I know if my geo is insufficient for summer cooling on the second floor, I might still use the 2nd floor window A/Cs for annual costs of $175, but that only very slightly extends somewhat the time to break even..
But what I'm REALLY CONCERNED ABOUT is if I will be WARM ENOUGH in the winter, since oil is the commodity that is going to kill me in the next 10 years, not electric.
As you stated, for a 4 ton geothermal heat unit technically I should have 16 individual 6" dia. ducts in the house. Looking back to my previous comment, I currently have only 11 ducts plus one extra in basement, not 16.
With only 11 ducts, will a 2 stage 4 ton geothermal fail to push enough hot air and therefore require greatly increased supplemental resistance heat that will up my electric bill over every winter? Is it worth the money to run a bit more ductwork for heating or heating and cooling, given the age of the house.
My gut feeling is that I am only about 4 ducts short of what I need. What if I install 1 extra duct in each of the two upstairs bedrooms, plus install a passive vent to help return air in each upstairs bedroom on the wall adjacent to the hallway stairway landing, which is a direct route back down for return air. That way each upstairs bedroom has 2 ducts, not one.
But that is not planning for increasing the main trunk size from which these direct vertical 3"x9" ducts branch off and come directly upwards through a middle wall and exit the FLOOR in the middle of each bedroom. Weird place for heat vents but that is how the existing 2 ducts are laid out, why not 2 more just like them?
And 2 extra ducts upstairs achieves only half way to the 4 ducts needed to reach the goal of 16.
So for the first floor, I have to decide where I need the BTUH most. My bedroom already has 2 ducts (13'x13'), but the other 1st floor bedroom (10'x10') only has one. I could add one more duct to that room, since that room has sometimes been lukewarm in the winter anyway (one duct but 2 windows, northwest facing).
For the final extra ductwork, I might consider 1 or 2 more ducts to the addition area living room (15'x26'). I already have 2 ducts midway on the far outside wall. But the room has a 7 foot slider, an 8 foot picture window, and two small windows. It is warm with oil forced air, but might need more air with lukewarm geo heat, I don't know.
Anyway, in order to avoid running cross-ways against the rafters and avoid digging up the finished basement ceiling, I could add 2 more ducts right along the wall that borders the addition versus older part of house. One near the slider, where there is heat loss, and one behind the woodstove, which is the greatest distance from all of the existing heat ducts in living and dining area.
So this would give me a total of 11 existing + 2 upstairs + 3 mainfloor = 16 ducts plus 1 in basement.
I wonder whether this would "BREAK THE BANK", cost efficiency speaking, to add to the geo install total price by adding 4 or 5 ducts in this manner (two from the main trunk going straight up through a middle wall of the 1st floor into the middle of rooms on 2nd floor, also one duct from main trunk straight up also inside the middle wall to the 1st floor bedroom, and finally 2 ducts only about 3 to 5 feet direct off the furnace room itelf through the former outside wall of house, to the living room inside wall.
I'm considering asking for a caveat price not-to-exceed on retrofit of 4-5 ducts, price per duct in certain rooms. I want to know all of this before pulling the trigger on something that is NOT BASED ON NEED FOR COMFORT, PURELY BASED ON LONG TERM NEED TO SAVE ENERGY (MAINLY OIL) COSTS. The loan costs will wash out the savings in energy if the install becomes ridiculously expensive.
For return air, I'm not worried as long as air can get back to the basement. Is this incorrect? I have a door I leave open between main floor and basement.