7/ 6 inch ducts is only going to give you about 560 CFM. You need 750-800 CFM. If you keep your 8 inch duct open, you will gain another 180 CFM which will give you a 740 CFM capacity. This will barely get you there. This is assuming you have a TESP of .5 in your system.
The correct furnace would have a -08 designating a 2 ton blower. Whether you need a 45,000 BTU or a 70, 000 BTU can only be determined with a load calculation.
I had mentioned earlier that I have 7/6inch ducts and 1/8inch duct. I am able to disconnect the 1/8inch duct and remove it. It is not necessary since I added it to heat my basement (which really doesn`t work anyway)
With only 7/6inch flexy duct to feed ,the longest being about 20ft.. from the trunk. Should I have the installer put in the 2ton rated inside unit and be done with it?
...or could there be some advantages of having an oversized inside unit.
One disadvantage of having the larger inside unitis is that the system is loud and the air blows pretty hard out of the registers ...even when jumpering the blower to the lowest speed.
The best HVAC systems that I have seen have been silent and comfortable.
I cant find it.... Made or Dash can post the low speed tap info. I'm betting not only the drive is oversized but the heating capcity is probably to much also. I cant imagine much over 50,000 for that house. Obviously we need to know the true load calcs.
oops, maybe that furnace only has 3 speeds. sorry for not knowing that model number, must be a basic 80%.