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I'm designing my next home, small load surprised even me! (case study)

David ButlerDavid Butler Posts: 3,884
edited July 8 in Heating & cooling
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 as is customary on stucco homes here
* R15 bibs basement wall 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 95/76 (corrected)

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. there's only one west-facing window, for egress in guest bedroom.

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 (the smallest ducted mini-split head is 9k). I'm not willing to try to zone a single ducted head. Fortunately, Carrier recently added a 1-ton model to its VS18 series (5 speed inverter drive). The Carrier/Bryant Infinity zone control is arguably the best on the market. I imagine this will be the first whole-house application for the 1-ton model, at least for a home in the 3,000 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 needless to say, I won't need any supplemental heat. Moreover, the diversity benefit of zoning gives provides even more headroom for extreme conditions beyond design.

As I mentioned to a colleague this morning, a lot of things had to happen, experience-wise, over the past 2 decades to give me the confidence necessary to go with a 1-ton system for this home. The only thing I would do differently if this were 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'll need ~3.5 kilowatts of PV to achieve net-zero site energy, including conventional electric DHW.


  • I looked into the details on that Carrier system, David, and am surprised that the lowest cfm on the air handler is only 300cfm (from what I can see), which results in a pretty high sensible heat ratio when the compressor is on lowest speed. You're probably not worried about it for your household or climate, but what are your comments about this for mixed-humid and hot-humid climates? I have to deal with the same kind of issues with ducted mini-splits in my climate.
  • Jake VierzenJake Vierzen Posts: 82
    Sounds awesome David! Glad to know about the tiny heat pump. About time.
  • David ButlerDavid Butler Posts: 3,884
    @John, according to the AHU product data book, the FE4ANF002 can ramp down to 150 CFM (see pg 8, version FE4A-10PD), which would be the equivalent of 272 CFM/ton at 95/75/63, although there's no indication that the system would actually ramp down that low. The minimum speed for the 1 ton model is 175 CFM, but you have to dig deep to find that information.

    I'm guessing you were looking at the the heat pump product data book. The table on page 4 (25VNA8 - 06PD) indicates 300 CFM min cooling airspeed (DH, zoning). That table is confusing, but I believe the 'Minimum Cooling' column refers to minimum CFM for stage 5 operation. Note how the maximum airflow data is broken into two columns - Stage 1 and Stage 5, but the minimum column isn't.

    Or were you looking at the expanded capacity tables? The table on pg 14 indicates the stage 1 airspeed at 95/75/63 is 300 CFM. However, the 105F and 115F columns show 200 CFM with footnote indicating the system will upshift to stage 2 in that case. But course, these tables don't account for the various airflow modes (MAX efficiency, Comfort, DH)

    Confused yet? Perhaps the following excerpt from the Infinity Zone Guide will bring some clarity. The minimum airspeed for the 1-ton model is 175 CFM. Note that references to "per ton" refer to nameplate capacity, not staged capacity. This logic explains how Carrier is able to avoid the need for bypass.


    If the system determines that with the selected airflow limit, the allowable airflow into a zone is not sufficient for the equipment to operate correctly, and that zone has a call for conditioning, the system will take the following 4 steps:
    1. Reduce airflow if possible
    . . .a. Minimum of 275 cfm per ton minimum in high stage cooling (325 if Dehum airflow set to High),
    . . . . .175 cfm per ton in low stage cooling. (225 if Dehum airflow set to High)
    . . .b. Comfort Heat airflow is minimum for heat pump heating (3.5 x outdoor temp + 137) cfm /ton
    . . .c. Use heat pump comfort airflow as minimum if AC airflow is efficiency or MAX
    . . .d. No adjustment for furnace heating
    2. Dump air to zones set to Unoccupied
    . . .a. Unoccupied zones may be conditioned up to the most conditioned setpoint.
    3. Dump air to zones with less conditioned setpoints
    . . .a. Zones with less conditioned setpoints may be conditioned to within 3 deg of the most conditioned setpoint.
    . . .b. Increase or decrease all zones an additional 0.75 deg
    4. Stage down equipment
    . . .a. Equipment will stage down or shut off if necessary
    . . .b. Last 10 System Events will record an AIRFLOW LIMITED event
    . . .c. If shut down occurs, other zones need to call before equipment will resume operation
  • Thanks, David. Yes, quite confusing. I was looking at the expanded capacity table.
  • DavCamrasDavCamras Posts: 59
    One of my questions is: what if you sell the home and the next owner wants to keep at at 72 degrees not 77? What does MJ say then for cooling capacity? Will the system make it?
  • David ButlerDavid Butler Posts: 3,884
    edited July 8
    @Dav, I erred in my original post. I designed for 95/76, yielding a sensible load of 7,800 btuh. I typed as 77 because that's my typical cool setpoint. At 95/72, the sensible load is only 1200 btuh higher (9k), well within the system's capacity in this climate. Also keep in mind that for well built homes, MJ is known to overstate accurately modeled cooling loads by at least 10%. And zone control provides at least an additional 10% diversity buffer that's not fully accounted for in MJ8. Lastly, if I went by the book and used the 1% design temp (92), the load @ 72 would be ~8,200 sensible, so I definitely haven't designed myself into a corner on resale.
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