Cooling system sizing

Cooling Design tab on Model Options dialog.

Design margin

The Cooling Design margin is a 'safety factor' used to multiply calculated cooling loads in each zone to give a recommended maximum cooling equipment capacity. It accounts for the additional cooling which might be required to cool the building down in a reasonably short pre-cool period and allows you to be confident that comfort conditions will be maintained in all but the most extreme summer conditions. The risk depends on the Summer Design Weather Data options selected at site level.

The default design margin is 1.15 which means that the cooling system will be oversized by 15%. This is value comes from ASHRAE recommendations.

Sizing method

Sizing methods available are:

• 1-ASHRAE - standard 'Heat Balance' method implemented in EnergyPlus.
• 2-Unitary DX - calculations are made through a detailed EnergyPlus simulation using Unitary DX equipment.

Note: you do not need to have the Compact HVAC model option set to use the detailed cooling load sizing option 2-Unitary DX above.

Airflow calculation method

When calculating the design airflow rate from the cooling load DesignBuilder can use one of 2 methods:

• 1-Sensible only the default option where the airflow rate is simply calculated from the cooling load, the supply air temperature and the zone air temperature setpoint. This method is widely used in the industry to calculate the airflow rate in cases where the latent load is a relatively low proportion of the total.

Design flow rate = SizingFactor . DesignCoolingLoad / (T_zoneairdb - T_supplyair) . CP_air . Density_air

 

where:

 

CP_air is the specific heat capacity of air

T_zoneairdb is the zone dry bulb temperature

 

T_supplyair is the supply air dry-bulb temperature

• 2-Sensible + latent where a similar enthalpy calculation is used. This method may be adopted in some cases where latent loads predominate:

Design flow rate = SizingFactor . DesignCoolingLoad / (E_zoneair - E_supplyair) . Density_air

 

where:

 

E_zoneair is the zone air enthalpy

 

E_supplyair is the supply air enthalpy