# Heating & Cooling Design Temperatures (99%, 1% / 0.4%)

> What the 99% heating and 1% / 0.4% cooling design temperatures mean, why they are percentiles rather than record extremes, and how engineers use them to size HVAC.

- Canonical: https://harth.build/resources/climate-data/heating-and-cooling-design-temperatures/
- Last updated: 2026-08-14
- Data source: Definitions follow the ASHRAE Handbook — Fundamentals design-condition methodology. Any specific temperatures are typical/illustrative; the official ASHRAE Climatic Design Conditions tables (and the values the AHJ-adopted code references) are authoritative for sizing. Official references: [ASHRAE standards and guidelines](https://www.ashrae.org/technical-resources/standards-and-guidelines/titles-purposes-and-scopes).

## Key facts

| Fact | Value |
| --- | --- |
| Heating design temp | 99% or 99.6% (annual percentile exceeded) |
| Cooling design temp | 1% or 0.4% (dry-bulb / wet-bulb) |
| Purpose | equipment sizing (not record extremes) |
| Source | ASHRAE climatic design tables (by weather station) |

## Direct answer

What the 99% heating and 1% / 0.4% cooling design temperatures mean, why they are percentiles rather than record extremes, and how engineers use them to size HVAC. A key reference is heating design temp: 99% or 99.6%; check the cited source, edition, units, and applicability before using it for design, compliance, procurement, or approval. Heating design temp is 99% or 99.6%; annual percentile exceeded. Cooling design temp is 1% or 0.4%; dry-bulb / wet-bulb. Purpose is equipment sizing; not record extremes. A **design temperature** is the outdoor condition an engineer sizes heating or cooling equipment to hold the indoor setpoint against. Rather than the record extreme, ASHRAE defines these as **annual percentiles** so equipment is sized for nearly all operating hours without being grossly oversized for a handful of extreme ones.

*Content and cited inputs reviewed by [Harth team](https://www.linkedin.com/company/harth-build) · Published 2026-08-14 · Updated 2026-08-14.*


![Climate-data selection and QA diagram: Heating & Cooling Design Temperatures (99%, 1% / 0.4%)](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/climate-data/heating-and-cooling-design-temperatures/diagram-1344.webp)

*What the 99% heating and 1% / 0.4% cooling design temperatures mean, why they are percentiles rather than record extremes, and how engineers use them to size…*

## Which climate inputs control the design?

A **design temperature** is the outdoor condition an engineer sizes heating or cooling equipment to hold the indoor setpoint against. Rather than the record extreme, ASHRAE defines these as **annual percentiles** so equipment is sized for nearly all operating hours without being grossly oversized for a handful of extreme ones.

## Heating: the 99% / 99.6% conditions

The **heating design temperature** is a low percentile of annual dry-bulb temperature:

- **99%** — the temperature exceeded (warmer than) 99% of annual hours; colder about 1% of the year.
- **99.6%** — a more conservative value, colder only 0.4% of hours.

Heating equipment is sized so it can maintain the indoor setpoint down to this outdoor temperature. Colder codes and critical facilities tend to use the 99.6% value.

## Cooling: the 1% / 0.4% conditions

The **cooling design conditions** are high percentiles, published as a **dry-bulb temperature** with a **coincident wet-bulb**:

- **1%** — dry-bulb exceeded only 1% of annual hours.
- **0.4%** — a more conservative value, exceeded only 0.4% of hours (the common commercial default).

Because cooling load includes humidity, the accompanying **wet-bulb** value lets the engineer size the **latent (dehumidification)** load as well as the sensible load.

## Illustrative comparison

| Condition | Percentile | What it sizes | Note |
| --- | --- | --- | --- |
| Heating dry-bulb | 99% / 99.6% | Heating capacity | Higher percentile = more conservative |
| Cooling dry-bulb | 1% / 0.4% | Sensible cooling | 0.4% typical for commercial |
| Coincident wet-bulb | with cooling DB | Latent / dehumidification | Matters most in moist (A) zones |

*Values above describe the method, not a specific site.*

## How architects use them

You generally won't run the load calculation yourself, but the design conditions shape the brief: they justify the mechanical engineer's equipment size, drive envelope trade-offs (a better envelope shrinks the peak load and the equipment), and feed peak heating/cooling estimates during schematic design. Always pull the authoritative values from the **ASHRAE Climatic Design Conditions** for the nearest representative weather station.

## Heating & Cooling Design Temperatures (99%, 1% / 0.4%): climate-input traceability record

| Input | Project entry | Source record |
| --- | --- | --- |
| Site coordinates and elevation | — | Survey / authoritative location source |
| Climate zone and governing edition | — | Official map, lookup and adoption record |
| Weather station / grid cell | — | Identifier, distance, elevation and fit |
| Observation period or weather-file vintage | — | Named dataset and release |
| Variable, percentile and units | — | Exact table / file / method |
| Intended use | — | Context, simulation, code or plant sizing |
| Local hazard source | — | Hazard-specific authority; not a zone proxy |
| Preparer, checker and date | — | Project QA record |

**Project-use verification:** never substitute a monthly normal for a design condition or a climate zone for a separate hazard lookup. Keep each input tied to its intended use.

## Where can you explore related guidance?

- [Explore all climate data guidance](https://harth.build/resources/climate-data.md)
- [ASHRAE / IECC Climate Zones Explained (Zones 1–8, A/B/C)](https://harth.build/resources/climate-data/ashrae-climate-zones-explained.md)
- [IECC Envelope Requirements by Climate Zone](https://harth.build/resources/climate-data/iecc-envelope-requirements-by-climate-zone.md)

## Sources and method

Definitions follow the ASHRAE Handbook — Fundamentals design-condition methodology. Any specific temperatures are typical/illustrative; the official ASHRAE Climatic Design Conditions tables (and the values the AHJ-adopted code references) are authoritative for sizing. Official references: [ASHRAE standards and guidelines](https://www.ashrae.org/technical-resources/standards-and-guidelines/titles-purposes-and-scopes).

### Source links

- [ashrae.org](https://www.ashrae.org/technical-resources/standards-and-guidelines/titles-purposes-and-scopes)

## Frequently asked questions

### Why not size to the record low or record high?

Sizing to an all-time extreme would oversize equipment for conditions that occur for a few hours a decade, hurting efficiency, comfort (short-cycling), and cost the rest of the time. The percentile design conditions capture nearly all real operating hours while keeping equipment right-sized. A modest safety margin covers the rare excursions beyond them.

### What does a 99% heating design temperature mean?

It is the outdoor dry-bulb temperature that the location is at or above 99% of the annual hours — i.e. it is colder than this value only about 1% of the year (roughly 88 hours). A 99.6% value is more conservative (colder is exceeded only 0.4% of hours). Heating equipment is sized to hold the indoor setpoint down to this temperature.

### Why do cooling conditions include a wet-bulb value?

Cooling load has both a sensible part (dry-bulb temperature) and a latent part (humidity). ASHRAE publishes cooling design conditions as a dry-bulb with a coincident (mean) wet-bulb, plus separate wet-bulb percentiles, so the engineer can size for both the sensible and dehumidification loads — critical in moist (A) zones where latent load is significant.

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