A “thermal zone” is the unit element or the building block of a building energy model. A “thermal zone” represents a fully enclosed volume of indoor air. Therefore, each BEM is a collection of thermal zones which represent a set of individual or groups of architectural/building spaces that share similar characteristics. The key point here is to understand the distinction between an architectural space/room and its representation as a thermal zone in building energy model. Based on a number of “grouping” or “ungrouping” criteria a number of architectural spaces can be combined into a single thermal zone or a single space can be divided into multiple thermal zones. The grouping and ungrouping actions can be performed by the modeler at geometric as well as non-geometric input levels.
C.EE: Environmental controllability of spaces (the need for a separate thermostat, humidity, air quality, etc. sensors). Different set-point set-back temperatures requires different thermal zones.
C.EE: HVAC system type serving the spaces (a VAV with reheat vs. DOAS with 4-pipe FCU).
C.EE: Status of Mechanical Conditioning and Ventilation (fully conditioned vs. unconditioned spaces or unconditioned but ventilated vs. unconditioned and unventilated spaces)
C.EC: Variations of internal loads (due to people, lights and internal equipment as generators of internal heat gain to the spaces). Having the same magnitude of internal loads is not a good justification for combining different spaces, attention should be focused on how these internal loads vary by time (temporal profiles). Are they having peaks and dips in the morning, noon or afternoon times? Or Are the profiles just flattened over the course of the day? The temporal characteristics of load profiles (internal loads, heating-cooling loads, OA ventilation loads dictate the selection and operation control of HVAC systems serving the thermal zones.
C.EC: Variation of space use type (office space vs. print room vs. conference room). This will impact the temporal profile of the people density, required temperature set-points for comfort, HVAC type and even the status of conditioning and ventilation. For some building projects designed for multiple tenants, a distinction has to be made between shared spaces, public spaces, and the rented ones.
C.EC: Ventilation Loads (also related to the people density and has impacts on HVAC system type. For instance, a high-occupancy space should have its own air handler to save fan energy and to optimize humidity controls). Ventilation Load: The OA flow rate (m3/sec or CFM) for a space
C.EC: Solar Exposure. Spaces facing similar orientation on the same facade (within 45-degree angle) can be combined into a single thermal zone as long as other criteria for thermal zoning are satisfied.
C.EC: Infiltration Effects. Even though most (perimeter) spaces are tried to be protected from wind-driven or buoyancy-driven infiltration effects by the use of active positive or neutral air pressurization, there may spaces that still impacted by infiltration and needs to be separate as distinct thermal zones. These spaces are entrance areas, lobbies, garage spaces, loading-unloading spaces, spaces equipped with manually operable windows, etc.
C.EC: Proximity to the Facade which faces the ambient conditions. That is also known as the “perimeter vs. core” thermal zoning approach. Perimeter spaces that should go into perimeter zones are usually defined as being within a maximum of 4.5m (15ft) of an external fenestration surface for a typical commercial office space with a ceiling height of 3m (10ft). The perimeter depth can be less than 4.6m if the ceiling height is less (the rule of thumb is 1.5 times the ceiling height). When we define the perimeter zones the rest will be the core ones buffered with the perimeter zones. For a rectangular floor plate, the perimeter vs. core zoning approach will result in a total of 5 thermal zones per floor (4 perimeter zone facing N-S-E-W and a single core zone in the middle). This method is suggested by the ASHRAE 90.1-2016 Energy Standard for building projects where internal layout has not been finalized by the design teams. This approach is also known as the Five-Zone approach. Note that the core zone of a top floor underneath the roof would experience different environmental exposures than the core zone of an intermediate floor or the core zone of a floor on ground.
C.EC: The Building Thermal Mass. Thermally massive spaces (the ones having concrete or masonry external walls) should de differentiated from light mass spaces (the ones having steel or wood framed external walls). It is a fact that the construction assemblies (and even interior finishes and furnishings) have thermal masses (due to their density and specific heat capacities) and interact with some dynamic thermal processes such as when the heat from solar radiation and internal gains are stored and released and how these affect the spaces’ characteristics on heat gain and loss in the overall heat balance.
C.EC: Top floor vs. Underground Floor. The top floor and its spaces under the roof is exposed to a relatively intensified solar radiation gains (due to solar angles and mostly unobstructed surfaces) as well as increased wind speeds (due to height). The intensity of exposure is pronounced in case the roof have horizontal fenestration systems (the skylights). The underground floors on the other hand have ground contact They don’t have direct exposure to outdoor air and solar conditions (N0 Sun and No Wind concept). However, these spaces transfer heat and moisture with the soil and through the below-grade basement walls. Tendency in thermal zoning is to group all underground spaces into a single or a few number of thermal zones to simplify the energy model (remember that underground spaces are not affect by change of orientation which simultaneously changes the solar exposure and radiation gain profiles).
C.EC or C.EE:? What could be other criteria of Environmental Controllability and Environmental Exposure in determining the thermal zones out of architectural/building spaces?
Note that thermal zones don’t show up in the construction drawings or MEP drawings and schedules or even in the as-built drawings. Developing thermal zone layout (simplified sketches over the project drawings) and creating then in the model is the responsibility of the energy modeler and needs to conducted by communicating with both the design and the engineering team members. Below are some dictionary definitions of the “Zone” concept: ASHRAE Terminology Web-Page (Free Access) https://xp20.ashrae.org/terminology/index.php?term=Zone&submit=Search HVAC zone a space or group of spaces, within a building with heating, cooling, and ventilating requirements, that are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor). Zone (1) a separately controlled heated or cooled space. (2) one occupied space or several occupied spaces with similar occupancy category, occupant density, zone air distribution effectiveness, and zone primary airflow per unit area. (3) space or group of spaces within a building for which the heating, cooling, or lighting requirements are sufficiently similar that desired conditions can be maintained throughout by a single controlling device. Zone (Control Zone) space or group of spaces within a building with heating or cooling requirements sufficiently similar that comfort conditions can be maintained by a single controlling device. Occupied Zone the portion of the space that is normally occupied. The occupied zone is typically defined as encompassing all space from the floor level, excluding the space from the floor to 0.25 ft (0.076 m) above the floor, to 6 ft (1.83 m) above the floor and excluding the space from the wall to 2 ft (0.61 m) away from any wall. Interior Zone any space not affected by exterior loads CO2: Carbon Dioxide OA: Outdoor Air ASHRAE: American Society of Heating Refrigeration and Air Conditioning HVAC: Heating Ventilating and Air Conditioning VAV: Variable Air Volume DOAS: Dedicated Outdoor Air System FCU: Fan Coil Unit Geometric Input: the shape, form and dimensions and orientation of spaces. Non-Geometric Input: the environmental boundary conditions, thermo-physical properties of building envelope assemblies, occupancy, equipment and lighting power densities, mechanical-electrical systems, operational profiles, etc. N-S-E-W: North-South-East-West Omer T. Karaguzel, PhD
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