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 and it can also be made to vary according to simple rules based on temperature difference between inside and outside and wind speed.. 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.  

  5. 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. When this option is selected there are options to override the default EN 12831 wind exposure and building height coefficients to account for national standards. See below under Coefficient Override options for "n50 - ac/h at 50 Pa".

 

 

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 a constant design infiltration rate. The exact usage of this schedule (Fschedule) is shown in the equation in the section below.

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).

DeltaT and Wind Speed Coefficients

During the simulation, the infiltration flow of outside air into the zone is calculated for each timestep from the equation below:

 

Infiltration = (Idesign)(Fschedule)[A + B |(Tzone - Toutside)| + C (Wind Speed) + D (Wind Speed)2

 

The coefficients A, B, C and D can defined by entering data for the 4 fields described below.

 

By default A is 1 and B, C and D are 0 so that the infiltration rate is calculated simply as the entered design infiltration flow rate value multiplied by the schedule value (0 - 1), unmodified by temperature difference or wind effects. However if you need to model the impact of inside to outside temperature difference or wind speed in a simple way then you can enter appropriate values for terms, A-D as described below.

 

More advanced infiltration calculations are possible using Calculated natural ventilation through the EnergyPlus AirflowNetwork model for natural infiltration driven by wind when the HVAC system does not operate and/or driven by wind and forced air for times when the HVAC system operates. Exfiltration (the leakage of zone air to the outside) is generally handled better as zone exhaust air in the zone equipment description.

 

Constant

The constant term, A, in the above equation, set to 1 by default.

Temperature

The temperature difference term, B, in the above equation, set to 0 by default..

Velocity

The velocity term, C, in the above equation, set to 0 by default.

Velocity squared

The velocity squared term, D, in the above equation, set to 0 by default.