The fundamental concepts of (architectural) rooms, (BEM) spaces, and (HVAC) zones form the basis of geometric and non-geometric modeling hierarchies. These concepts are interrelated and often confused by energy modelers particularly during the building energy model development of complex building projects. Let’s have a focused look at these concepts with the help of following schematic depiction of the entire interrelationships.
At the root of everything lies the architectural room as designed by the architect and depicted on a schematic or detailed architectural plan layout. Then comes the BEM space. As seen in the diagram, a single BEM space can be linked to a specific architectural room (Config #1) or this individual BEM space can contain multiple rooms within it (Config #2). For instance, a closed office room at the corner of a plan can be linked to a single BEM space with the same name or a series of closed offices of similar areas and occupancy capacities and facing the same orientation can all be grouped together and linked to a single BEM space that represents the entire grouping. BEM spaces have individual geometries as well as occupancy densities, lighting power densities, or equipment power densities, and all other non-geometric input parameters. BEM space (irrespective of its underlying architectural room grouping) represents a single well-mixed volume of air (with uniform temperature unless intentionally stratified by the modeler) to be thermally simulated by the BEM software/engine.
BEM spaces can be grouped together to provide a higher level of modeling control (see Space Group 01, Space Group 02). This grouping can be created based on an arbitrarily defined user criteria such as BEM spaces’ location on the building floor, ground contact characteristics, service water heating system branches, etc. But this grouping is only arbitrary and does not indicate a geometric unification or non-geometric similarities. Grouped BEM spaces can be selected at once and certain properties can be updated by single high-level data entry by the modeler.
HVAC Zone as the name implies is the unit controllable entity that be served by a given HVAC system configuration. Briefly, and given HVAC zone contains a single thermostat (or humidistat or any other environmental sensing & control device). Similar to the previous interrelationship (of architectural room to BEM space), a single BEM space can be associated with a single HVAC zone (Config #1, Config #4) or a given HVAC zone can contain a group of BEM spaces (Config #2, #3). The key point to pay attention here is that in case multiple BEM spaces are connected to a single HVAC zone only one of these spaces can contain the environmental sensing & control device (~the thermostat). Therefore, only one of the BEM spaces in a group becomes the master space (where you see the “M” in the BEM space notations). The thermostat located in the master BEM space basically controls all the other BEM spaces in the same HVAC zone.
HVAC zones can also grouped together and linked a single HVAC Zone group. THe physical equivalent of the HVAC zone group concept is the Air Handling Unit (AHU) of an HVAC system (Config #4). An example of this set up can be a roof top unit (RTU) that is serving a group of zones or multi-zone AHU that is serving all the spaces-zones in a specific floor of a multistory office building. Please note that a single HVAC zone can be directly linked to an individual HVAC zone group (~AHU) (e.g., packaged single zone HVAC systems) (Config #1, #2).
As expected at the top level of hierarchy lies the HVAC plant which usually serves multiple AHUs in the same building. HVAC plant contains all the primary system elements (e.g., boilers, chillers, cooling towers) (Config #5). Finally, a single AHU can be interlinked with a single HVAC plant for small sized buildings (Config #6). There is no perfect way of connecting all these modeling elements and there will always be unique hierarchal connectivities for each building project. Here are some example as depicted in the diagram.
Configuration #1: Single HVAC zone and zone group is linked with a single BEM space which is also linked with a specific architectural room. Example: A corner office unit is defined as a distinct BEM space which is defined as a specific HVAC thermal zone with its own thermostat.
Configuration #2: Single HVAC zone and zone group is linked with a single BEM space which represents multiple (lumped) architectural rooms. Example: Office 102 and 103 are both single-occupant closed office spaces with the same dimensions and they are facing the same orientation. These rooms are represented with a single BEM space and a single HVAC zone. The BEM space named as Office 2+3 and it has its own thermostat.
Configuration #3: A single HVAC zone and zone group is linked to multiple BEM spaces which are linked to multiple but disjoint architectural rooms. Example: An office room (104) and an adjacent meeting room (105) are defined as two distinct BEM spaces with their own occupancy densities and usage patterns. However, these two BEM spaces are linked to single HVAC zone. This means that only one of these spaces will be the Master Space (M) (like Meeting 1 in the diagram) and the space conditioning of the Office 4 space will be controlled by the Meeting 1 room. This would not be an ideal space conditioning configuration. Since temperature profiles of the office space will be dictated by the conditions of the meeting space which will not align with the occupancy pattern of the office space in the first place.
Configuration #4: Multiple HVAC zones (connected to a single AHU) are linked to multiple but distinct BEM spaces which are also linked to disjoint architectural spaces. Example: This setup solves the environmental controllability problem that is seen in Config #3. Since each BEM space (Office 5 & Meeting 2) becomes their own masters (Master Space - M) with their own (individual) thermostats since there are defined as different HVAC zones. There is no problem of connecting multiple HVAC zones to a single zone group (AHU) since indoor temperatures can be controlled at the zone level (e.g., zone level reheat units with air dampers).
Configuration #5: Represents a common AHU to HVAC plant connection configuration where individual and multiple AHUs are linked to (or served by) a single (central) HVAC plant with its own boiler and chiller set.
Configuration #6: In this configuration, all HVAC zones (of different set ups) are connected to a single AHU for the entire building (AHU-Main) and this main AHU is linked to a single HVAC plant with its own boiler and chiller set. This can be observed for small scale building projects and their energy models.
Omer T. Karaguzel, PhD
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