Weather

Weather conditions determine the outdoor design temperatures used for heating and cooling load calculations. HVAKR supports ASHRAE and CIBSE weather data.

Weather Station Selection

Automatic Selection

When you enter a project address, HVAKR automatically selects the nearest weather station from the selected data source and year. New projects use ASHRAE 2025 by default. Projects that existed before ASHRAE 2025 became the default stay on ASHRAE 2021 until you change the year.

Data Source and Year

Use the two Weather Data Source selectors below the Project Address field to choose a weather dataset — the first picks the source, the second picks the year. The current options are:

• ASHRAE — 2025, 2021
• CIBSE — 2015

When you change the data source or year, HVAKR loads nearby stations from that dataset and selects the nearest station.

Choosing a Different Station

The nearest weather stations are shown on a map, each connected to your project location by a line. To use a different station:

1. Navigate to Basis of Design > Weather
2. Click a station marker on the map to open its preview
3. Review the station’s latitude, longitude, elevation, climate zone, and design temperatures
4. Click Select in the preview to use that station

Your project’s latitude, longitude, and elevation — along with the selected station’s climate zone and name — appear in the read-only summary below the Weather Data Source selectors. Change the Project Address to load a different set of nearby stations.

Design Conditions

Cooling Design Temperatures

The outdoor conditions used for cooling load calculations. HVAKR uses the 0.4% monthly design condition by default — for each month, the outdoor dry-bulb exceeds this value during only 0.4% of that month’s hours, per ASHRAE’s monthly design tables. Summer Design DB is the highest of those monthly values; Summer Coincident WB is the mean coincident wet-bulb at that peak month. The RTS load method uses the full monthly design-day temperature profiles behind those values.

Heating Design Temperature

The outdoor dry-bulb temperature used for heating load calculations. HVAKR uses the 99.6% annual design condition by default, meaning outdoor temperatures are below this value only 0.4% of the year.

Design Percentiles

Summer and winter percentiles are set independently:

• Summer Design Percent — 0.4%, 2%, 5%, or 10% (default 0.4%). A lower percentile is more conservative.
• Winter Design Percent — 99% or 99.6% (default 99.6%). A higher percentile is more conservative.

Changing a percentile updates the looked-up Summer Design DB, Summer Coincident WB, and Winter Design DB for the selected station.

CIBSE Future Design Temperatures

When the weather dataset is CIBSE and the selected station has future-year data, the CIBSE Future Temperature Adjustment selector appears below the weather dataset controls. It applies one future scenario to both cooling and heating design conditions:

• Default — use the selected station’s design temperatures
• 2020 / 2030 — use 2020 summer and 2030 winter design temperatures
• 2050 or 2080 — use the matching summer and winter design temperatures
• Custom — shown when the entered summer and winter temperatures do not match one scenario

Future summer values are 0.4% dry-bulb and coincident wet-bulb from CIBSE 2016 DSY1 (high emissions, 50th percentile). Future winter values are 99.6% dry-bulb from CIBSE Guide A (2015) Table 2.25 (high emissions, 50th percentile). Selecting a future scenario writes those values into Summer Design DB, Summer Coincident WB, and Winter Design DB. Selecting Default clears the overrides.

Weather Data Display

Monthly Peaks

The Monthly Peaks dialog shows a table of each month’s peak cooling dry-bulb and wet-bulb temperatures, useful for understanding seasonal variations. The DB (ASHRAE) and WB (ASHRAE) rows use the station’s ASHRAE design-day values. The DB (User-Adjusted) and WB (User-Adjusted) rows include your design condition overrides.

Psychrometric Chart

A psychrometric chart plots the summer and winter outdoor design points. When the station has typical meteorological year (TMY) data, a heatmap underlay visualizes the relative time spent at each condition during a typical year.

The Cooling Design Temperatures grid shows the 24-hour design-day dry-bulb profile for each month. Each cell uses the active display unit system. The annual heating design dry-bulb remains in the Winter Design DB field above.

Elevation

The project’s elevation, which affects air density calculations. HVAKR geocodes this from the Project Address and falls back to the selected weather station’s elevation when the project has no geocoded elevation — for example, before the address is geocoded or when the elevation lookup fails.

Impact on Calculations

Weather conditions affect:

• Transmission loads - Heat transfer through walls, roof, and windows
• Infiltration loads - Heat loss/gain from air leakage
• Ventilation loads - Conditioning of outdoor air
• Solar loads - Based on the station’s latitude, longitude, and monthly ASHRAE clear-sky optical depths

Custom Design Conditions

For locations where the looked-up values don’t match local conditions, you can override the design temperatures directly. Edit the Summer Design DB, Summer Coincident WB, or Winter Design DB fields with your own values; each field shows the station’s looked-up value as a default you can reset to.

Caution

Custom design conditions should be based on local meteorological records or engineering judgment. Inaccurate weather data will result in undersized or oversized equipment.