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Description

With the Psi-Value tool kind 2 Constructions you can calculate Ψ-values of constructions, which have 2 connected constructions. You can also calculate joint Ψ-values (ΨTJ-Values) according to EN ISO 12631 using this tool.

The Psi-Value tool allows you to calculate the Ψ-values (previously linear k-value). The Ψ-value, which quantifies additional energy, is a correction value that is caused by linear shaped thermal bridges, and is not taken into account in 1 dimensional energy calculations with U-values.

The magnitude of the Ψ-value depends on several values: the quality of the construction, the quality of the dimensions, as well as the U-value which calculates the loss of heat of the undisturbed component. Hence, it is possible that a poorly insulating flat roof protection has a better Ψ-value than a well insulating window-wall junction, because in the case of the former, the calculation of the Ψ-value is done with exterior measurements through which part of the energy is already covered by thermal bridge energy losses by flat roof and wall measurements. Ψ-values can also be negative.

The calculation of Ψ-values is explained in detail in tutorial 5.

How do I activate it?

The Psi-Value tool can be activated with the menu command Psi-Value in the menu Results or by clicking on the icon in the  Toolbox flyout. Then select the kind 2 Constructions in the tool properties (cf. figure 2). When the tool is active, the mouse becomes a .

How do I use it?

To calculate the Psi-value three points are required: the first two points define the surface line, through which the effective heat flow should be calculated. You then have to define a third reference point, which divides the surface line in two parts with different U-values.

The surface line is defined by the input of the start and endpoints:

First click on the spot where the surface line should begin. The boundaries of the construction, which come into question for the surface line, will be highlighted as a dashed line. Move your mouse counterclockwise to the desired endpoint. The dashed line now shows the segment through which the heat flow will be calculated. As soon as you click on the endpoint the line will be defined.

If the SHIFT key is pressed while you define the surface line, then the first click sets start and endpoints automatically at the adjacent adiabatic edges (e.g. construction sections).

The Psi-value  will be displayed, and the start and end points of the surface line will be labeled.

If you want to interrupt the line input process, then you can use context menu (right click) command Cancel or you can hit the ESC key.

You can change the reference lengths of the base constructions using the Scale, Move, Select tool or Edit tool respectively, by moving the start-, end and reference point. You can also change the text positions of the help objects, of the U-Values of the base construction and the dimension lines using the same tool.

The base values of the Ψ-value calculation, which have a significant influence on the result (U-values of the constructions, length of the components, if the heat flow through the sections at the start- and endpoint should be considered and the temperature difference), can be adjusted afterwards in the Properties flyout (cf. figure 1).

Figure 1: Properties flyout of a Ψ-value object based on 2 constructions

Defining Styles

The presentation of a Psi-value object can be defined in the Style list in the Psi-Value tool properties (see figure 2). The list shows all styles for Psi-value objects that have previously been determined in the Styles flyout.

A new Psi-value object will always be created with the styles currently marked in the Style drop down list. You can either change the selection of the styles in the Styles flyout or in the Style drop down list in the Psi-Value tool properties.

The styles can also be adjusted afterwards by using the Assign Properties tool or by using the Drag&Drop function (see tutorial 1).

Figure 2: Psi-Value tool properties

Accurate Results

To receive accurate results, the following conditions must be met:

The type of label (e.g. number of post-decimal place holders) as well as the graphical properties of the cross section line can be adjusted with the corresponding styles in the Styles flyout.

Figure 3

Physical Explanation

The Ψ-value is calculated as follows:

and calculated as follows if the heat flux through the section should be taken into account (cf. Properties flyout):

where:

ΦA-C: heat flow from A to C (counterclockwise) [W/m]

ΦB-A: heat flow from B to A (counterclockwise) [W/m]

ΦC-D: heat flow from C to D (counterclockwise) [W/m]

ΔT: if there are only two boundary condition temperatures: Temperature difference TA - TB of the boundary condition temperatures TA and TB at the points A and B [K]

ΔT: if there are more than 2 boundary condition temperatures, then the temperature difference will be calculated according to the rule defined in the Properties flyout.

UB-A: U-value for the whole construction from A to E [W/m2K]

UC-D: U-value for the whole construction from E to C [W/m2K]

lA-E: length of the projection of the line segment AE on the perpendicular to the section line AB [m]

lC-E: length of the projection of the line segment CE on the perpendicular to the section line CD [m]

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