# Residential heating and cooling load estimate, Lot 308 Black Ridge Cove Phase 7

Residential heating and cooling load estimate following the ACCA Manual J 8th edition procedure. Design weather derived from published station data. This software is not ACCA-approved. First-pass estimate for review by a qualified designer.

Prepared 2026-09-18 by loads.py (this folder). All numbers in this report are written by the script from its own arithmetic. Imperial units throughout: Btuh, sf, cfm, F, grains per lb.

## 1. Project and site

| Item | Value |
| --- | --- |
| Project | Proposed residence for Black Ridge Cove LLC, Lot 308 (plans mislabel it Lot 708), Black Ridge Cove Phase 7, St. George, Washington County, Utah |
| Plan set | BRC - Floorplan Lot 308.pdf, 13 sheets, dated 11-25-24, cites 2015 IECC and 2021 IRC |
| Cover-sheet address | 1965 S Hammon St, Colorado City, AZ. Wrong per Alan; the house is at the base of Tech Ridge, St. George |
| Latitude / elevation | about 37.1 N / about 2,800 ft (station 37.09 N, 2,940 ft) |
| Climate zone | IECC 3B (hot dry) |
| Conditioned floor area | 2,413 sf plan living area (16 rooms, 2,455.1 sf face-of-stud in rooms.csv) |
| Storeys | 2, slab on grade, flat TPO parapet roof, attached 2-car garage as unconditioned buffer |
| Orientation | front (grid A) faces azimuth 159, rear 339, west side 249, east side 69 (assumptions A3) |
| HVAC | ductless mini-splits with ERV (mechanical notes 4, 5); duct loss zero |
| Occupants / bedrooms | 4 / 3 |

## 2. Design conditions

Sources and derivations are in design-conditions.md. The station is St. George Regional Airport, the only ASHRAE station in the valley.

| Condition | Value | Source |
| --- | --- | --- |
| Outdoor heating, 99% | 27.9 F | ASHRAE 2025 station table (-2.3 C); 99.6% is 25.3 F |
| Indoor heating | 70 F | MJ8 standard |
| Heating dT | 42.1 F |  |
| Outdoor cooling, 1% | 103.8 F | ASHRAE 2025 station table (39.9 C) |
| Coincident wet bulb | 65.5 F | ASHRAE 2025 station table (18.6 C) |
| Indoor cooling | 75 F, 50% RH | MJ8 standard |
| Cooling dT | 28.8 F |  |
| Daily range | 25.4 F (class H, borderline) | ASHRAE mean DB range of the hottest month; the 1% design day itself shows 28.3 F |
| Grains difference, 50% RH | -28.8 gr/lb | derived: outdoor 42.9, indoor 71.7 gr/lb at 13.27 psia |
| Altitude factors | sensible 0.993, latent 0.614 | 1.10 and 0.68 x density ratio 0.903 at 2,800 ft |
| Garage buffer | 49.0 F heating, 89.4 F cooling | midpoint rule (assumed) |
| ENERGY STAR county limits | cooling 109 F, heating 7 F | Washington County composite of other stations; reference only, station values sit inside the limits |

## 3. Construction and assemblies

| Id | Assembly | U (Btuh/sf F) or F (Btuh/ft F) | Status |
| --- | --- | --- | --- |
| W1 | Exterior wood-frame wall: 2x6 @16 o.c. w/ R-20 cavity, 7/16 waferboard, 1in EPS foam, stucco; 1/2 gyp inside | 0.049 | sourced R-values, U calculated |
| W2 | Exterior wood-frame wall: 2x6 @16 o.c. w/ R-20 cavity, 7/16 waferboard, weather barrier, inverted board-and-batten or cultured stone veneer (no EPS shown); 1/2 gyp inside | 0.060 | sourced R-value, U from IECC table |
| W3 | Partition wall between house and garage: 2x6 wood frame, 5/8in Type X gyp garage side, 1/2 gyp house side; cavity insulation not called out | 0.060 | assumed |
| R1 | Flat roof/ceiling: prefab wood trusses @24 o.c., R-38 in roof, roof sheathing, 60-mil TPO membrane, 1/2 gyp ceiling; no vented attic (parapet roof) | 0.030 | sourced R-value, U from IECC table |
| F1 | Floor over unconditioned garage: wood floor trusses @24 o.c., 1-1/4in T&G plywood, R-19 in floor, 5/8in Type X gyp ceiling (garage side) | 0.047 | sourced R-value, U from IECC table |
| F2 | Floor/ceiling exposed to outdoors (ceiling under covered porch, under open decks, cantilever): wood floor or deck trusses, R-19 assumed in floor, stucco soffit on 7/16 OSB | 0.047 | assumed location of R-19 |
| S1 | Slab on grade: 4in concrete slab w/ 6in turndown, no edge or under-slab insulation shown; F-factor (Btu/h per lineal ft of exposed edge per F), not U | 1.358 | assumed (no insulation) / MJ8 default soil |
| G1 | Glazed fenestration: vinyl windows w/ insulated glass; sliding glass doors treated the same | 0.350 | assumed code minimum |
| D1 | Opaque exterior entry door, 3'-0 x 8'-0 (sidelite counted separately under G1) | 0.350 | assumed code maximum |
| D2 | House-to-garage door, 3'-0 x 8'-0, 20-minute fire-rated, self-closing; solid-core assumed | 0.400 | assumed |

Areas are gross face of stud (Alan decision 4). Main-floor exterior walls are floor-to-floor 11.69 ft so the floor-truss band is charged as wall; upper walls are 10.08 ft; parapets are outside the envelope. Garage common walls R-20 U 0.060 (decision 2); deck-truss ceilings R-19 U 0.047 (decision 5); EPS under stucco only (decision 6); windows U 0.35 SHGC 0.25 (decision 7); slab F 1.358 heavy moist soil (decision 8).

## 4. Window and door schedule

| Id | Room | Face | Type | Nominal | Qty | Area sf | Asm | SHGC | External shading | Beam shaded at house peak | Sky view | Heating Btuh | Cooling cond Btuh | Cooling solar Btuh at house peak |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| O01 | M1 | W | window fixed | 2'-6" X 6'-0" | 1 | 15.0 | G1 | 0.25 | none | 0.00 | 1.00 | 221 | 143 | 818 |
| O02 | M1 | S | exterior door | 3'-0" X 8'-0" | 1 | 24.0 | G1 | 0.25 | covered porch, soffit at main ceiling line | 0.00 | 0.82 | 354 | 229 | 146 |
| O03 | M1 | S | sidelite obscure tempered | 30" X 96" | 1 | 20.0 | G1 | 0.25 | covered porch, soffit at main ceiling line | 0.00 | 0.82 | 295 | 191 | 122 |
| O04 | M1 | E | fire door 20-min | 3'-0" X 8'-0" | 1 | 24.0 | D2 | opaque | none | 0.00 | 1.00 | 202 | 138 | 0 |
| O05 | M4 | W | window fixed | 4'-0" X 6'-0" | 2 | 48.0 | G1 | 0.25 | none | 0.00 | 1.00 | 707 | 458 | 2,619 |
| O06 | M5 | N | sliding glass door | 12'-0" X 8'-0" | 1 | 96.0 | G1 | 0.25 | 2 ft awning at upper floor line | 0.05 | 0.99 | 1,415 | 916 | 1,709 |
| O07 | M5 | N | window fixed | 6'-0" X 5'-0" | 1 | 30.0 | G1 | 0.25 | 2 ft awning at upper floor line (assumed to run the rear wall) | 0.08 | 0.99 | 442 | 286 | 524 |
| O08 | M5 | E | window fixed | 6'-0" X 5'-0" | 1 | 30.0 | G1 | 0.25 | none | 0.00 | 1.00 | 442 | 286 | 199 |
| O09 | M6 | E | window fixed | 4'-0" X 1'-4" | 2 | 10.7 | G1 | 0.25 | none | 0.00 | 1.00 | 158 | 102 | 71 |
| O10 | U1 | W | transom fixed | 6'-0" X 2'-0" | 2 | 24.0 | G1 | 0.25 | none | 0.00 | 1.00 | 354 | 229 | 1,309 |
| O11 | U1 | N | exterior door | 3'-0" (x 8'-0" per note 6) | 1 | 24.0 | G1 | 0.25 | none | 0.00 | 1.00 | 354 | 229 | 440 |
| O12 | U2 | N | sliding glass door | 8'-0" X 8'-0" | 1 | 64.0 | G1 | 0.25 | 8 ft covered deck under main roof | 1.00 | 0.72 | 943 | 611 | 447 |
| O13 | U2 | W | window fixed | 3'-0" X 5'-0" | 1 | 15.0 | G1 | 0.25 | none | 0.00 | 1.00 | 221 | 143 | 818 |
| O14 | U3 | E | transom fixed | 6'-0" X 2'-0" | 1 | 12.0 | G1 | 0.25 | none | 0.00 | 1.00 | 177 | 115 | 79 |
| O15 | U4 | E | window fixed tempered obscure | 3'-0" X 5'-6" | 1 | 16.5 | G1 | 0.25 | none | 0.00 | 1.00 | 243 | 158 | 109 |
| O16 | U7 | E | transom fixed | 6'-0" X 2'-0" | 1 | 12.0 | G1 | 0.25 | none | 0.00 | 1.00 | 177 | 115 | 79 |
| O17 | U7 | S | sliding glass door | 6'-0" X 8'-0" | 1 | 48.0 | G1 | 0.25 | 5 ft covered deck under main roof | 0.00 | 0.87 | 707 | 458 | 300 |
| O18 | U9 | W | transom fixed | 6'-0" X 2'-0" | 1 | 12.0 | G1 | 0.25 | none | 0.00 | 1.00 | 177 | 115 | 655 |
| O19 | U9 | S | window casement | 3'-0" X 5'-0" | 1 | 15.0 | G1 | 0.25 | 2 ft awning, assumed at upper ceiling line | 0.00 | 0.97 | 221 | 143 | 98 |
| O20 | U10 | S | window fixed | 5'-0" X 2'-0" | 1 | 10.0 | G1 | 0.25 | 1 ft awning, assumed at upper ceiling line | 0.00 | 0.99 | 147 | 95 | 66 |

O02 (entry door) and O11 (loft deck door) are glazed clear glass on assembly G1 per Alan decision 10. Face letters are the 8-point names; azimuths S 159, E 69, N 339, W 249. Beam shaded fraction and solar gain are shown at the house peak hour (16:00 solar time); the per-room table uses each room's own peak hour.

## 5. Cooling equivalent temperature differences

Manual J takes opaque cooling loads from ETD tables by daily range and colour. Those tables are not reproduced here. Instead the script computes, for every hour of the July 21 clear design day, the ASHRAE sol-air temperature for each wall orientation and the roof, and passes it through a first-order lag (time constant 3 hours for the stucco-and-EPS frame walls, 2 hours for the membrane roof) to represent the damping and delay a light assembly gives; ETD is that response less the indoor temperature. The outdoor hourly profile is the design DB less the daily range times the ASHRAE fraction-of-range for that hour. Walls: light stucco, alpha/h_o 0.15. Roof: white TPO, alpha/h_o 0.117 and the 7 F long-wave correction. Deck ceilings and floors over the porch see shaded outdoor air. Values at the house peak hour:

| Surface | ETD at house peak (F) |
| --- | --- |
| wall S (az 159) | 35.4 |
| wall E (az 69) | 34.0 |
| wall N (az 339) | 35.5 |
| wall W (az 249) | 50.7 |
| roof, white TPO | 46.0 |
| shaded outdoor (deck ceilings, floors over porch) | 25.4 |
| glazing conduction (outdoor air at hour) | 27.3 |
| garage buffer | 14.4 |

Expected divergence from the MJ8 tables: MJ8 uses one wall ETD for all orientations, so the west wall here (51 F) sits above the MJ8 figure and the south, east and north walls (34 to 36 F) below it; on this house the west walls are about a third of the exterior wall area, so the net effect is small, of order 500 to 1,000 Btuh either way. No licensed table was available to check against; the undamped sol-air peaks (west wall 137 F, roof 130 F) are ordinary values for a clear 104 F day, and the damping applied is the conservative end for a stucco-over-EPS frame wall. The white-roof ETD of 46 F is the one to question if the TPO is allowed to grey (sensitivity row in section 11).

## 6. Room loads

Heating is steady state at the design dT. Cooling sensible is the room's own peak hour (solar time) of conduction plus solar plus internal plus air loads; the house block total is the peak hour of the sum, which is hour 16. Latent is occupants only: the infiltration and ventilation latent terms are negative (drying) at St. George's design grains and are taken as zero, per the MJ8 convention; section 7 shows the raw values.

| Room | Name | Floor sf | Heating Btuh | Heat Btuh/sf | Cooling sensible Btuh (room peak) | Peak hour | Cooling sensible at house peak | Cooling latent Btuh | Cool Btuh/sf | AED excursion |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| M1 | Entry / Stair Hall (incl. under-stair  | 186 | 3,732 | 20.0 | 2,797 | 15 | 2,743 | 0 | 15.0 | yes |
| M2 | Powder | 37 | 816 | 22.3 | 317 | 17 | 312 | 0 | 8.7 | no |
| M3 | Broom Closet | 17 | 382 | 22.4 | 149 | 17 | 146 | 0 | 8.7 | no |
| M4 | Living Room | 327 | 4,103 | 12.5 | 4,788 | 16 | 4,788 | 400 | 14.6 | yes |
| M5 | Dining Room | 257 | 5,953 | 23.2 | 6,116 | 17 | 5,420 | 400 | 23.8 | yes |
| M6 | Kitchen (incl. pantry and refrigerator | 304 | 2,662 | 8.7 | 2,586 | 8 | 2,221 | 0 | 8.5 | yes |
| U1 | Loft (incl. open stairwell footprint) | 238 | 2,360 | 9.9 | 3,583 | 16 | 3,583 | 0 | 15.1 | yes |
| U2 | Master Bedroom | 224 | 2,209 | 9.9 | 3,083 | 18 | 2,831 | 0 | 13.8 | yes |
| U3 | Master W.I.C. | 91 | 1,103 | 12.1 | 1,119 | 8 | 842 | 0 | 12.3 | yes |
| U4 | Master Bath (incl. toilet room, linen, | 183 | 1,044 | 5.7 | 1,346 | 8 | 884 | 0 | 7.4 | yes |
| U5 | Laundry | 74 | 151 | 2.1 | 152 | 14 | 142 | 0 | 2.1 | no |
| U6 | Upper Hall + Linen | 44 | 123 | 2.8 | 113 | 14 | 107 | 0 | 2.6 | no |
| U7 | Bedroom #3 (incl. closet) | 208 | 2,446 | 11.8 | 2,391 | 13 | 2,070 | 0 | 11.5 | yes |
| U8 | Bath (hall) | 59 | 164 | 2.8 | 151 | 14 | 142 | 0 | 2.6 | no |
| U9 | Bedroom #2 | 158 | 1,746 | 11.1 | 2,118 | 15 | 2,109 | 0 | 13.4 | yes |
| U10 | W.I.C. (Bedroom #2) | 49 | 849 | 17.2 | 776 | 11 | 654 | 0 | 15.7 | yes |
| HOUSE | block at hour 16 | 2,455 | 29,843 | 12.2 | 28,993 | 16 | 28,993 | 800 | 11.8 |  |
| SUM | sum of room peaks | 2,455 |  |  | 31,585 |  |  |  | 12.9 |  |

### 6a. Heating by component (Btuh)

| Room | Wall | Glass | Roof/ceiling | Floor to outdoors | Garage side | Slab edge | Infiltration | Ventilation | Total |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| M1 | 489 | 869 | 0 | 0 | 399 | 1,382 | 495 | 97 | 3,732 |
| M2 | 189 | 0 | 0 | 0 | 0 | 448 | 160 | 19 | 816 |
| M3 | 88 | 0 | 0 | 0 | 0 | 210 | 75 | 9 | 382 |
| M4 | 608 | 707 | 0 | 236 | 102 | 1,677 | 601 | 171 | 4,103 |
| M5 | 502 | 2,299 | 0 | 364 | 0 | 1,954 | 700 | 134 | 5,953 |
| M6 | 484 | 158 | 0 | 0 | 229 | 1,201 | 430 | 159 | 2,662 |
| U1 | 619 | 707 | 300 | 0 | 0 | 0 | 610 | 124 | 2,360 |
| U2 | 274 | 1,164 | 283 | 0 | 0 | 0 | 371 | 117 | 2,209 |
| U3 | 402 | 177 | 115 | 0 | 0 | 0 | 362 | 48 | 1,103 |
| U4 | 219 | 243 | 231 | 0 | 40 | 0 | 215 | 96 | 1,044 |
| U5 | 0 | 0 | 93 | 0 | 20 | 0 | 0 | 39 | 151 |
| U6 | 0 | 0 | 56 | 0 | 44 | 0 | 0 | 23 | 123 |
| U7 | 476 | 884 | 262 | 0 | 205 | 0 | 510 | 109 | 2,446 |
| U8 | 0 | 0 | 75 | 0 | 58 | 0 | 0 | 31 | 164 |
| U9 | 530 | 398 | 199 | 89 | 31 | 0 | 416 | 82 | 1,746 |
| U10 | 319 | 147 | 62 | 14 | 42 | 0 | 239 | 26 | 849 |
| HOUSE | 5,199 | 7,754 | 1,676 | 703 | 1,171 | 6,870 | 5,186 | 1,284 | 29,843 |

### 6b. Cooling sensible by component at each room's peak hour (Btuh)

| Room | Hour | Wall | Glass cond | Glass solar | Roof | Floor to outdoors | Garage side | Internal | Infiltration | Ventilation | Total |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| M1 | 15 | 503 | 595 | 1,140 | 0 | 0 | 273 | 0 | 220 | 67 | 2,797 |
| M2 | 17 | 233 | 0 | 0 | 0 | 0 | 0 | 0 | 71 | 13 | 317 |
| M3 | 17 | 109 | 0 | 0 | 0 | 0 | 0 | 0 | 33 | 6 | 149 |
| M4 | 16 | 654 | 458 | 2,619 | 0 | 143 | 70 | 460 | 267 | 117 | 4,788 |
| M5 | 17 | 424 | 1,378 | 3,232 | 0 | 219 | 0 | 460 | 311 | 92 | 6,116 |
| M6 | 8 | 310 | 37 | 581 | 0 | 0 | 157 | 1,200 | 191 | 109 | 2,586 |
| U1 | 16 | 692 | 458 | 1,750 | 328 | 0 | 0 | 0 | 271 | 85 | 3,583 |
| U2 | 18 | 265 | 628 | 1,727 | 218 | 0 | 0 | 0 | 165 | 80 | 3,083 |
| U3 | 8 | 200 | 42 | 652 | 31 | 0 | 0 | 0 | 161 | 33 | 1,119 |
| U4 | 8 | 140 | 58 | 896 | 63 | 0 | 27 | 0 | 96 | 65 | 1,346 |
| U5 | 14 | 0 | 0 | 0 | 112 | 0 | 13 | 0 | 0 | 26 | 152 |
| U6 | 14 | 0 | 0 | 0 | 67 | 0 | 30 | 0 | 0 | 16 | 113 |
| U7 | 13 | 406 | 578 | 661 | 305 | 0 | 141 | 0 | 226 | 74 | 2,391 |
| U8 | 14 | 0 | 0 | 0 | 90 | 0 | 40 | 0 | 0 | 21 | 151 |
| U9 | 15 | 537 | 272 | 759 | 235 | 52 | 21 | 0 | 185 | 56 | 2,118 |
| U10 | 11 | 252 | 80 | 230 | 56 | 5 | 29 | 0 | 106 | 18 | 776 |
| HOUSE block | 16 | 5,002 | 5,023 | 10,610 | 1,830 | 424 | 801 | 2,120 | 2,303 | 879 | 28,993 |

### 6c. AED check

| Room | Glass solar, 8 AM to 7 PM average | Glass solar, peak hour | Peak / average | Excursion (over 1.30) | Excursion Btuh |
| --- | --- | --- | --- | --- | --- |
| M1 | 722 | 1,140 | 1.58 | yes | 202 |
| M4 | 1,224 | 2,619 | 2.14 | yes | 1,027 |
| M5 | 1,990 | 3,232 | 1.62 | yes | 645 |
| M6 | 175 | 581 | 3.32 | yes | 354 |
| U1 | 904 | 1,750 | 1.94 | yes | 574 |
| U2 | 960 | 1,727 | 1.80 | yes | 479 |
| U3 | 196 | 652 | 3.32 | yes | 397 |
| U4 | 270 | 896 | 3.32 | yes | 545 |
| U7 | 577 | 1,032 | 1.79 | yes | 282 |
| U9 | 467 | 759 | 1.63 | yes | 152 |
| U10 | 129 | 270 | 2.10 | yes | 103 |

What was done: MJ8 flags a room (or zone) whose peak hourly fenestration gain exceeds 1.30 times the 8 AM to 7 PM average, and adds the excess to a load otherwise built on the daily average gain. This engine carries every room at its own peak hour instead, so the excursion is already inside each room figure; the table only shows how peaky each room's glass is. Every room with glass on a single orientation trips the 1.30 test, as expected: east rooms (M6, U3, U4) peak at 8 AM, west rooms (M4, U1, M1) at 3 to 4 PM, the master bedroom at 6 PM. For one mini-split head serving the open living, dining and kitchen area, size on the block of those three rooms at the house peak hour (12,428 Btuh), not the sum of their separate peaks (13,490 Btuh).

## 7. House summary

Heating (Btuh):

| Component | Btuh | Share |
| --- | --- | --- |
| wall | 5,199 | 17% |
| glass | 7,754 | 26% |
| roof ceiling | 1,676 | 6% |
| floor out | 703 | 2% |
| garage | 1,171 | 4% |
| slab | 6,870 | 23% |
| infiltration | 5,186 | 17% |
| ventilation | 1,284 | 4% |
| Total heating | 29,843 | 12.2 Btuh/sf |

Cooling sensible (Btuh):

| Component | Btuh | Share |
| --- | --- | --- |
| wall | 5,002 | 17% |
| glass cond | 5,023 | 17% |
| glass solar | 10,610 | 37% |
| roof ceiling | 1,830 | 6% |
| floor out | 424 | 1% |
| garage | 801 | 3% |
| internal | 2,120 | 7% |
| infiltration | 2,303 | 8% |
| ventilation | 879 | 3% |
| Total sensible, block at hour 16 | 28,993 | 11.8 Btuh/sf |
| Sum of room peaks | 31,585 | 12.9 Btuh/sf |

Cooling latent (Btuh):

| Component | Raw Btuh | Design Btuh (negative air latent taken as zero) |
| --- | --- | --- |
| occupants | 800 | 800 |
| infiltration | -1,423 | 0 |
| ventilation | -905 | 0 |
| Total | -1,527 | 800 |

Total cooling at the block hour: 28,993 Btuh sensible plus 800 Btuh latent = 29,793 Btuh (2.48 tons). Sensible heat ratio 0.97. Duct loss zero.

Hourly house sensible on the design day (Btuh):

| Hour | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Btuh | 18,049 | 19,958 | 20,926 | 21,482 | 22,575 | 24,956 | 27,176 | 28,207 | 28,993 | 28,522 | 25,301 | 15,923 | 13,658 |

## 8. Infiltration and ventilation

| Item | Value |
| --- | --- |
| Envelope volume | 26,565 ft3 (rooms.csv areas x 10.08 ft, plus the 1.60 ft truss band on the main floor) |
| Blower door | 3.0 ACH50 = 1,328 cfm50 |
| Effective leakage area at 4 Pa | 447 cm2 (69.2 in2), flow exponent 0.65 |
| Model | ASHRAE Fundamentals basic (LBL) model, two storeys, shelter class 3 normal: Cs 0.000290, Cw 0.000231 |
| Heating infiltration | 124 cfm at 42.1 F dT and 15 mph = 0.28 ACH natural |
| Cooling infiltration | 81 cfm at 28.8 F dT and 7.5 mph = 0.18 ACH natural |
| Infiltration sensible | heating 5,186 Btuh; cooling 2,303 Btuh |
| Infiltration latent | -1,423 Btuh (negative: outdoor air is drier than 50% RH indoor) |
| Ventilation | ASHRAE 62.2-2016, 0.03 x 2,413 + 7.5 x 4 = 102.4 cfm continuous, balanced ERV |
| ERV | sensible effectiveness 0.70, latent 0.50 (assumed) |
| Ventilation sensible | heating 1,284 Btuh; cooling 879 Btuh (after ERV recovery) |
| Ventilation latent | -905 Btuh (after ERV recovery) |
| Apportioning | infiltration by each room's share of gross exposed wall (2,957 sf); ventilation by floor area |

The station's own coincident winter wind is only 2.9 mph (calm inversions), so the MJ8 15 mph default is conservative for St. George; the sensitivity table shows the effect.

## 9. Assumptions register

| Term | Value used | Source | Status |
| --- | --- | --- | --- |
| 99% heating dry bulb | 27.9 F | ASHRAE Handbook Fundamentals 2025 ch. 14 station table for St. George Regional AP (WMO 724754), reproduced in the climate.onebuilding.org STAT/DDY files (-2.3 C) | sourced |
| 1% cooling dry bulb | 103.8 F | same station table (39.9 C) | sourced |
| Mean coincident wet bulb at 1% DB | 65.5 F | same station table (18.6 C) | sourced |
| Daily range | 25.4 F, class H (borderline) | ASHRAE 2025 station table, mean DB range of the hottest month (14.1 C); the 1% design day in the DDY file shows 28.3 F. MJ8 class boundaries L under 16, M 16 to 25, H over 25 F | sourced value, class boundaries from memory of MJ8 |
| Indoor design | 70 F heating; 75 F, 50% RH cooling | brief; MJ8 standard indoor conditions | sourced |
| Site elevation and pressure | 2,800 ft; 13.27 psia; density ratio 0.903 | Alan (elevation); ICAO standard atmosphere | derived |
| Sensible air factor | 0.993 Btuh per cfm F | 1.10 x density ratio | derived |
| Latent air factor | 0.614 Btuh per cfm grain | 0.68 x density ratio | derived |
| Grains difference at 50% RH | -28.8 gr/lb (outdoor 42.9, indoor 71.7) | ASHRAE psychrometric equations at site pressure from the 1% DB / MCWB pair | derived |
| Unconditioned garage temperature | midpoint: 49.0 F heating, 89.4 F cooling | Midpoint of indoor and outdoor. The MJ8 buffer-space procedure was not available to check; a UA balance would be the next step | assumed |
| Slab F-factor | 1.358 Btuh per ft F | assemblies.csv (MJ8 Table 4A heavy moist soil, no edge insulation, as quoted on a public page); Alan decision 8 | sourced (reference value) |
| Blower door | 3.0 ACH50 on 26,565 ft3 envelope volume | Alan decision 9 (2015 IECC CZ3 limit); volume from rooms.csv with the main-floor truss band | sourced |
| ACH50 to design infiltration | ELA 447 cm2 (69.2 in2); 124 cfm heating (0.28 ACH), 81 cfm cooling (0.18 ACH) | ASHRAE Fundamentals ch. 16 basic (LBL) model: Q = ELA sqrt(Cs dT + Cw V^2), Cs 0.000290 and Cw 0.000231 for two storeys, shelter class 3 (normal); flow exponent 0.65; wind 15 mph heating, 7.5 mph cooling (MJ8 defaults) | derived |
| Infiltration apportioning | by gross exposed above-grade wall area (2957 sf total) | MJ8 apportions infiltration to rooms by exposed wall area; rooms with no exterior wall get none | sourced (procedure from memory) |
| Ventilation rate | 102.4 cfm | ASHRAE 62.2-2016: 0.03 x 2,413 sf + 7.5 x (3 + 1) | sourced |
| ERV effectiveness | sensible 0.70, latent 0.50 | typical residential ERV; no product selected | assumed |
| Ventilation apportioning | by floor area | ERV supply assumed distributed to rooms in proportion to floor area | assumed |
| Cooling ETD, opaque | sol-air method, see report section 5 | ASHRAE sol-air temperature with light-colour alpha/h_o 0.15 (walls), 0.117 (white TPO roof), 7 F long-wave correction on the roof; passed through a first-order lag (time constant 3 h walls, 2 h roof) for light-frame damping; hourly outdoor profile from the ASHRAE daily-range fractions | derived |
| Daily temperature profile | ASHRAE fraction-of-range table, peak at 3 PM | ASHRAE Fundamentals climatic design chapter; values reproduced from memory (about 0.05 tolerance) | reference |
| Clear-sky solar | ASHRAE 2017 tau model, July taub 0.388 taud 2.339 | station STAT file (2025 Handbook monthly optical depths); July 21, latitude 37.09 N | sourced |
| Glazing solar gain | SHGC 0.25 x angular factor x beam + 0.87 x SHGC x diffuse | angular curve is the ASHRAE double low-e pattern (1.00 at 0 to 40 deg, 0.85 at 60, 0.40 at 80); ground reflectance 0.20 | derived |
| External shading | shade-line geometry per hour | shadow depth = overhang depth x tan(profile angle); shaded fraction applied to beam only; diffuse reduced by a sky-view factor under the deck roofs and awnings. Same geometry MJ8 tabulates as shade line factors | derived |
| Interior shading | none | brief | sourced |
| Internal gains | 4 occupants at 230 s / 200 l (2 living, 2 dining); 1,200 Btuh kitchen appliances | brief (MJ8 defaults) | sourced |
| AED check | peak hourly glass solar vs 1.30 x 8 AM to 7 PM average, per room | MJ8 adequate exposure diversity rule as remembered; excursion reported, and the room peak already includes it | procedure from memory |
| Duct loss | 0 | ductless mini-splits, Alan decision 3 | sourced |
| Latent, negative grains | infiltration and ventilation latent taken as zero when negative; occupants carried | MJ8 treats a negative design-grains latent load as zero rather than a credit; the raw negative values are reported for information | procedure from memory |

The surface schedule's own assumptions A1 to A25 (run-a-all-at-once/assumptions.md) and Alan's decisions 1 to 10 (decisions.md) apply on top of this list.

## 10. Open questions

- Window product: U 0.35 / SHGC 0.25 are 2015 IECC maximums, not a selection. A stock low-e vinyl unit (U 0.30, SHGC 0.23) cuts the loads by the amounts in section 11.
- Awning positions for O19 and O20 (upper front windows) were taken at the upper ceiling line; the front elevation should confirm the awning band height. The rear awning was assumed to run across O07 as well as O06.
- Garage buffer temperature: midpoint used. The garage holds the furnace, water heater and softener, which warm it in winter; a UA balance with those gains would raise the winter garage temperature and lower the garage-side heating loss (about 1,171 Btuh at midpoint).
- Deck-truss ceilings under the two upper decks and the floor over the porch are assumed R-19; if uninsulated the living and dining ceilings roughly quadruple their loss (section 11).
- Slab soil: heavy moist F 1.358 stands until the geotech report; St. George soils are usually light and dry, which lowers the slab term (6,870 Btuh, the second largest heating component).
- Blower-door target 3 ACH50 is the code limit, not a tested value. A tested 2 ACH50 house or a 5 ACH50 house moves the air loads in proportion.
- ERV effectiveness is assumed at 0.70 sensible / 0.50 latent; the selected unit's rated values should replace them.
- Whether the ERV supplies all rooms or only some (apportioning by floor area assumes all).
- Build location: Alan confirmed St. George; the cover-sheet Colorado City address is wrong. Colorado City (5,000 ft, CZ 4B or 5B) would change every design condition.
- Design day: July 21 with July optical depths. West and south glass peak later in the year at lower sun angles; a September check would raise the west-room solar slightly but at a lower outdoor temperature.

## 11. What would move the numbers most

| Case | Heating Btuh | Change | Cooling sensible block Btuh | Change |
| --- | --- | --- | --- | --- |
| Windows U 0.30 SHGC 0.23 (stock low-e vinyl) | 28,735 | -1108 | 27,427 | -1566 |
| Blower door 5 ACH50 instead of 3 | 33,300 | +3457 | 30,529 | +1535 |
| Blower door 2 ACH50 instead of 3 | 28,114 | -1729 | 28,226 | -768 |
| Deck ceilings and porch floor uninsulated (F2 U 0.20) | 32,133 | +2290 | 30,374 | +1381 |
| Garage walls uninsulated (W3 U 0.20) | 31,075 | +1232 | 29,836 | +843 |
| Garage at outdoor temperature (no buffer credit) | 31,013 | +1171 | 29,794 | +801 |
| ERV sensible effectiveness 0.50 | 30,699 | +856 | 29,579 | +586 |
| No ERV recovery (exhaust-only ventilation) | 32,840 | +2997 | 31,043 | +2050 |
| Station coincident winds (2.9 mph winter, 12.8 mph summer) | 28,008 | -1834 | 29,681 | +688 |
| Dark walls (alpha/h_o 0.30) and grey roof (0.23) | 29,843 | +0 | 31,860 | +2867 |
| 99.6% heating design (25.3 F) | 31,753 | +1910 | 28,993 | +0 |

Read this table with the register: the base case carries code-minimum windows, code-limit air tightness, heavy moist soil and an assumed ERV. The first three rows are the ones a product selection or a test result would settle.

## 12. Reconciliation to the earlier heating rollup

See checks.md. Summary: rerun at the rollup's conditions (26 F outdoor, 48 F garage, conduction only):

```
room       wall   glass  roof/clg  floor_out  garage    slab   conduction  rollup  diff
M1           511     909         0          0     417    1444        3281    3281    +0
M2           197       0         0          0       0     468         665     665    +0
M3            92       0         0          0       0     219         312     312    -0
M4           636     739         0        247     107    1753        3481    3481    -0
M5           525    2402         0        381       0    2042        5349    5349    +0
M6           506     165         0          0     240    1255        2165    2165    +0
U1           647     739       314          0       0       0        1699    1699    +0
U2           286    1217       296          0       0       0        1798    1798    +0
U3           420     185       120          0       0       0         725     725    -0
U4           229     254       241          0      42       0         766     766    -0
U5             0       0        97          0      21       0         118     118    -0
U6             0       0        58          0      46       0         104     104    +0
U7           497     924       274          0     215       0        1910    1910    +0
U8             0       0        78          0      61       0         139     139    -0
U9           554     416       208         93      33       0        1304    1304    -0
U10          333     154        65         14      44       0         610     610    +0
HOUSE       5434    8103      1751        735    1223    7180       24427   24427    +0
```