OT- HVAC questions

The size of the heater and the a/c coil obviously depends upon where you live.

To keep duct noise down select the duct velocity in both the Mains and Branch Ducts around 1000 feet/minute.

Keep in mind that the 'effective area' of a rectangular duct does not match the mathematical cross sectional area.

For example, a 8" Round duct (.35 sf) moves 350 cfm @ 1000 fpm; whereas, it takes a 14" x 4" Rectangular duct (.38 sf) to do the same.

In the HVAC professional world a mechanical Calculator, called a "Ductulator" is used to select the proper size duct.

However, assuming that you don't have one, and you don't need great accuracy, assume that the 'effective area' of a rectangular duct is only 90% of the mathematical area.

As the flow into the Branch ducts gets smaller so does the duct to keep the velocity around 1000 fpm.

Hope this helps.

I have one more question, does the size of the returns = size of supply (cross sectional area) or is there some sort of factor involved?
Thanks again

Keep Return Air Grille at least 20' from furnace, and preferrably with a couple of duct turns, to mitigate fan noise.

Preferrable to add a small duct at Return Air Plenum on fan coil unit that brings in some fresh outdoor air. Install damper in this duct so that approximately 10-15% of Supply air is fresh air. Equal amount of air not returned to furnace will pressurize house to keep drafts out at doors and windows.