EPC Convention 4.08 Modelling a zone within a zone

The simplification of guidance (paragraph 4.08 NDEPC Conventions) from the NDEPC Conventions Group has instigated a review of our guidance on this. The guidance here is intended to provide improved modelling but does not replace the guidance given previously.

Multi height zones can be drawn using either Add Surface or Boolean tools. This article explains how this can be done with the Add Surface tool:

This method avoids having to draw a tall space using multiple blocks and merging the blocks using holes drawn between them. This article shows how to do this using two blocks one representing the complete building, the other representing the single-height parts of the building. Note there is more information on 'Add Surface' in the online help.

1. The complete block may be drawn extruded from a horizontal or vertical base depending on which is more complex. For a rectangular plan, it is better to draw from a vertical base as the pitched or curved roof can be produced without multiple operations.

2. The vertical base is best drawn with the horizontal base line drawn first and the second edge drawn to snap the vertical axis. Protractor and centre snap tools help when drawing a simple pitched roof.

3. The added surface (really a removed section) is to correspond with the geometry of the single-height spaces and is best drawn with its base on the base of the complete block if the perimeter is polygonal. Click on the Add Surface tool and draw the perimeter. 

When complete drag upwards to set the height, entering the height manually or using increment snap.

4. The surface that has been removed needs to be replaced by a separate block. Make sure that the base is visible and Add Block is following the perimeter.

5. The main geometry is now complete.

6. Referring to the two cases described in Section 4.08 of the Conventions: 

not accessible floor area

If the floor above the office space is not accessible space then navigate to that floor element and on the construction tab (under adjacency header) select 'Exclude this surface from total zone area' (image shows DesignBuilder switched to non-learning mode).

Is accessible floor area

If the space above the floor is accessible then you will need to draw virtual partitions around its perimeter.

7. Set the zone height to match EPC Conventions (Section 10.03). In this case, it will be the height of the building (top of ground floor to eaves). DesignBuilder's automatic computation of zone height is generally the average height of walls and would need correcting for a space with a pitched roof. In cases where there is floor space excluded or holes drawn in the floor then the calculated zone height will be higher and could result in less accurate results.

Note that for DSM calculations you do not need to input the zone height manually, as the EnergyPlus variant of DesignBuilder (unlike SBEM) will automatically calculate the correct heat loss areas based on the geometry drawn and other relevant data inputs.