EN 15241 2007 64 e stf BRITISH STANDARD BS EN 15241 2007 Incorporating corrigendum 2011February Ventilation for buildings — Calculation methods for energy losses due to ventilation and infiltration in[.]
Basis of the calculation method
Starting from the airflows, the aim of the procedure is to calculate:
Temperature and humidities of the airflows entering the heated or cooled areas
Energy devoted to the air treatment.
Air entering through infiltration, passive air inlets or windows
It is basically considered that the air characteristics are the outdoor air ones
Preheated air inlets and ground coupling are part of this standard
If the air is taken in an adjacent space the air temperature in this space shall be calculated according to prEN ISO 13790.
Air entering through balanced or supply only system calculation
Duct heat losses
6.3.2.1 Heat transfer through the parts of duct situated in the heated/conditioned area
It has to be evaluated if these losses are significant in respect to the accuracy required for the calculations
They can be neglected for systems not providing heating and cooling
When ducts are located outside the conditioned area but the surrounding air temperature matches the zone temperature, the equations remain unchanged However, if the heat transfer from the zone to the air in the duct is considered, the energy balance of the room must be adjusted, as the heat transferred to the air will result in a loss from the zone.
6.3.2.2 Heat transfer through the parts of duct situated out the heated/conditioned area
The air temperature is modified in the duct as follows: θ 2 = θ 1 + ∆ T duct x 2 = x 1 where
The ∆T duct represents the temperature difference in air between the inlet and outlet, measured in Kelvin (K) At the duct's inlet, the air temperature and humidity are denoted as θ1 and x1, respectively, expressed in degrees Celsius (°C) and grams per kilogram of dry air (g/kg) Conversely, at the outlet, the air temperature and humidity are represented as θ2 and x2, also in °C and g/kg of dry air.
∆ θ θ s where θsurduc is the temperature of the air surrounding the duct, equal in this case to the outdoor air temperature, in °C
H duct is the heat loss from the duct to the surrounding, in W/K q vduct is the rate of air flow in the duct, in (m³/h)
Duct flow losses
The infiltred or exfiltred flow into or from the duct is calculated according to EN 15242
If the air is exfiltred, there is no change in air characteristics in the duct (but a difference in air flows)
If the air is infitred, the outdoor air is mixed to the air entering the duct.
Fan
The air temperature is increased by the fan of a ∆T fan value vfan r f, fan
∆T fan is the increase of air temperature caused by fan, in K,
F fan is the fan power, in W,
The fan power recovered ratio, denoted as \$R_{f,r}\$, is an important metric in evaluating fan efficiency The product of air density and specific heat, represented as \$\rho_c\$, is typically valued at 0.34 Wh/(m³·K) at a temperature of 20 °C Additionally, the airflow through the fan, denoted as \$q_{vfan}\$, is measured in cubic meters per hour (m³/h).
NOTE 1 EN 13779 provides a classification of fan power
The fan power recovered ratio (R f,r) measures the proportion of electrical energy from the fan that is converted into air energy Default values are provided in Table 4, and in cases where the position is uncertain, the least favorable value should be applied, which is the motor positioned in airflow for cooling and out of airflow for heating.
Motor in airflow 0,9 Motor out air flow 0,6
In demand controlled ventilation (DCV) or a VAV system utilizing 100% outdoor air without any recirculation, it can be assumed that the average fan power consumption aligns with the fan power level determined at the average airflow of C cont.q v, simplifying the calculation process.
NOTE 2 Other assumptions may be made if they are described For instance, if the fan power at maximum speed and minimum speed has importance on the overall result, another calculation method of the average fan absorbed power may be used taking it into account
In VAV systems utilizing air recirculation, the constant C is influenced by the operation of the outdoor air damper, while the power consumed by the fan is determined by the average supply air ratio in relation to its maximum capacity.
For DCV and VAV systems with 100 % outdoor air: Airflow ratio = C cont
For VAV systems with recirculation, the airflow ratio is equal to the weighted average airflow in the system divided by the maximum air flow in the system
X volume flow VC Variable control BC Backward curved
Y power input VP Variable pitch FC Forward curved
DP Damper control SP Speed control
BP By-pass control SL Slip control
Figure 3 — Example of fan absorbed power against air flow
In a DCV system where the C cont value is determined to be 0.5, it can be inferred that the fan power consumption corresponds to a 50% operational ratio, which translates to 30% of the maximum power when speed control is applied.
Therefore, the following Table 5 summarises the ratio that may be applied to the fan power at maximum speed depending on C cont and regulation type
Table 5 (informative) — Example of fan power ratio depending on regulation and airflow ratio
( at speed Fanpower tio FanPowerRa Power
Damper control on forward blades centrifugal fan 55 % 75 % 90 % 100 %
Damper control on backward blades centrifugal fan 50 % 55 % 70 % 100 %
heat exchanger
6.3.5.1 “sensible heat only” heat exchangers
For equal supply and extract airflows, temperature variations can be determined using the equations: \[\theta_s^2 = \theta_s^1 + \Delta T_{HEsup}\]\[\theta_e^2 = \theta_e^1 + \Delta T_{HEextr}\]where \(\theta_e^1\) and \(\theta_s^1\) represent the air extract and supply characteristics before the heat exchanger, respectively.
Eff HE is the Heat Exchanger efficiency for a given set of equal or almost supply and extract airflows
For single residential supply and exhaust units tested per EN 13141-7, the overall efficiency accounts for the fan temperature increase when recoverable Consequently, this increase should be set to 0 in calculations, as it is already factored into the efficiency term.
6.3.5.2 Sensible and latent heat exchanger
Writing equations that separate the effects of temperature and humidity is feasible; however, product standards currently provide only a single testing point for hygroscopic units, which is insufficient to fully characterize the influence of both factors.
In both cases, the θe 2 value is limited to a θe 2min value
The following default values θ e 2min can be used for if no national information is available:
Default value for θsetdefrost : 5 °C: a ) Direct defrosting control:
A correction value ∆(∆ T HEext)a shall be applied on θ e 2
∆(∆T HEext)a = max(0; θ e 2min –θ e 2) if exhaust and supply flow are equal, the same correction has to be applied to θs 2
The corrected value of θs 2 is lower than the initial ones, which corresponds to the heating penalty devoted to the defrosting b) Defrosting coil
The outdoor air is warm up to a θ setdefrost value It is required in this case to heat directly the air
P defrost the heating power, in W, required to warm up the air is calculated by
To determine the θsetdefrost value for the heat exchanger, it is essential to calculate the θe 2min value This calculation is based on the condition that the supply and extract air flows are equal, leading to the formula: \[\theta_{setdefrost} = \theta_{e1} + \frac{\theta_{e2min} - \theta_{e1}}{Eff \, HE}\]
NOTE The θset defrost increases when the heat exchanger efficiency increases
The air charateristics are calculated by θs1 = θext x s1 = xext θs2 = max(θσ1, θsetdefrost) x s2 =x s1
6.3.5.4 Free cooling - Limitation of supply temperature
Only valid in case of the presence of a by-pass provision
The θs2 temperature can be limited to a θs2max value in order to prevent air heating in a cooling period The ∆T HEsup shall be corrected by a value
∆(∆T = min(0; max (θs -θs ; θs – θs ) ) if no limitation, it is possible to apply the same formula by setting θs 2max to a high value (for example 100 °C)
The new value of θs2 with control (θs2c) is then equal to θs2c = θs2 + ∆∆T HEsupa + ∆∆T HEsupb
Mixing boxes
The supply air is a mix of outdoor air and recirculated air Mixing is made in the mixing box (or recirculation box) with dampers
The airflows to and from the building are predetermined, and the recirculation process primarily affects the outdoor airflows The modified supply airflow can be expressed as \$q_{vs1} = (1 - R_{rec}) q_{s2}\$, while the exhaust airflow is given by \$q_{ve2} = (1 - R_{rec}) q_{e1}\$ The temperatures and humidity levels are adjusted accordingly, with \$\theta_{s2} = R_{rec} \theta_{e1} + (1 - R_{rec}) \theta_{s1}\$ and \$x_{s2} = R_{rec} x_{e1} + (1 - R_{rec}) x_{s1}\$ The temperature of the extract air before the mixing box is denoted as \$\theta_{e1}\$, and its humidity is \$x_{e1}\$ After the mixing box, the extract air's temperature and humidity are represented as \$\theta_{e2}\$ and \$x_{e2}\$, respectively The airflow rates before and after the mixing box are \$q_{ve1}\$ and \$q_{ve2}\$, respectively, while the supply air parameters before the mixing box are \$\theta_{s1}\$, \$x_{s1}\$, and \$q_{vs1}\$ The control of recirculation is also a critical factor in this process.
As for a heat exchanger, the recirculation air ratio can be controlled for saving energy, mainly by increasing the outdoor air when it is beneficial.
Pre-heating
The supply air is warmed up to a θsetPH value for comfort reasons The heating power required P preheat and the temperature and humidity are calculated by
P preheat = max (0; 0,34 q vPH (θSetPH – θ1) θ2 = max(θ1, θsetPH) x 2 =x 1
The air flow through the preheating coil, denoted as \$q_{vPH}\$, is measured in m³/h The set point for preheating is represented by \$\theta_{SetPH}\$ in °C The air temperature before the preheating coil is indicated as \$\theta_{1}\$ in °C, while the temperature after the coil is \$\theta_{2}\$ Additionally, the air humidity before the preheating coil is expressed as \$x_{1}\$ in g/kg of dry air, and the humidity after the coil is \$x_{2}\$.
Example values for θsetPH are 12 15 °C depending on the application.
Pre-cooling
The supply air is cooled down to a θsetPC (°C) value for comfort reasons The cooling power
The airflow through the precooling coil, denoted as \$q_{vPC}\$, is measured in m³/h The air temperature before the precooling coil is represented by \$\theta_1\$ in °C, while the temperature after the coil is \$\theta_2\$ in °C The humidity of the air before the precooling coil is indicated by \$x_1\$ in g/kg of dry air, and after the coil, it is \$x_2\$ in g/kg of dry air The values of \$x_2\$ and \$\theta_2\$ are determined using the equations \$x_2 = x_1 + \Delta x_{PC}\$ and \$\theta_2 = \theta_1 + \Delta T_{PC}\$.
∆x PC = min(0; x coil –x 1) ⋅ (1- BP avfactor ) x coil = EXP(18,8161-4110,34/(θcoil+235)) θcoil: coil temperature with a default value of 8 °C
BP avfactor = min( 1; (θ2-θcoil) / (θ1-θcoil) )
The BP avfactor is an averaged Bypass factor taking into account the temperature control and can therefore be higher than the actual coil bypass factor.
Humidifying in winter
The air is humidified to a x sethum (g/kg of dry air) value
P humid required heating power to humidify the air at constant temperature is calculated by
Where q vhum is the air flow through the humidifier, in m 3 /h x 1 is the air humidity before the humidifier, in g/kg of dry air
After passing through the humidifier, the air characteristics are defined by the equations \( \theta_2 = \theta_1 \) and \( x_2 = \max(x_1, x_{\text{sethum}}) \) Here, \( \theta_1 \) represents the air temperature before humidification in °C, while \( \theta_2 \) is the temperature after humidification, also in °C Additionally, \( x_2 \) indicates the air humidity following the humidifier, measured in g/kg of dry air.
NOTE It is assumed that the air temperature remains constant (water vapour production) or that the air is warmed up to keep it constant (wet pad humidification)
Dehumidification
The calculation is done only if x setdeshum (g/kg of dry air) humidity set point value is lower than x 1, humidity level before dehumidification coil
If the bypass factor of the cooling coil BP coil is known, the w coil is calculated by
) ( coil coil 1 setdeshum coil BP
If the Bypass factor is not known, It is set to 0
The coil and set coil temperatures are calculated by θcoil =(4110,34/(18,8161-ln(x coil ))-235 θsetcoil = -θcoil
Powers have to be summed for each hour over the considered period.
General
The general fields of application are as follows:
Before implementing the calculation procedure, the type and performance of control has to be defined in accordance with prEN 15232.
Hourly method
In the absence of air entering through a balanced or supply-only system, the air characteristics are determined as specified in section 6.2 Additionally, it is essential to consider the energy consumption of the fan, if present.
In other cases, on the basis of the components impact, the calculation is done as follows:
1 Define at the beginning of the yearly calculation the system characteristics, except set points and indoor/outdoor climates
• The outdoor air characteristics (θext,w ext);
• The indoor air characteristics (θint, w int) In order to avoid loops, it is allowed to use the values calculated at the previous hour;
• The set points to be used;
• Calculation of extract air characteristics and before heat exchanger
Outdoor Duct (heat and mix with infiltred air)
• Calculation of supply air before heat exchanger
• Calculation of extract and supply air after heat exchanger
• Calculation of additional treatment on supply air a) Fan b) Outdoor duct heat losses c) Preheating d) Precooling e) Humidifying
This order may not be the actual one, but is correct considering the calculation of temperatures, humidities and energies with the following assumptions:
Control of preheating and precooling is done on the air supplied to the heat/conditioned zone The duct losses and fan impact are therefore compensated;
Temperature set point for precooling is lower than the set point for preheating (should be mandatory!);
Humidity set point for humidifying is lower than the saturation humidity for cool coil (or running of both should be forbidden).
Monthly methods
System with no or low humidity impact
The method involves analyzing the monthly distribution of outdoor temperatures, considering specific ranges and their occurrences By calculating each temperature range, we can estimate the corresponding indoor temperatures effectively.
The final results is the yearly (monthly) values of energy for preheating, precooling and auxiliaries
System with medium or high humidity impact
The final results are the yearly (monthly) value on energy for preheating, precooling and auxiliaries.
Statistical approach to be applied at national level
It is allowed to define on a national basis simplified approaches based on a statistically analysis of results
The following rules shall be fulfilled:
Field of application shall be specified (for example, detached houses, specified ventilation system…);
Specific assumptions (such as indoor temperature) or data (for example climate) shall be clearly described;
Set of cases used for the statistical analysis shall be clearly described;
Remaining inputs data for the simplified approach shall be the same as the ones described in the steady state calculation, or part of them;
For the input data of the steady state calculation not taken into account, the conventional value used shall be specified (for example, no defrosting in a mild climate);
Results of the simplified approach shall be compared to the reference ones for the set of cases taken into account in the statistical analysis
A report shall be provided with two parts
1) Description of the statistically based simplified approach defining
The main aim is to make it possible to redo and check the calculation starting from this steady state calculation
Definition of the cases taken into account for the statistical analysis, including:
Conventional values for the input data not kept in the simplified method
Range of values for the input data kept in the simplified approach
Results of the different test cases (called reference results)
Description of the simplified approach and comparison of the reference results
Indication on the level of accuracy based on the comparison
A simplified model of a Ground to Air Heat Exchanger
Background and summary
This is a simplified model to calculate air preheating due to supplying air through ducts lying in the ground
leaving air temperature of the heat exchanger;
heat flux between ground and air in duct;
pressure losses depending on the air velocity and the specific duct parameters
This simplified model is based on the "Handbook of Passive Cooling" and incorporates specific duct parameters along with the ground's inertia, which varies according to the depth of the ducts embedded in the ground.
Also the ground material is taken into account by a correction factor for the ground temperature
In this simplified model the ground temperature depends on two parameters: the annual mean outside air temperature and the depth of ducts
The ground temperature is modelled as a sinus curve based on the annual mean outside air temperature The depth of ducts corrects the sinus curve in two ways:
1 The amplitude decreases in function of the depth
2 The ground temperature is retarded in function of the depth It means the inertia of the ground increases in function of the depth
Overview of program links, variables, parameters and constants
TAirIn "Temp of entering air"
MAir "Dry air massflow rate"
PAirOut "Pressure of leaving air"
TAirOut "Temp of leaving air"
Q "Heat flux from soil to air" dp "Pressure losses"
TG "Soil temperature" hi "Int surf coefficient"
VAir "Volume flow" v0 "Velocity in duct"
The parameters relevant to duct specifications include the "number of ducts," which indicates how many ducts are present, and "depth," referring to the depth of the duct in the ground Additionally, "length" denotes the total length of the ducts, while "duct inside diameter" specifies the internal diameter of the duct The "duct wall thickness" is crucial for structural integrity, and "roughness of duct surface" affects flow characteristics Furthermore, "conductivity of the duct" plays a significant role in thermal performance, and the "ground material factor" is essential for understanding the interaction between the duct and surrounding soil.
TAM "Annual mean outside temperature"
AS "Surface Area" do "Duct outside diameter"
CP_Air “Specific heat capacity”
Physical description of the ground to air heat x-change model
A.3.1 U-Value of the air duct
A.3.1.1 Volume flow and air velocity
A.3.1.2 Inside surface coefficient θm is the arithmetic mean value of entering and leaving temperature To avoid iteration, Equation A.3 can be simplified by setting θ m = TairIn
The ground temperature is influenced by the annual mean and the range of outside air temperature fluctuations at the building site, as well as the depth of the duct buried in the ground To account for the ground's thermal inertia, adjustments are made to the outside air temperature using factors such as AH, VS, and gm.
AH corrects the amplitude, depending on the depth of the ducts lying in the ground
VS correct the ground temperature by a time shift, depending on the depth of the ducts lying in the ground
The amplitude of the annual outside air temperature swing, denoted as ∆T A, can be calculated by taking the difference between the maximum monthly mean temperature (typically in July) and the minimum monthly mean temperature (usually in January), and then dividing that value by 2.
Table A.1 — gm values for soil materials
Ground Material Conductivity [W/mK] Density
A.3.4 Heat flux from ground to air
A.3.5 Pressure losses of the heat exchanger
The pressure losses are calculated as for any other duct, depending on material properties, size and velocity
[1] EN 1886, Ventilation for buildings — Air handling units — Mechanical performance
EN 13141-7 outlines the performance testing standards for mechanical supply and exhaust ventilation units, including heat recovery systems, specifically designed for single-family dwellings This standard is crucial for ensuring effective residential ventilation and optimizing indoor air quality.
[3] EN 13465, Ventilation for buildings — Calculation methods for the determination of air flow rates in dwellings