ASHRAE Sensible Design Load Calculator

Build UA by summing U-factor times area for surfaces exposed to the same outdoor condition. UA has units of BTU per hour per degree Fahrenheit. Adjacent conditioned spaces need separate temperature differences.

Enter actual station winter and summer design temperatures. An IECC climate zone does not establish site-specific ASHRAE design percentiles. Defaults are a worked scenario, not weather data.

Both indoor setpoints affect the results. The per-degree value supports temperature sensitivity comparisons. This steady-state model excludes solar timing, people, equipment and moisture; it is not the ASHRAE radiant-time-series or heat-balance load method.

For a deeper room-by-room breakdown that adds window orientation and internal gains, use the Manual J calculator. The assumptions and rounding behind every number here are documented on the methodology page.

Using these inputs, the results are winter sensible loss: 40,800 BTU/h; summer sensible gain: 16,320 BTU/h; load change per degree: 816 BTU/(h F). Defaults are example assumptions, not universal design conditions.

The arithmetic: the combined conductance is UA + 1.08 × CFM = 600 + 1.08 × 200 = 816 BTU/(h·°F). Heating ΔT is 70 − 20 = 50°F, so the winter sensible loss is 816 × 50 = 40,800 BTU/h. Cooling ΔT is 95 − 75 = 20°F, so the summer sensible gain is 816 × 20 = 16,320 BTU/h. The winter figure is more than twice the summer one purely because the temperature difference is larger, not because the house is any different.

Where do 600 and 200 come from? As an illustration, a poorly insulated 2,000 sq ft single-story home might have 1,800 sq ft of wall at U-0.20 (360), 1,200 sq ft of ceiling at U-0.08 (96), 250 sq ft of windows at U-0.50 (125), and 40 sq ft of doors at U-0.50 (20), which sums to about 600. A volume of 18,000 cu ft changing at about 0.67 air changes per hour is 18,000 × 0.67 ÷ 60 ≈ 200 CFM. Use your own measured areas and U-factors, which the U-value calculator can help derive, and see the CFM calculator for airflow.

Because the model is linear, you can see the effect of any input change by hand. With the default UA of 600 and 200 CFM (816 BTU/(h·°F) combined), moving one design temperature or setpoint at a time gives:

Every degree moves the result by 816 BTU/h in this example. Changing the envelope or airflow moves it too: keeping the same temperatures, cutting infiltration to 100 CFM lowers the combined conductance to 708 and the winter loss to 35,400 BTU/h, while 400 CFM raises it to 1,032 and 51,600 BTU/h. That is why air sealing and insulation show up in every load comparison.

The table applies the same default conductance (816 BTU/(h·°F)) to the site's illustrative zone profiles, using a 70°F heating setpoint and a 75°F cooling setpoint. These temperatures are site assumptions for comparison, not ASHRAE station data; use your own station values for a real project. Notice how the winter figure climbs and the summer figure falls as the climate gets colder.

Values are in BTU/h for the same UA and airflow in every row. Real homes differ by climate in insulation, glazing, and leakage too, and the summer column excludes solar, internal, and latent loads, so it is not a cooling load.

• Do not add the same outdoor air stream as both infiltration and ventilation.
• Do not select cooling from a temperature-only sensible subtotal.
• Have a qualified commercial designer complete internal, solar and latent loads and equipment selection.
• Sizing to record-low or record-high temperatures instead of percentile design values, which can oversize equipment substantially.
• Mixing up the design temperatures: using the winter value in the summer field, or pulling the wrong column from the ASHRAE data.
• Ignoring latent (moisture) load in humid climates, which this tool does not include.
• Forgetting duct losses in unconditioned attics or crawlspaces; Manual J accounts for them explicitly.
• Oversizing cooling for 'a hot day': it short-cycles, leaves humidity high, and ends up less comfortable than a right-sized unit.
• Assuming heat-pump heating capacity equals its AHRI 47°F rating; capacity drops at the heating design temperature, so check manufacturer data.
• Neglecting infiltration on tight new builds. Even at low air-change rates it is a non-trivial part of the heating load.

Continue with the commercial load calculator for a separate part of the design.

The numbers behind design temperatures trace back to the climatic design data tables in the ASHRAE Handbook of Fundamentals (Chapter 14, Climatic Design Information, in recent editions). Those tables are not casual estimates: they are derived from decades of hourly weather observations at thousands of stations, processed into percentile values. When you see a 99% heating design temperature, it is the dry-bulb value that the location equals or exceeds 99% of the 8,760 hours in a year, leaving roughly 88 colder hours. The 1% cooling design condition works the same way from the warm end and is published alongside a coincident wet-bulb so that latent load can be estimated rather than guessed.

This percentile approach is the quiet reason a load calculation can be trusted. Designing to a record low, the kind of figure that makes local news once a decade, would force every duct, register, and compressor to be sized for an event that lasts a few hours every several years. The method accepts a small, bounded comfort drift during those rare extremes in exchange for equipment that runs at sensible part load most of the season. That is why a furnace sized to the 99% value normally still keeps a house comfortable on the coldest morning: thermal mass and a degree or two of setpoint tolerance cover the gap. Read how HVAC design temperatures are chosen and what the IECC climate zones represent for the background.

ASHRAE refreshes these climatic tables across handbook editions as more observation years accumulate, so a design temperature for a given city can shift slightly between releases. For most residential work the change is small, but it is worth citing which edition a number came from when a result feeds a permit set or an engineered drawing.

A simple conduction-and-airflow model handles winter heat loss well because that loss is nearly steady and driven mostly by the design ΔT. Cooling is harder, and this is where ASHRAE's more advanced methods come in. The radiant time series (RTS) method, one of the cooling-load procedures in the Handbook of Fundamentals alongside the heat balance method, recognizes that solar energy striking a wall or passing through glazing does not become a cooling load instantly. It is absorbed by mass, stored, and released over the following hours, so peak cooling frequently lags peak outdoor temperature. RTS captures that delay with conduction and radiant time factors, then sums the hourly results to find the true design peak.

This calculator does none of that. It deliberately keeps to steady-state sensible transfer so you can see temperature sensitivity cleanly. The components a complete cooling analysis tracks are still worth knowing, because they explain why two houses with the same UA can land far apart in summer.

• Sensible conduction through walls, roof, and glazing, proportional to the design ΔT and the assembly U-value (this is what the tool computes).
• Solar gain through windows, which depends on orientation, shading, and the glass solar heat gain coefficient (SHGC).
• Infiltration and ventilation, both sensible and latent, set by air changes and the coincident wet-bulb.
• Internal gains from people, lighting, and appliances; modest in homes, dominant in many offices.
• Latent moisture load, which the coincident wet-bulb in the ASHRAE 1% data makes possible to size in humid climates.

It is easy to treat ASHRAE and ACCA Manual J as rival methods, but they are layers of the same system. Manual J is the residential load procedure published by the Air Conditioning Contractors of America, and the IECC points to it, or another approved method, for sizing heating and cooling equipment in new homes. Manual J draws its outdoor design conditions from published climatic data that is based on ASHRAE weather-station analysis, typically the 99% heating and 1% cooling values. In other words, the design temperatures this tool asks you to enter are the same kind of inputs a Manual J report opens with.

The difference is resolution. Manual J multiplies the design ΔT across a room-by-room takeoff: each wall by orientation, each window by SHGC and shading, the duct system by its location, and a detailed infiltration model. The output then feeds ACCA Manual S for equipment selection against manufacturer capacity data and Manual D for duct design. This calculator gives you a quick view of how the temperature-driven sensible load responds to your assumptions; a full Manual J turns that into the granular, permit-ready document. If you want the room-level version, the Manual J calculator applies design conditions surface by surface, and the what is Manual J article explains the full procedure.

Understanding this shared lineage changes how you use a quick estimate. When a contractor's proposal lists a load that is far above what you expect, a common cause is not a different method but a different design temperature, such as a record extreme substituted for the 1% value, or a generous safety pad layered on top. Knowing the ASHRAE source lets you ask the right question and keep the equipment honestly sized. For whole-house sizing context, see how to calculate HVAC load.

ASHRAE Handbook - Fundamentals provides heat-transfer, air-property and load-calculation background. Example inputs are stated scenarios, not prescribed handbook values.

For residential design, ACCA Manual J, S and D address load calculation, equipment selection and duct design. This calculator does not certify a complete design or code compliance.