12 ,8" branch ducts may or may not be okay.Being they are all 5" ,it sounds like the duct system was not designed correctly,seldom if ever would all branch runs be the same size,as they are feeding different rooms/areas.
What is the physical relationship between the indoor unit,existing home,and the adddition?how large is the addition?
What size/tons is the old system?
I really like that you promote this, Nice to see someone out there is doing it correctly!
The only decision in life is to decide what to do with the time given to you
Currently, we have 14" main flex duct and 8" flex duct for outlets. Measurements were taken from outside sides so probably the inside airflow diameters are little smaller. We are doing an addition and the ac contractor said that we need a 4 ton after doing computer calculations. He also said that the existing duct is not big enough. We are on a tight budget.
Is the existing duct enough for a 4 ton ac?
i would find some place else to cut corners and do the ac and duct work correctly nothing worse than a nice new addition that you sit and sweat your a$$ off in
I think the HO needs to look at the pro's & Con's on doing the job right from the start or having it fixed later after the fact, Which one is cheaper When you look at the over all job and what is is going to cost to install the proper ductwork from the start vs. have it work just ok, but then being uncomfortable while using this room and the possibiliies of damaging the equipment i.e replacing one of those newer ECM blower motors, replacing a compressor on a new 4 ton unit Those seem to be a bit more expensive than a few bucks for the right ductwork. If your going to skimp on this job buy some cheap paint, and cheap curtains, or a lower grade flooring to make up the difference for better ductwork you'll thank yourself later for doing so as the other items are easier and cheaper to replace later by you like paint,curtains, ect......