ISO 15927-5:2004

Overview

ISO 15927-5:2004 - "Hygrothermal performance of buildings - Calculation and presentation of climatic data - Part 5: Data for design heat load for space heating" specifies how to define, calculate and present the climatic data used to determine the design heat load for space heating. The standard covers winter external design air temperatures and the associated coincident wind speed and dominant wind direction where relevant. It explains acceptable data sources, averaging periods, return periods and how to report results for building heating design.

Key topics and requirements

• Design temperature types
  • n‑day mean design temperature (n = 1–4 days): calculated from consecutive-day averages; priority of data resolution: hourly → 3‑hourly → selected daily observations → daily max/min.
  • Hourly mean design temperature: defined as the hourly mean exceeded in 99% of hours during the coldest month (coldest month = month with lowest 20‑year average monthly mean).
• Return period and records
  • Values should be derived from a continuous record where possible (recommended minimum 20 years). The n‑day mean design temperature is specified with a defined return period (standard practice in the document).
• Wind data
  • Coincident wind speed: use the 95th‑percentile wind speed from periods/hours at or colder than the design temperature; wind speeds referenced or corrected to 10 m above ground.
  • Dominant wind direction: analyse wind by non‑overlapping 30° segments; a segment is dominant if wind blows from it ≥ 40% of the time. Report main and secondary segments if applicable.
• Presentation requirements
  • Report air temperature (°C), wind speed (m/s), direction (°). Provide qualifiers: reference to ISO 15927‑5:2004, return period/frequency, period of records, reference altitude. Round temperatures to 0.1 °C and wind speeds to nearest 0.5 m/s; report “calm” for <1 m/s.
• Altitude and local corrections
  • Specify reference altitude and apply a local lapse rate to convert design temperatures between elevations. Seek meteorological advice where local effects (air drainage, heat islands) are significant.

Applications and users

ISO 15927-5 is used by:

• HVAC and building services engineers for sizing heating systems and radiators
• Building energy modellers and thermal performance analysts
• Architects and façade designers assessing thermal loads
• Meteorological agencies and data publishers preparing climatic design datasets
• National standards bodies preparing local design temperature maps and isopleths

Practical benefits include consistent, comparable climatic inputs for heat‑load calculations, better balancing of system cost versus reliability, and standardized reporting for regulatory compliance.

Related standards

• ISO 13370 - heat transfer via the ground (ground heat loss)
• ISO 6243 - symbols for climatic data
• WMO Guide No. 8 - methods for meteorological measurements

Keywords: ISO 15927-5:2004, design heat load, winter external design temperature, coincident wind speed, dominant wind direction, hygrothermal performance, climatic data, building heating design.