Natural Ventilation

Archsim can simulate natural ventilation in 3 different modes.

• Scheduled
• Simple wind and stack equations
• Airflow network

Scheduled Ventilation

The first option is great for coupling Archsim with other tools that return a schedule of air-changes in hourly format. Just link this schedule in the zone settings under the ventilation tab or specify a fixed air change rate.This model does not model airflow from zone to zone.

Wind and stack equation

Buoyancy

The second option uses simple formulas explained on the slides below. This model is extremely robust – especially for buoyancy driven flow.

$Q_{buoy} = F_{schedule} * C_{d} * A_{opening} * \sqrt{2 * g * \Delta H_{NPL} * \frac{|T_{zone} - T_{odb}|}{T_{zone} }}$

$C_{d} = 0.4 + 0.0045 * |T_{zone} - T_{odb}|$

Wind

Wind driven flow always assumes an ideal exhaust and basically simulates cross-ventilation – even if you only model one window – this can easily lead to overestimation of the wind driven ventilation potential. This model does not model airflow from zone to zone.

$Q_{buoy} = F_{schedule} * C_{w} * A_{opening} * V$

$C_{w} = 0.55$ for perpendicular wind and $C_{w} = 0.3$ for diagonal wind interpolated for in-between angles.

Airflow Network

The third option is the airflow network. It provides the possibility to model natural ventilation and more complicated airflow paths through the building. Thus, this is the preferred option – however – it might slow down your simulations.

What it can do:

• Air flow through exterior or interzone surfaces
• Different models for horizontal vertical and tilted surfs
• Buoyancy effects and wind pressure.
• Wind direction and surface orientation dependent.
• Bi-directional flow through large openings

What it cannot do:

• Air circulation and/or air temperature stratification within a thermal zone. For example, you should not try to divide a high space, such as an atrium, into subzones.

Source and further reading: Energy Plus Input Output Reference

Infiltration

EnergyPlus's basic infiltration model can adjust the infiltration rate for delta T and windspeed, using the following formula:

$Infiltration = I_{design} * (A + B (T_{zone} - T_{outdoor}) + C*windspeed + D*windspeed^2 )$

1. WIND setting (Coefficients from BLAST (Building Loads Analysis and System Thermodynamics))
   • A = 0.606, !- Constant Term Coefficient
   • B = 0.03636, !- Temperature Term Coefficient
   • C = 0.1177, !- Velocity Term Coefficient
   • D = 0; !- Velocity Squared Term Coefficient

1. CONSTANT setting (Results in a constant ACH with no modification)
   • A = 1, !- Constant Term Coefficient
   • B = 0, !- Temperature Term Coefficient
   • C = 0, !- Velocity Term Coefficient
   • D = 0, !- Velocity Squared Term Coefficient