Airtightness (Scheduled Natural ventilation)

Construction tab in model data under Airtightness header

 

Model infiltration

Infiltration is included by default but you can switch infiltration off by unchecking the Model infiltration checkbox.

Infiltration rate

With the Scheduled natural ventilation option set the design infiltration rate (rate of entry of unintentional air from outside through cracks, holes and through the porosity of the fabric) is assumed to be constant throughout the simulation though it can vary with time through a schedule. The design infiltration rate can be set in one of 4 different units depending on the setting of the Infiltration units model option:

 

  1. ac/h - the default option where infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour under standard operating pressures. An input value is entered equivalent to the actual anticipated air changes due to infiltration during normal operation under typical pressures, not under pressure test conditions. It will operate according to its schedule (below). Note that if you input a value of 0.6 (and assuming a schedule value of 1) then in the outputs you will see 0.6 +/- a small amount as the air temperature (and hence density) varies slightly.

  2. m3/h-m2 at 50 Pa infiltration is defined as flow rate in m3/h per unit exposed surface area, when the pressure difference between inside and outside is 50 Pa. Unit commonly used in the UK. Note that this is a rate under pressure test conditions and therefore the actual rate under normal operating conditions (at much lower pressures) will be a lot lower.

  3. m3/h-m2 at 4 Pa infiltration is defined as flow rate in m3/h per unit exposed surface area, when the pressure difference between inside and outside is 4 Pa. Units commonly used in France and Belgium. DesignBuilder calculates the actual resultant value under Standard Pressure of 1atm or 101325 Pa using formulae from EN12831 (section 7.2.2):

    4.  

    Infiltration airflow [m3/h] = 2*ZoneVolume*n50*ei*epsilon

     

    The epsilon coefficient is the building height class and is calculated by DesignBuilder based on the building height:

    epsilon:

    The ei - wind exposure class [coefficient] is determined based on the façade exposure of each zone according with the table below.

    Exposure description Heated zone no façade exposed to outside Heated zone with single façade exposed Heated zone with multiple façades exposed
    Open site (sea shores, open country...) 0.01 0.03 0.05
    Medium sheltered site (towns, city periphery) 0.01 0.02 0.03
    Well sheltered site (city centre) 0.01 0.01 0.02

     

  4. n50 - ac/h at 50 Pa - infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour, when the pressure difference between inside and outside is 50 Pa. Unit commonly used in Europe.

 

The air flow rate in m3/s at standard pressure is calculated from the ac/h data using:

 

m3/s = ac/h * ZoneVolume / 3600

 

where ZoneVolume is the total air volume of the space including the volume of floor and ceiling constructions and any raised floor and suspended ceiling voids (if selected in Separate constructions) and including half the volume of partition walls.

 

Note: this total zone volume value may be different from the value used for calculating air flow rates for natural ventilation and mechanical ventilation which have model options for excluding floor/ceiling constructions.

 

The ac/h at standard pressure is calculated from other units using formulae in EN12831 and the m3/s calculated using the above formula.

Schedule

The schedule that modifies the design infiltration rate. The schedule should contain fractions between 0.0 and 1.0 and is used for Cooling design and Simulations (when the Scheduled natural ventilation option is set) but not for Heating design calculations which use the constant design infiltration rate.

 

Note: Both the Infiltration rate and Schedule are only used in Simulations when the Scheduled natural ventilation model option is set. However this data is still used when Calculated natural ventilation is set for Cooling design calculations (Infiltration rate and schedule) and Heating design calculations (Infiltration rate only).