I'm seeking advice evaluating options that different HVAC contractors and consultants have presented for fixing a 2nd floor climate control problem. We live in the mixed-humid zone (near Washington, DC).
The building contractor who renovated our 65-year-old house and added an addition screwed up the HVAC very badly (long story, photos available). Many of the problems have been fixed, but the 2nd floor is still screwed up. Prior to our renovation, climate control on the 2nd floor was fine. The HVAC sub replaced the original two upstairs bathroom air supplies, which had been separately ducted from the furnace with strong air flow from wall registers at floor level. These 2 bathrooms are now supplied by one smaller duct that travels a tortuous route from the basement up through 2 stories and into the attic where it then splits into two flex ducts that make further twists and turns across the attic floor to the ceilings of each bathroom. As a result, the bathrooms receive virtually no air. We now have a 40% reduction to the air supply to the 2nd floor, leading to problems not only in the bathrooms but the entire 2nd floor -- too hot in summer, too cold in winter. Our contractor has agreed to resolve this problem, but exactly what to do has not been determined yet.
We have been presented with the following possible options. Some of these are not mutually exclusive.
1. Put a booster fan in the duct to the bathrooms.
2. Raise the wall returns in the 3 bedrooms from floor level to near the ceilings.
3. Create a separate zone for the 2nd floor by adding a 2nd plant (Trane heat pump, etc.) in the attic with a 2nd Trane condenser for A/C. (Our main plant is a Carrier Infinity system.)
4. Install damper-controlled zoning for the entire house using the existing plant (2, maybe 3 zones).
5. Increase the size of the duct running to the bathroom supplies.
6. Reduce the size of the registers in the 2 bathrooms. (I frankly don't understand this option, which was proposed by an HVAC subcontractor from an established, registered company. When I asked how it would increase air flow, he told me that "The air is getting lost in the register box.")
Our 2-story house is now about 3000 square feet, with a finished basement and a 1st-floor, single-story addition over a conditioned crawlspace. All ceilings are 8-ft, except for a cathedral ceiling in the 800-sq. ft. addition that is 13-ft. at the highest point.
The original plant was replaced during the renovation, and we now have a 5-ton Carrier Infinity, one zone, in the basement. The contractors and consultants have all agreed that the current plant has sufficient capacity to do the whole house.
The 2nd floor is about 700 total sq. ft. with three bedrooms and 2 baths. Each bedroom has a supply and return at the base of the walls, the bathrooms have no returns. Attic is unconditioned, but it has a well-insulated floor, a ridge vent, and a powered attic fan at one of the two gable vents. It has no soffits due to the roof's design and is in effect a crawlspace.
Basically, I need to know the pros and cons of the different options. I also have some specific questions:
1. Assuming for the moment that all options are feasible, is there any one that would be the best way to go to achieve the best combination of good climate control, energy efficiency, and operating and maintenance costs? I've heard that two plants are cheaper and more efficient to run than one, but also more expensive to run if you include maintenance costs.
2. Will a 2nd plant work in an unconditioned attic? I've read that it's a bad idea, especially given our attic's very high temperatures (120+) in the summer, but I can't find any solid analysis on this. Also, since a heat pump's efficacy is significantly reduced by extreme summer/winter exterior temperatures, I'm having trouble seeing how a heat pump will provide us with good, efficient climate comfort for the 2nd floor.
3. Does adding a 2nd plant require shutting off the current supplies and returns to that floor, or is it possible to add something to supplement the existing climate control? If the 2nd floor needs to be self-contained, how can we control humidity for the 2nd floor in winter? Do we need a 2nd humidifier?
4. Assuming a proper (e.g., fantech) booster fan is installed in the duct, how noisy are they and what are the operating costs?
5. If available space in the walls and joist bays prevents simply running another bathroom supply duct up to the attic, are there other ways to get more air up there? For example, figuring out a way to straighten the existing duct, replacing the current oval duct in the walls with a rectangular duct that fills the stud bay?
6. Are there other options the contractor should be considering? For example, would it make sense to close in and condition the attic?
I realize this is a lot of info and questions, but your advice, insights, and suggestions on any part of this would be greatly appreciated.