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There's definitely some confusion here-
-a system: one furnace, one condenser. Could be used for one floor or multiple floors.
-zone system: one system like above but with dampers that can control the amount of air that is supplied to an individual room, area, or floor. Each room, area, or floor has it's own t/stat that controls the air handler and the damper(s).
The house you're describing would definitely be a candidate for duel systems. And more so with that area-RTA. But, it's possible that with a manual J calc a zone system maybe adequate- but I think a straight single system would be extremely inadequate, regardless of the tonnage. I'd probably do a manual d calc also. I'd also see what the seer rating is for the system they're installing- figure the annual cost versus the seer rating/annual cost of two smaller systems.
Bonus rooms over garages have always been problematic for HVAC guys. When in fact it's not their doings- it's the builder's minimum standard/code compliance doings. It's not so much about the amount of insulation, as it is about making it air tight. Due to the nature of construction techniques used for framing a habitable space above a garage, certain procedures need to be addressed to complete the "envelope" of the space. Without going into a lot of details- a foamed floor over a garage is the best defense against extreme cold and heat of that particular space.
Of course, all of this is basically mute because you're dealing with a national builder that only builds to minimum standards/ code compliance. And they generally don't want to, or won't deal with other subs outside their approved list.
Right now, energy efficiency should be at the top of every builders list. Yes, of course it will cost more to build the house- the cost gets past to the consumer. It's how they sell it that matters. If a house is a long term "investment", shouldn't everything you get out of it be the same.(?)
what Knwbldr said is spot on about the rooms over garages at least from my research. I am in the sheetrocking stage of my renovation.
the biggest problem with rooms over garages is the floor in the room ends up freezing due to no insulation below, or inadequate insulation. Since we have some flexability, we could spec our insulation in each space and went with sprayfoam in the garage ceiling as well as any areas where the second floor was cantilevered past the first floor.
does it cost a lot? well, in the grand scheme of things....... no. in comparison to the other insulation? yes, it was 40% of the bill for 10% of the space. the total spray foam bill was 1700 for the garage ceiling (single bay) and the 2ft overhang.
Just a follow up. After talking with the builder it is one unit but it has two thermostats (one for each floor) and an electronically controlled damper to direct air upstairs or downstairs.
This particular builder is pretty much cookie cutter so I can't go crazy with customizations. I am thinking if we build we will see how livable the bonus room is. If too hot in summer we might consider a radiant barrier mentioned above. If the floor is too cold in winter then we will consider extra insulation. Hopefully a mini-split is the last resort, while convenient I am hoping it doesn't get to that point.
Thanks again for the suggestions, I appreciate everyone's input.
I am thinking if we build we will see how livable the bonus room is. If too hot in summer we might consider a radiant barrier mentioned above. If the floor is too cold in winter then we will consider extra insulation. Hopefully a mini-split is the last resort, while convenient I am hoping it doesn't get to that point.
Don't bother with any "after-the-fact". The cost will not be worth it.
The best radiant barrier on the market right now is radiant barrier sheathing. If, you were to add "it" later it would require removing the existing shingles and decking- not worth the hassle. Any after market, post framing type of barrier is pretty much just $$$ down the crapper.
Adding "extra" insulation may not be the answer. And if it were, it would mean removing all the drywall from the ceiling to install. Then replacing all the drywall. More money down the crapper!
A split system would require a 220v line. What are the chances that would be easy to do "after-the-fact"?
I don't think it's as much about "if" you build- but who you build with.
When you say "cookie cutter builder", All I hear is, Builder builds to the minimum standards that are required for the area. Nothing more, nothing less. I hope you make a wise decision.
Manual J determines the minimums (heating and cooling).
After that, it appears to me you can do as many zones as you want (even window A/C) as along as you meet the minimums.
My preference would be a zone dedicated to what gets the most traffic, and another zoned for least traffic. Manual J for zone 1 ... and Manual J for zone 2 = Manual J for the facility.
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