Alot of could be here. First the fan is designed to move a certain range of air volumn. If you should restrict the air flow enough, that would cause the fan to work harder and overheat to the point the fan would start a fire. Without proper air flow measurements, you wound not know how much air flow is safe to restrict and be within the design air flow range. It's called balanceing the air flow. Another bad thing is that the people whom installed the vent system don't know either. They just slap in some vent and it works some what. Not a well design system at all. When you change vent size it's hit or miss. Miss can cause a fire.
emtee - try over on the Tech Issues forum - as I recall there have been several career heating & a/c folks over there - - one with ID of something like "HVAC".
Also - over there - enter "ducting" in search = 4 pages.
Measureing the temp rise IS one of the most important measurements - it proves if there IS enough airflow and anybody can operate a thermometer. The furnace mfgr will say what is the max temp rise but it's usually about 75 degrees - 65 degrees is pretty normal. BTW, I've been doing hvac work for 33+ years. There actually are some old residential systems that used 2" supplies - I know where there is still one in existance. Coleman used to make residential furnaces (slightly better quality than their mfgr'd home and rv junk) and they liked to run 4" supplies, for some reason. It works but not cost effective to have to install so blasted many supplies! Craig
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