# Design conditions, St. George, Utah

For the Lot 308 load calculation (loads.py). Station: St. George Regional Airport, WMO 724754, 37.09 N, 113.59 W, 896 m (2,940 ft). The site at the base of Tech Ridge is about 2,800 ft, roughly 1.5 miles from the airport; no correction was made for the 140 ft difference (about 0.5 F of dry-bulb lapse, inside the rounding).

## Values used

| Term | Value | SI as published | Source | Status |
| --- | --- | --- | --- | --- |
| Heating dry bulb, 99% | 27.9 F | -2.3 C | ASHRAE Handbook Fundamentals 2025, ch. 14 station table, as carried in the station STAT and DDY files below | sourced |
| Heating dry bulb, 99.6% | 25.3 F | -3.7 C | same | sourced, reference only |
| Cooling dry bulb, 1% | 103.8 F | 39.9 C | same | sourced |
| Mean coincident wet bulb at 1% DB | 65.5 F | 18.6 C | same | sourced |
| Cooling dry bulb, 0.4% / MCWB | 106.7 F / 66.2 F | 41.5 / 19.0 C | same | reference only |
| Cooling dry bulb, 2% / MCWB | 101.5 F / 64.4 F | 38.6 / 18.0 C | same | reference only |
| Daily range (mean DB range of hottest month, July) | 25.4 F | 14.1 C | same; MJ8 class H (over 25 F), borderline with M | sourced |
| Daily range on the 1% design day | 28.3 F | 15.7 C | DDY file 1% design-day object | reference only |
| Coincident wind, 99.6% heating | 2.9 mph from 80 deg | 1.3 m/s | same | reference; MJ8 default 15 mph used |
| Coincident wind, 0.4% cooling | 12.8 mph from 250 deg | 5.7 m/s | same | reference; MJ8 default 7.5 mph used |
| Dehumidification dew point 1% / HR / MCDB | 62.2 F / 94 gr/lb / 77.9 F | 16.8 C / 13.4 g/kg / 25.5 C | same | reference only, not used |
| July clear-sky optical depths, taub / taud | 0.388 / 2.339 | | same table (monthly optical depths) | sourced, used for the design-day solar |
| Indoor heating | 70 F | | brief, MJ8 standard | sourced |
| Indoor cooling | 75 F, 50% RH | | brief, MJ8 standard | sourced |
| Site elevation | 2,800 ft | | Alan | sourced |
| Site pressure | 13.27 psia | | ICAO standard atmosphere at 2,800 ft | derived |
| Grains difference at 50% RH | -28.8 gr/lb | | derived, see below | derived |
| Altitude correction | density ratio 0.903; sensible 0.993 Btuh/cfm F, latent 0.614 Btuh/cfm gr | | 1.10 and 0.68 times the density ratio | derived |

## Where the numbers come from

1. The ASHRAE station tables are not free on the web as text. The same values are carried, verbatim and labelled "Displaying Design Conditions from 2025 ASHRAE Handbook -- Fundamentals - Chapter 14 Climatic Design Information", in the STAT and DDY files that ship with every EnergyPlus weather file from climate.onebuilding.org. Downloaded on 2026-09-18 from
   https://climate.onebuilding.org/WMO_Region_4_North_and_Central_America/USA_United_States_of_America/UT_Utah/
   files USA_UT_St.George.Rgnl.AP.724754_TMYx.2011-2025.zip and USA_UT_Saint.George.Rgnl.AP.724754_TMY3.zip. Both packages carry the identical design-conditions block (the design conditions are ASHRAE's, not the TMY period's). The heating row reads `Heating 12 -3.7 -2.3 ...` and the cooling row `Cooling 7 14.1 41.5 19.0 39.9 18.6 38.6 18.0 ...`. The 1% design-day object in the DDY reads `Maximum Dry-Bulb Temperature 39.9 C, Daily Dry-Bulb Temperature Range 15.7 C, Wetbulb at Maximum Dry-Bulb 18.6 C`.
2. ENERGY STAR Certified Homes Design Temperature Limit Reference Guide (2019 edition, rev. 11/01/2019), which "utilize[s] the 1% cooling and 99% heating design temperatures in the ASHRAE 2017 Handbook of Fundamentals and Manual J Design Conditions 8th Edition", gives Washington County, UT a cooling limit of 109 F and a heating limit of 7 F, but those are the county composite of Beaver Dam, Arizona (Manual J station) and Cedar City Regional (ASHRAE station); the guide does not use a St. George station. The station values above sit inside those limits, so an ENERGY STAR rater would accept them. Reference only.
   https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20(2019%20Ed)%20-%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf
3. Manual J Table 1A for St. George was not found as text on any public page. The Utah Energy Code article that reproduces the Utah portion of Table 1A does so as an image (utahenergycode.com, "ACCA Residential HVAC Standards Part 2"), and only the Salt Lake City row is quoted in prose (14 F / 95 F). A local contractor page quotes 103 F as the St. George 1% cooling design temperature, which agrees with the ASHRAE 103.8 F. No table was copied.
4. Edition note: the 2025 Handbook values are the newest. Older editions for this station will differ by a degree or so; the ENERGY STAR guide (2017 Handbook basis) is consistent with them.

## Derived grains difference

MJ8 tabulates design grains for 45, 50 and 55% RH. Instead, from the 1% DB / MCWB pair at site pressure (ASHRAE Fundamentals ch. 1 psychrometric equations, computed in loads.py):

| | Dry bulb | Wet bulb or RH | Humidity ratio | Grains per lb |
| --- | --- | --- | --- | --- |
| Outdoor, 2,800 ft | 103.8 F | 65.5 F WB | 0.00613 | 42.9 |
| Indoor, 2,800 ft | 75 F | 50% RH | 0.01024 | 71.7 |
| Difference | | | | -28.8 |

For comparison at sea level the same pair gives 32.7 gr/lb outdoor and 64.6 gr/lb indoor (difference -31.9). A negative grains difference means outdoor air at the cooling design condition is drier than the indoor set point, so infiltration and ventilation remove moisture; MJ8 treats that latent load as zero rather than as a credit. Expected divergence from the MJ8 table value: MJ8's Table 1A grains are computed at the station elevation from the same ASHRAE pair, so they should agree within 2 to 3 gr/lb; the sign and the conclusion (no latent air load) do not change.

## Altitude correction

Density ratio at 2,800 ft (standard atmosphere): p = 14.696 x (1 - 6.8754e-6 x 2800)^5.2559 = 13.27 psia; ratio 0.903. Sensible factor 1.10 x 0.903 = 0.993 Btuh per cfm per F; latent factor 0.68 x 0.903 = 0.614 Btuh per cfm per grain. MJ8's altitude table (about 0.90 at 3,000 ft) agrees to the second decimal.

## Not used

Colorado City, AZ (the cover-sheet address): about 5,000 ft, climate zone 4B or 5B, with a much lower heating design temperature. Alan confirmed the house is in St. George, so no values were pulled for it.