Thanks for your replies, even though most of them are discouraging.
Based on the HVAC-Calc PC application, the heat loss for that house is
The furnace is at 80% efficient rate, so it requires minum 47,500 BTU
So I picked up the 50,000 BTU furnace.
For the 4 rooms that require less than 60 CFM, I ran 4" flexible ducts
to each of the rooms.
For the 2 bigger rooms that require around 133 CFM, I ran 6" flexible
ducts to them.
These are based on the HVAC-Calc calculations.
All the ducts are underneath the crawl space will connect directly to
the sheet metal transitional box on the discharge end of the furnance.
The vent pipe is 3", which is indicated on the furnace manuals.
I will run the vent pipe straight through the ceiling of the furnace
room and stays up on the roof for more than 1 foot to create the draft.
My dad, a licensed electrician will run the electrical cables to the
furnace for me.
I am now stuck at the return air duct, that some of you did give me
My new question is do I need a return air duct for each of the living
rooms or one big one should be OK?