The OEM's have engineered designs for ductwork. BUT! The actual install is up to production people that are NOT told the specs and why the install has to be to the engineering prints. That is why over the years I have seen horrible installs on Roof AC ducting and furnace floor ducting. But, until the customer complains I do not dive into the ducting to see the problems. One exception was over 10 years ago when we sold a certain brand if inexpensive Trailers. ALL the roof AC ducting was not installed and sealed especially at the AC control box plenum. HUGE gaps. It was so bad(customer complaints under warranty), that we in service told our PDI dept to repair every AC ducting of that brand during PDI, which the OEM paid for under warranty. AS I stated, most customers never realize that the furnace is LIMITING during operation and if they hear the burner kick off and on, they assume that is normal. Doug
