors, cubic this and that, and on and on. Air has to evenly come and go. It really is a design best done with computer programs.
Older homes had heating systems that blew air around with small, metal ducts. Metal ducts are famously leaky.
Someone got the idea to wrap some joints and such with tape - the infamous duct tape. And it worked, basically, but only until the tape dried out and the leaking began again.
Now HVAC science uses a combination of metal ducts and flexible ducts. Flexible ducts are nothing more than huge slinkies with plastic wrapped around them. Some are insulated. They can be extended and fitted and wrapped around things to send air where ever. But it all must be tight! No leaks!
The latest method of sealing seams and joints in metal duct work is a latex paste.
It is a thick material, like painting with peanut butter. When it dries it seals cracks such that when the ducts expand and contract with temperature changes it can move to. It is working well. You can see it in this photo.
But it doesn't work so well when the plumber cuts through the duct to run a drain line! That drain line passes COMPLETELY through the metal duct!
THIS WAS GOING TO BE SEALED WITH DRYWALL IN A COUPLE OF DAYS!
It would forever have been a duct that weakly transferred air to many HVAC registers. Those rooms would never have been properly conditioned. The HVAC company would have been called and called to measure and test and nobody would have been able to figure out why those registers did not blow much air.
The reason, of course, is that the duct would have been blowing its air
INSIDE THE WALL!
It might never have occurred to anyone to call a thermographer (who are all really cute) to see if there might be leaking inside the wall anywhere.
This is what leaky metal ducts look like inside a wall traveling from basement to upper level to condition the bedrooms upstairs. That warm air looks orange and yellow in the thermal image. The inside of that wall is comfy warm! The bedrooms in that house weren't so comfy.
The hole in the metal duct above would have leaked terribly! It would have lost a large portion of its air pressure. And forever!
Had we not done a pre-drywall inspection the interesting plumbing arrangement above would not have been handled and drywall would have gone up. The supervisor didn't see it or didn't care. The county approved the HVAC and didn't see it or didn't care.
BUT THE HOME INSPECTOR JUST HAPPENED TO NOTICE IT ON THE PRE-DRYWALL INSPECTION.
My recommendation: it's an exceptional idea to have a pre-drywall inspection done on new construction. You never ever know what the home inspector might come up with! The home inspector is an objective voice and had an objective eye. What he finds on the inspection won't simply be blowin' in the wind. He'll note anything important! And if he's a really cute thermographer, well, even better!