HVAC Draught Rate (DR) Calculator
HVAC Design Thermal Comfort Parameter
Draft Rate (DR) Calculator
Draft Rate (DR): -
What is Draft Rate (DR)?
Draft Rate (DR) is an index in HVAC thermal comfort parameter, used to predict the percentage of people who feel uncomfortable due to unwanted local cooling, commonly referred to as “draft.” Unlike PMV, which evaluates overall thermal comfort, DR focuses specifically on local discomfort caused by air movement, especially around the neck, head, and ankles. It is widely used in standards such as ISO 7730 and ASHRAE 55 to assess the impact of air velocity fluctuations in indoor environments. Even in thermally neutral conditions, high air speed combined with turbulence can cause discomfort, making DR an important parameter in HVAC design.
What is Draft Rate (DR)?
Draft Rate (DR) is an index used to predict the percentage of people who feel uncomfortable due to unwanted local cooling, commonly referred to as “draft.” Unlike PMV, which evaluates overall thermal comfort, DR focuses specifically on local discomfort caused by air movement, especially around the neck, head, and ankles. It is widely used in standards such as ISO 7730 and ASHRAE 55 to assess the impact of air velocity fluctuations in indoor environments. Even in thermally neutral conditions, high air speed combined with turbulence can cause discomfort, making DR an important parameter in HVAC design.
Typical Input Parameters
The Draft Rate model depends on three main variables:
- Air Temperature (Ta): Typically between 20–26°C in indoor environments. Lower temperatures increase sensitivity to draft.
- Air Velocity (Va): Usually in the range of 0.05–0.3 m/s for occupied zones. Higher velocities significantly increase draft risk.
- Turbulence Intensity (Tu): Commonly around 10–60%, depending on airflow type (laminar vs. diffused air supply).
In practice, draft discomfort becomes more critical when air velocity is high and temperature is low, especially in spaces with poorly distributed airflow.
Typical Design Values
In HVAC design, DR is used to ensure occupant comfort by limiting draft risk:
- DR < 10% → High comfort (recommended)
- DR < 20% → Acceptable (commonly used design limit)
- DR > 20% → Likely uncomfortable
Designers aim to keep DR below 20% in occupied zones by carefully controlling diffuser placement, airflow velocity, and supply air temperature. This is especially important in offices, classrooms, and control rooms where occupants remain stationary for long periods.
For more accurate evaluation of draft discomfort in real environments, simplified DR calculations may not fully capture the complexity of airflow patterns. In practical applications, factors such as diffuser placement, room geometry, and localized turbulence can significantly influence draft sensation. By using tensorHVAC-Pro, you can simulate detailed airflow fields within actual geometries, allowing precise prediction of air velocity distribution and turbulence intensity. This enables engineers to assess draft risk more reliably and optimize HVAC designs to maintain comfortable conditions throughout the occupied space.