An ACCA Manual J load calculation is only as accurate as the outdoor design conditions you feed it. The same 2,400 square foot house needs a very different system in Phoenix than it does in Chicago, and the reason is local climate: the winter and summer design temperatures, the coincident humidity, the climate zone, and the altitude all change the heating and cooling loads. This page collects the real, published design conditions for five high demand US metros so you can see how location drives the number. Every design temperature below is drawn from the ASHRAE Handbook of Fundamentals climatic design conditions (the same data set ACCA Manual J references), not a rule of thumb.
Why the Design Temperature Decides the Size
Manual J sizes equipment to a statistical extreme, not the worst day on record. The 99% heating (winter) design temperature is the outdoor temperature that a location stays at or above for 99% of the hours in a year. The 1% cooling (summer) design temperature is the dry bulb that is exceeded only about 1% of the hours in a year. Designing to these percentiles keeps equipment from being oversized for a handful of hours it will rarely see, which is exactly what causes short cycling, poor humidity control, and wasted energy. In humid metros the coincident wet bulb matters just as much as the dry bulb, because it drives the latent (moisture removal) portion of the cooling load.
Those percentile temperatures pair with your IECC climate zone, which sets the envelope and equipment efficiency baseline your permit will be checked against. Efficiency ratings like SEER2, EER2, and HSPF2 come into the picture at the equipment selection stage; our guide to HVAC efficiency ratings (SEER, SEER2, EER, and HSPF) explained covers how those numbers are read and how they interact with regional minimums. The load calculation always comes first, and it always depends on where the house is.
Design Conditions by Metro
Phoenix, Arizona (Climate Zone 2B, hot-dry)
Phoenix is a cooling-dominated, low-humidity market. Per the ASHRAE climatic design conditions for Phoenix Sky Harbor International Airport:
- 99% winter design temperature: about 41 degrees F (the more conservative 99.6% value is near 39 degrees F).
- 1% summer design temperature: about 108 degrees F dry bulb, with a coincident wet bulb near 70 degrees F.
- IECC climate zone: 2B, hot-dry.
Because the coincident wet bulb is relatively low, the Phoenix cooling load is overwhelmingly sensible (dry heat) rather than latent, so peak equipment performance at high outdoor temperature matters more than aggressive dehumidification. The very high 1% dry bulb also makes EER and EER2 (efficiency measured at a hot 95 degree F test point) a meaningful comparison when equipment is selected. On the code side, Arizona has no single statewide residential energy code; adoption happens locally, and the City of Phoenix administers its own building and mechanical permits, so Manual J and Manual S documentation should follow the locally adopted code edition.
Houston, Texas (Climate Zone 2A, hot-humid)
Houston is the humid counterpart to Phoenix. Per the ASHRAE climatic design conditions for Houston Bush Intercontinental Airport:
- 99% winter design temperature: about 33 degrees F (99.6% near 29 degrees F).
- 1% summer design temperature: about 95 degrees F dry bulb, with a coincident wet bulb near 77 degrees F.
- IECC climate zone: 2A, hot-humid.
That high coincident wet bulb near 77 degrees F is the story in Houston: a large share of the summer load is latent, so a Manual J that undercounts moisture, or a system selected without checking latent capacity in Manual S, leaves a house cool but clammy. Texas administers a statewide residential energy code based on the IECC, and the City of Houston requires permits and load calculations for new and replacement mechanical work, so the latent side of the calculation should be documented, not assumed.
Denver, Colorado (Climate Zone 5B, cold and dry, high altitude)
Denver adds a variable the coastal metros do not: elevation. Per the ASHRAE climatic design conditions for Denver International Airport (roughly 5,430 feet):
- 99% winter design temperature: about 7 degrees F (the 99.6% value is near 1 degree F).
- 1% summer design temperature: about 92 degrees F dry bulb, with a coincident wet bulb near 60 degrees F.
- IECC climate zone: 5B, cold and dry.
At Denver's altitude the air is less dense, which reduces the capacity that equipment can actually deliver, so Manual S selection has to apply an altitude correction to the manufacturer's rated performance. Skip that step and a unit that looks correctly sized on paper falls short on the coldest and hottest days. The dry summer (low coincident wet bulb) means the cooling load is mostly sensible, while the cold winter makes heating capacity, and for heat pumps the low-temperature output, the governing concern. Colorado has moved to statewide model energy code adoption, and the City and County of Denver administers its own building code, so confirm the current edition when you submit.
Chicago, Illinois (Climate Zone 5A, cold and humid)
Chicago is heating-dominated with a genuinely humid summer. Per the ASHRAE climatic design conditions for Chicago O'Hare International Airport:
- 99% winter design temperature: about 2 degrees F (the 99.6% value is near minus 4 degrees F).
- 1% summer design temperature: about 89 degrees F dry bulb, with a coincident wet bulb near 73 degrees F.
- IECC climate zone: 5A, cold and humid.
The low winter design temperature means the heating load usually sets the equipment size, and for a heat pump it drives the question of supplemental heat and low-temperature performance. At the same time the summer coincident wet bulb near 73 degrees F is high enough that latent cooling still needs attention, so a Chicago design cannot ignore either season. Illinois enforces a statewide energy code based on the IECC, and the City of Chicago administers its own construction code, so both the load calculation and the equipment selection should be prepared to that framework.
Miami, Florida (Climate Zone 1A, very hot and humid, hurricane zone)
Miami is the extreme humid, minimal-heating case, and it carries a wind requirement no other metro on this list does. Per the ASHRAE climatic design conditions for Miami International Airport:
- 99% winter design temperature: about 52 degrees F (99.6% near 48 degrees F).
- 1% summer design temperature: about 91 degrees F dry bulb, with a coincident wet bulb near 78 degrees F.
- IECC climate zone: 1A, very hot and humid.
Heating is almost an afterthought here; the design challenge is a very high latent load driven by that 78 degree F coincident wet bulb, so moisture removal and correct dehumidification capacity dominate the Manual S selection. Miami-Dade also sits in the High-Velocity Hurricane Zone (HVHZ), where structures are designed for very high wind speeds (on the order of 175 mph for typical risk category buildings) and rooftop or ground-mounted mechanical equipment must be anchored to resist wind loads under the Florida Building Code and ASCE 7. That anchoring and product-approval context is part of a permit-ready Miami mechanical package, on top of the load calculation itself.
How Local Conditions Flow Into Manual J, S, and D
The design temperatures above are the first input, not the whole job. A complete permit package moves from load to equipment to distribution, and each step inherits the local climate:
- Manual J takes the 99% and 1% design temperatures, the coincident wet bulb, and the home's envelope to produce room-by-room and whole-house heating and cooling loads in BTU per hour.
- Manual S selects equipment that meets those loads at the local design conditions, applying corrections such as Denver's altitude derate and verifying latent capacity in humid metros like Houston and Miami.
- Manual D sizes the duct system to deliver the required airflow to each room within the equipment's static pressure.
If you want the fuller walkthrough of how these three standards fit into a single permit submission, see Manual J, Manual S, and Manual D: right-sizing HVAC for permit approval. For the software side of running the calculation, our overview of the CoolCalc web-based Manual J app explains why the quality of the inputs, including the correct local design conditions, matters more than the tool. And if your project is a heat pump upgrade in New York rather than one of the metros above, the incentives in the NYS Clean Heat Program hinge on a correctly sized system, which again starts with an accurate Manual J.
FAQs
Do I use the 99% or the 99.6% winter design temperature?
Both are published for every station, and the choice depends on the code or program your project follows. The 99% value is the common Manual J default; the 99.6% value is more conservative and is required or preferred in some jurisdictions and by some programs. The figures above list the 99% value first and note the 99.6% value in parentheses so you can match whichever your building department specifies. When in doubt, confirm with the authority having jurisdiction before you submit.
Why do two nearby weather stations show different design temperatures?
Design conditions are tied to a specific weather station, and a metro can have several (an airport, a smaller field, a military base) at slightly different elevations and exposures. The values here use the primary metropolitan airport station for each city, which is the station most Manual J software defaults to. A project on the edge of a metro may be closer to a different station, so the exact figures can shift by a few degrees. That is one more reason a Manual J should be run to the project's actual location rather than copied from a neighboring city.
My city is not on this list. Can you still do the calculation?
Yes. These five metros illustrate how climate zone, humidity, and altitude change the load, but Manual J works for any US location using the same ASHRAE data set. We run load calculations nationwide to the design conditions for your specific site and the code your building department enforces.
Need a permit-ready Manual J for a project in one of these metros or anywhere else? We run ACCA-compliant Manual J load calculations, Manual S equipment selection, and Manual D duct design to your site's real design conditions, most reports back in 24 to 48 hours. Contact us to get started.