I'm finishing up the design of my next home. A bit of background... When we moved to Arizona (from Charlotte) 10-1/2 years ago, it was my intent to develop a 53-home net-zero 'green' subdivision, where I would ultimately reside. The subdivision would feature onsite wastewater treatment as well as a small community center that would serve double-duty as a regional green building education and resource center. Our zoning regs allow for 3/4 acre rural lots with onsite wastewater treatment, although no one has ever done this. In early 2008, I purchased a 40-acre parcel about 10 minutes north of Fort Huachuca, the area's dominate economic engine, in an area that was destined to be the next hot building market. That was then.
The influence of the fort protected us from the deepest effects of the real estate collapse. But the wind-down from two wars and years of defense budget sequestration have since plunged Cochise county into a prolonged economic depression as other areas of the state and country have recovered nicely from the Great Recession. So my grand plan to become a developer is out the window and that very expensive piece of land I bought has virtually no value for the foreseeable future. Ouch.
Still wanting to build a new home (hopefully our last), I decided not to split off my building lot from the 40. In Arizona, lots must be min. 4 acres to avoid subdivision regs, I purchased another lot for my house. If and when the local real estate market recovers, my parcel will have significantly more value if it's kept intact. So I purchased a nice 4-acre parcel nearby for cheap.
I've been spending most of my spare time the last couple of months designing the house and preparing the construction drawings. I'm almost done, hoping to break ground end of July. The house is a 1,472 ft2 ranch over a fully below-grade finished basement (lot is flat), for a total of 2,944 ft2 CFA. I did the load calcs last week and the cooling load is so small, it surprised even me! The sensible is <8,000 BTUH (we have negligible latent loads).
My enclosure specs aren't all that special.
* R22 bibs wall cavity + R4 EPS continuous (customary on stucco homes here)
* R15 bibs basement call cavity + R2.5 XPS continuous behind frame walls
* R38 on underside of "flat" roof deck + R2.5 continuous XPS on top
* dual-pane windows (27/17 except south, 28/43)
* 2 ACH50
* cooling design temp is 92, although I designed for 95/77
Of course, ducts & ahu are in the floor trusses and the house is purpose-designed to be passive solar, optimized for this climate. But the main reason the load is so low is that the glazing represents less than 12% of CFA (due to basement being fully below grade). The basement has three 60"H twin casement egress windows plus six 24"H awning windows. On the main level, the garage is on the west. I only have one west-facing window (guest bedroom egress).
The house screams for zone control (motorized dampers) since the basement load, at 0.2 tons, is way too small for its own air handler (smallest ducted mini-split head is 9k). I'm not willing to try to zone a single mini-split ducted head. Fortunately, Carrier added a 1-ton model to its VS18 series (5 speed inverter). The Carrier/Bryant Infinity zone control is arguably the best on the market. I imagine this will be the first whole-house application for that model, at least for homes in the 3000 ft2 range.
According to MJ, my heat load is just under 16k, but I know from experience that MJ overstates heat loads, especially in this climate where winters are generally sunny, and especially for load-load homes in general. Keep in mind that MJ intentionally ignores solar gain as well as internal gains when calculating the head load. But when you crank down on the envelope load, those gains become rather significant and, to some extent, persistent. I estimate my true design heat load is closer to 6k so I won't be installing any supplemental heat. Moreover, the diversity benefit of zoning gives me even more headroom for extreme conditions beyond design.
As I mentioned to a colleague this morning, a lot of things have had to happen, experience-wise, over the past two decades to give me the confidence necessary to go with a 1-ton heat pump. The only thing I would do differently if this were in a humid climate would be to tighten the enclosure to 1 ACH50, thus avoiding the need for supplemental dehumidification.
The house is all electric (other than a propane outdoor grill). I estimate I need ~3.5 kW PV array to achieve net-zero site energy (includes conventional electric DHW).