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.  

  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. 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".

 

This note describes the methods used to convert entered infiltration values described above to the m3/s value provided to EnergyPlus. The equation used depends on the units selected as follows.

 

2-m3/h-m2 at 50 Pa

ac/h = 2 (m3/hr-m2 at 50 Pa) (Exposed_Area) (EI) (Epsilon) / (Zone_Volume)

 

3-m3/h-m2 at 4 Pa

ac/h = 2 (m3/hr-m2 at 4 Pa) (Exposed_Area) (EI) (Epsilon) / (0.18566355 (Zone_Volume))

 

4-n50 - ac/h at 50 Pa

ac/h = 2 (n50 - ac/h at 50 Pa) (EI) (Epsilon)

 

The air flow rate in m3/s at standard pressure provided to EnergyPlus is calculated from the ac/h calculated above using:

 

m3/s = (ac/h) (Zone_Volume) / 3600

 

where Zone_Volumeis the air volume of the space calculated based on the Zone volume calculation method selected under Geometry Areas and Volumes

 

The Exposed_Area value is the sum of the gross area of the zone exterior surfaces, excluding any ground and adiabatic surfaces. That includes external walls, flat and pitched roofs, external floors and all external windows. It does not include ground floors, semi-exposed or adiabatic surfaces.

 

Epsilon is the building height class and accounts for height of building. It is calculated by DesignBuilder based on the building height using EN12831 rules:

epsilon:

 

The EI wind exposure class [coefficient] is determined based on the façade exposure of each zone and the Exposure to Wind setting according with the table from EN12831 below.

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

 

Note that the above EN 12831 Epsilon and EI values can be overriden for the 4-n50 - ac/h at 50 Pa option. See below.

Override wind exposure coefficient

Some national standards require a fixed value of wind exposure coefficient to be used instead of the default from EN 12831. To override the default, check this checkbox and enter the required coefficient in the text box below.

Wind exposure coefficient

Enter the wind exposure coefficient to override the default value from EN 12831.

Override height coefficient

Some national standards require a fixed value of building height coefficient to be used instead of the default from EN 12831. To override the default, check this checkbox and enter the required coefficient in the text box below.

Height coefficient

Enter the height coefficient to override the default value from EN 12831.

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.

 

The question of typical values for these infiltration coefficients is subject to debate. Ideally, one should do a detailed analysis of the infiltration situation and then determine a custom set of coefficients using methods such as those laid out in Chapter 26 of the ASHRAE Handbook of Fundamentals. As previously noted, the defaults are 1,0,0,0 which gives a constant volume flow of infiltration under all conditions.

 

BLAST (one of the EnergyPlus predecessors) used the following values as defaults: 0.606, 0.03636, 0.1177, 0. These coefficients produce a value of 1.0 at 0°C deltaT and 3.35 m/s (7.5 mph) wind speed, which corresponds to a typical summer condition. At a winter condition of 40°C deltaT and 6 m/s (13.4 mph) wind speed, these coefficients would increase the infiltration rate by a factor of 2.75.

 

In DOE-2 (the other EnergyPlus predecessor), the air change method defaults are (adjusted to SI units) 0, 0, 0.224 (wind speed), 0. With these coefficients, the summer conditions above would give a factor of 0.75, and the winter conditions would give 1.34. A wind speed of 4.47 m/s (10 mph) gives a factor of 1.0.

 

The source of the BLAST defaults is noted in the BLAST documentation as:

 

“Empirical equation and the coefficient default were determined from ASHRAE journal articles and other data on the effects of outdoor weather conditions.”

The source of the DOE-2 defaults is based on examining the infiltration relationships described in the ASHRAE Handbook of Fundamentals.

The EnergyPlus example files use all of the above, the BLAST defaults in some (e.g., GeometryTest), the DOE-2 defaults in some (e.g., 5ZoneAirCooled), and the EnergyPlus defaults in some (e.g., LgOffVAVDetCoil).

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.