What is the duct size of the trunk line you are cutting???

The main trunk is 12 1/2" high by 10" wide and is the same size from the air handler to the end.

Looking at the origional duct, it can handle 600 to 800 cfm. the system you have should be no larger than 2 tons of cooling. considering that you would like to cut off 2 " from the sides this would lower your cubic feet per minute to 500 cfm or alittle over a ton of cooling. now if you have a "T" style duct configuration, (furnace and duct trunk line going either way, it should be ok. If an "L" style, you may be compromising the far side of the system. I think you should get a local contractor to look at this set up because I do not know exactly what you are attempting to do. They will be glad to give you advice.
Each ton of cooling requires 400 cfm of airflow to pass through the coil to allow the system to operate properly.

[This message has been edited by HOLLYWOOD (edited September 05, 2000).]

Thanks, your advice is greatly appreciated.
It is a "T" system with one side extending about 8 feet and the side I want to alter extending about 12 feet.....both ends supply 3, 6 inch flex ducts and a main trunk supplying the 2nd story going up from the middle. What I need to do is gain an extra 2 inches of head room under the one end of the main trunk so I can install a set of French doors. I spoke to my contractor and he came up with upgrading my outdoor unit and my auxillary heat.....and all I wanted to know was whether or not I could get the extra 2 inches?! An estimate of $3500.00 just to gain 2 inches of headroom under 4 feet of trunk. Again, thanks for the advice and your time....I really appreciate your help!

Yes you can.........I needed to know the facts before I compromised your hvac system. Sorry it was drawn out.

Thanks again....it's great to be able to obtain an unbiased opinion.