Air System Design Load Summary Report
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Air System Design Load Summary Report The Air System Design Load Summary report provides a list of component cooling loads for the air system for one design cooling hour and a list of component heating loads heating loads for the design heating heating condition. condition. Cooling data on this report can be generated for the hour of the peak central central cooling cooling coil load, or for a month and hour chosen by the user. This report is typically used when investigating how different load components contribute to the system load. Length: 1 page. A sample of this report for a VAV Reheat system is shown below. Following this figure, the format and content of the re port is discussed in detail.
Report Format and Content. The Content. The Air System Design Load Summary report consists of one large table. Each row contains data for a different load component. Load components are divided into three sections: 1. Zone Loads are Loads are grouped in the upper portion of the table. These are loads occurring in the zones served by the system and include items such as wall loads, lighting loads and infiltration loads. 2. System Loads are Loads are grouped in the middle portion of the table. These are loads occurring outside the zones. They include items such as fan heat gain, ventilation loads and plenum loads. 3. Coil Loads Loads are grouped at the bottom of the table. These items show how the total system load is divided among the various coils in the system. Line items appearing in this section of the table vary with system type. The table contains seven columns: a column listing the load components, three columns containing cooling load data and three columns containing heating load data. Columns for load data are as follows: 1. Details contains Details contains useful information associated with the cooling or heating load data. This includes areas for walls or roofs, wattages for internal loads and airflow rates for ventilation loads and fan heat heat gains. gains. 2. Sensible contains Sensible contains the sensible component of each load. 3. Latent contains Latent contains the latent component of each load. Line items in the table are described below in three separate sections, one for each of the major groupings of load components. calculations.. For cooling, Component Zone Loads. The Loads. The first section of the table lists zone component loads. Zone loads are the result of the first stage in the sizing calculations zone loads are loads are obtained by calculating calculating the the heat heat gain gain for each component and then using the heat gains in the room transfer function procedures to derive the load. Per ASHRAE procedures, loads are calculated calculated assuming assuming that cooling is provided 24 hours per day and all zones are held exactly at the occupied cooling thermostat setpoint. For heating heating,, zone loads are loads are computed as instantaneous heat heat losses losses for the design heating condition. Zone loads serve as the basis for system simulation calculations whose calculations whose results are shown in the middle and lower sections of the table. Each component load in this section of the table is the sum of individual loads in all spaces in all zones served by the air system. Individual items are as follows: 1. Solar Loads include loads due to solar radiation entering zones through windows in wall exposures and skylights in roof exposures. The value in the "details" column represents the total area for all windows and skylights. 2. Wall Transmission Transmission lists the transmission load for all above-grade vertical wall exposures. The area in the "details" column is the net wall area. Only the wall transmission load reaching the zone will be listed. Any wall load assigned to a return plenum will be listed later in the system section of the table. Note: The load for below-grade portions of walls is included in the Floor Transmission line item (see below). 3. Roof Transmission lists Transmission lists the transmission load for all horizontal and sloped roof exposures. The area in the "details" column is the net roof area. Only the roof transmission load reaching the zone will be listed. Any roof load assigned to a return plenum will be listed later in the system section of this output. 4. Glass Transmission lists Transmission lists the transmission loads for all windows in wall exposures. The area in the "details" column is for all windows. 5. Skylight Transmission lists Transmission lists the transmission loads for all skylights in roof exposures. 6. Door Loads lists Loads lists the total load for the door. This includes transmission of heat through both the opaque and glass portions of the door and the solar load for door glass.
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Air System Design Load Summary Report
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7. Floor Transmission lists heat transmission through floors above unconditioned spaces, slab on grade floors and basement floors. When basement floors are involved, this item also includes heat transferred through below grade walls. Readers should note that slab and basement floor transmission are not considered for design cooling calculations. Finally, the area listed in the "details" column lists the total area for these three types of floors. Areas for floors above conditioned spaces and basement wall areas are not included in this total area. 8. Partitions lists heat transmission for partitions adjacent to unconditioned spaces. 9. Ceiling lists heat transmission for ceilings below unconditioned spaces. 10. Overhead Lighting lists the lighting load for overhead lighting fixtures only. The value listed is for the portion of the lighting load assigned to the zone. The portion of the lighting load assigned to a return plenum is provided later in the system load section of the table. The wattage listed in the "details" column is the total lighting heat gain, and includes both the portions of the heat gain assigned to the zone and to a return plenum. 11. Task Lighting lists the loads due to any task lighting fixtures. 12. Electric Equipment lists the loads due to electric equipment heat gain. 13. People lists the loads due to sensible and latent occupant heat gains. 14. Infiltration lists loads due to infiltration air. For the cooling calculation the "design cooling" infiltration airflow specified in space inputs is used. For the heating calculations the "design heating" infiltration airflow specified in space inputs is used. 15. Miscellaneous lists loads due to any miscellaneous sensible and latent heat gains specified by the user. 16. Safety Factor lists the safety BTU/h or W for each column of cooling and heating loads. The safety loads are computed by summing load quantities in all previous rows in the table and multiplying this total by the safety factor percentage specified by the user. In the cooling "details" column the safety factors specified by the user are listed in the following order: cooling sensible, cooling latent. 17. Total Zone Loads lists the sum of all load items in the zone load section of the table. Component System Loads. The middle section of the table lists system component loads. These are loads occurring outside zones in the system. They include items such as fan heat gains, the ventilation load and plenum loads. Loads in this section of the table are the result ofcalculations simulating the operation of the air system in response to cooling and heating demands in the zones. Items in this section are as follows: 1. Zone Conditioning is the actual amount of heat which is removed from the zones (for cooling) or added to zones (for heating). For sensible cooling the zone conditioning is based on the "total zone load" with corrections for the actual operating schedule (which may be less than 24 hours per day), for the use of set-up and set-back temperatures in the unoccupied period, and for the variation of zone temperatures within the thermostat throttling range. These corrections are made using the ASHRAE heat extraction procedures. Thus "zone conditioning" includes such heat quantities as pulldown loads, while "total zone load" does not. For latent cooling the zone conditioning will equal the total zone load since latent loads are instantaneous. For design heating, the zone conditioning will nearly equal to the total zone load because the transient behavior considered for cooling calculations is not considered for a standard design heating calculation. Note that small differences between zone conditioning and total zone load may be seen due mathematics of the system simulation process. These differences will generally be very small. If a large difference between the total zone heating load and the zone conditioning occurs, it is usually due to a system operating problem. In such a case, the System Psychrometrics report for the design heating condition should be generated to evaluate system performance in greater detail. 2. Plenum Wall Load lists the portion of the wall transmission load transferred to return plenum air. The "details" column lists the wall-load-to-plenum percentage specified by the user. 3. Plenum Roof Load provides the portion of the roof transmission load transferred to plenum air. The "details" column lists the roof-load-to-plenum percentage specified by the user. 4. Plenum Lighting Load lists the portion of overhead lighting heat gain transferred to return plenum air. The "details" column lists the lighting-load-to-plenum percentage specified by the user. 5. Return Fan Load lists the heat gain for a return fan, if one is used. The "details" column lists the return airflow rate. When a return fan is not used, the corresponding heat gains will be zero. 6. Ventilation Load lists the net heat gain or loss for ventilation air entering the system and exhaust air leaving the system. The "details" column lists the outdoor ventilation airflow rate. One of the common hand-calculation checks users perform involves the cooling and heating ventilation loads. To duplicate ventilation calculations by hand successfully, users are strongly encouraged to refer to the discussion of ventilation load calculations. 7. Supply Fan Load lists the heat gain for the system supply fan. The "details" column lists the actual airflow through the supply fan. 8. Hot Deck Supply Fan. For 2-Fan Dual Duct VAV systems, the heat gain for the hot deck supply fan will also be listed. The "details" column lists the actual airflow through the hot deck fan. 9. Space Fan Coil Fans lists the total heat gain for all fan coil units in zones served by the air system. For terminal systems such as fan coils or water source heat pumps, these values will be heat gains for the terminal unit fans. For other systems, heat gains for supplemental zone fan coil heating units will be listed. 10. Duct Heat Gain/Loss. For the cooling condition, this item lists supply duct heat gain. For the heating condition, the item lists supply duct heat loss. The percent duct heat gain/loss value specified by the user is provided in the "details" column. 11. Total System Loads contains the sum of system loads in each of the cooling and heating columns. These totals are computed starting with "zone conditioning" and working downward. The totals represent the net amount of heat which must be removed from the system (for cooling) or added to the system (for heating) to maintain comfort conditions in the zones. Component Coil Loads. The lower section of the table lists individual coil loads for the system. This section shows how theheat removal or heat addition values represented by the "total system loads" line are divided among different cooling andheating coils in the system. This section contains up to thirteen line items which differ depending on the type of air system is involved. Items in this section are as follows: 1. Central Cooling Coil: Sensible and latent loads for a central cooling coil or the cold deck coil in 2-Deck Multizone, 3-Deck Multizone and dual duct systems. 2. Central Heating Coil: Sensible loads for a central heating coil in a Single-Zone CAV system or the hot deck coil in 2-Deck Multizone, 3-Deck Multizone and dual duct systems. 3. Precool Coil: Sensible and latent loads for a precool coil. 4. Cooling Coil: Sensible and latent loads for a cooling coil in a common or tempering ventilation unit. 5. Preheat Coil: Sensible loads for a preheat coil. 6. Heating Coil: Sensible loads for a heating coil in a common or tempering ventilation unit. 7. Terminal Unit Cooling: The sum of sensible and latent loads for terminal cooling coils in all zones, such as those in terminal fan coil or water source heat pump units, or coils in 4-pipe induction terminals. 8. Terminal Unit Heating: The sum of loads for terminal heating coils in all zones, such as heating coils in fan coil or water source heat pump units. 9. Central Reheat Coil: Sensible loads for a central reheat coil used for dehumidification control. 10. Humidification Load: Latent load for moisture added to the supply air stream by the humidifier. 11. Terminal Reheat Coils: The sum of sensible loads for terminal reheat coils or heating coils in 4-pipe induction units for all zones in the system. 12. Zone Heating Unit Coils: The sum of heating loads for coils in supplemental zone heating units such as baseboard or fan coil units. 13. Total Conditioning: The sum of all items in the coil load portion of this output. Total conditioning values should closely match corresponding "Total System Loads" values. Small differences between corresponding values may be seen due to the iterative procedure needed to determine system operating conditions. Larger differences indicate a problem with system operation which should be investigated using the System Psychrometrics report. For latent cooling loads, differences are also usually small. These differences are the result of the iterative calculation used to estimate the steady-state humidity
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levels in the system. For most operating conditions this results in very small differences between the total system load and the total conditioning value. For unusually warm coil conditions, it can result in differences larger than 2%.
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