Set Sizing

Sizing refers to the process of determining the capacity of the Heating, Ventilation, and Air Conditioning (HVAC) equipment needed to heat and cool a building under peak conditions adequately.

Outdoor Air

Outdoor Air Method

The Outdoor Air Method refers to the amount of outdoor air (also known as fresh air) that must be supplied to different zones in a building. This is an essential part of ensuring good indoor air quality and can also significantly impact the energy use of a building's HVAC (heating, ventilation, and air conditioning) systems. There are different methods of calculating the required outdoor air:

Air Changes / Hour: Specifies the number of times outdoor air replaces the volume of a space per hour.

Flow/Area Method: Specifies the minimum outdoor airflow rate based on the floor area of the zone. This simple approach does not consider the number of people in the zone.

Flow/Person Method: Specifies the minimum outdoor air flow rate based on the number of people in the zone. This method can better account for variations in occupancy, but it requires accurate information about the occupancy schedule.

Flow/Zone Method: Specifies a fixed outdoor air flow rate for the zone, regardless of the floor area or the number of people. This is the simplest method, but it is also the least flexible.

Sum Method: Specifies the combined Flow/Area and Flow/Person methods. The minimum outdoor air flow rate is the sum of the amount calculated by the Flow/Area and Flow/Person methods.

Maximum Method: Specifies the combined Flow/Area and Flow/Person methods. However, this method uses larger amounts as the minimum outdoor airflow rate instead of summing the amounts.

Sizing Factor

The Sizing Factor refers to heating and cooling sizing factors or ratios. These ratios are applied at the zone level to all zone heating and cooling loads and airflow rates. Once these new loads and airflow rates are determined, they are used to calculate the system-level flow rates and capacities and in all component sizing calculations. These sizing factors allow users to oversize or undersize a zone's heating and cooling capacities, providing flexibility in the design and simulation of HVAC systems. Specifically, there are two kinds of sizing factors:

Zone Heating Sizing Factor: Specifies the zone-level heating sizing ratio. The design heating air flow rates and loads for a specific zone will be multiplied by the value input in this field.

Zone Cooling Sizing Factor: Specifies the zone-level cooling sizing ratio. The design cooling air flow rates and loads for a specific zone will be multiplied by the value input in this field.

Cooling Supply Air

Cooling Design Supply Air Temperature Input Method: Specifies the 'Supply Air Temperature' or 'Temperature Difference' method. The selected method will determine how the zone cooling design air flow rates are calculated by using user inputs from either 'Zone Cooling Design Supply Air Temperature' or 'Zone Cooling Design Supply Air Temperature Difference' fields.

Cooling Design Supply Air Temperature: Specifies the temperature (in degrees Celsius) at which air is supplied to the zone during the cooling design day simulation. This field is used only when the 'SupplyAirTemperature' method is chosen. The zone load is managed by adjusting the zone air flow rate.

Cooling Design Supply Air Temperature Difference: Specifies the temperature difference (in degrees Celsius) between the cooling design supply air and room air temperatures. This field is used only when the 'TemperatureDifference' method is selected. The zone load is managed by adjusting the zone air flow rate.

Zone Cooling Design Supply Air Humidity Ratio: Specifies the humidity ratio, measured in kilograms of water per kilogram of dry air, of the supply air in a zone during the calculation of the cooling design air flow rate. This parameter is used in EnergyPlus simulations to characterize the moisture content of the air supplied for cooling purposes in a specific zone.

Cooling Design Air Flow Method: Specifies the method for calculating the zone design cooling air flow rate. The options include 'Flow/Zone', 'DesignDay', and 'DesignDayWithLimit'. These methods either use the user input from the 'Cooling Design Air Flow Rate' field or perform calculations based on a design day simulation and possibly minimum airflow requirements.

Heating Supply Air

Zone Heating Design Supply Air Temperature Input Method: Specifies the method of determining zone heating design air flow rates through SupplyAirTemperature or TemperatureDifference. Supply Air Temperature uses the user inputs from the fields of Zone Heating Design Supply Air Temperature, while Temperature Difference uses inputs from Zone Heating Design Supply Air Temperature Difference.

Zone Heating Design Supply Air Temperature: Specifies the supply air temperature, in degrees Celsius, used during the heating design day simulation. The zone load is managed by varying the zone air flow rate, and the maximum zone flow rate is saved as the zone heating design air flow rate. This field is only used when the Zone Heating Design Supply Air Temperature Input Method is set to SupplyAirTemperature. An entered value less than zero prompts a warning message for the user to verify the input.

Zone Heating Design Supply Air Temperature Difference: Specifies the temperature difference between the heating design supply air temperature and room air temperature in degrees Celsius for the zone heating design air flow rate calculation. This field is only used when the Zone Heating Design Supply Air Temperature Input Method is set to Temperature Difference.

Zone Heating Design Supply Air Humidity Ratio: Specifies the humidity ratio of the supply air used in the zone heating design air flow rate calculation, defined as the kilograms of water per kilogram of dry air.

Heating Design Air Flow Method: Specifies the method for calculating the zone design heating air flow rate: Flow/Zone, DesignDay, or DesignDayWithLimit. The default method is DesignDay, where the program uses calculated design values subject to ventilation requirements.

Heating Design Air Flow Rate: Specifies the design zone heating air flow rate in cubic meters per second and is used when the Heating Design Air Flow Method is specified as Flow/Zone. This value is then multiplied by the global or zone sizing factor and by zone multipliers.

Heating Maximum Air Flow per Zone Floor Area: Specifies the maximum zone heating volumetric flow rate per square meter is used when the Heating Design Air Flow Method is set as DesignDayWithLimit. It sets an upper limit on the zone design heating air flow rate.

Heating Maximum Air Flow: Specifies the maximum zone heating volumetric flow rate in cubic meters per second. It is used when the Heating Design Air Flow Method is specified as DesignDayWithLimit and sets an upper limit on the zone design heating air flow rate.

Heating Maximum Air Flow Fraction: Specifies the maximum zone design heating volumetric flow rate expressed as a fraction of the zone design cooling volumetric flow rate. The maximum flow derived from Heating Maximum Air Flow per Zone Floor Area, Heating Maximum Air Flow, and Heating Maximum Air Flow Fraction sets a maximum heating supply air flow rate for the zone for VAV systems.

Latent Supply Air

Zone Load Sizing Method: Specifies the basis for sizing the zone supply air flow rate. Valid choices are Sensible Load, Latent Load, Sensible And Latent Load, and Sensible Load Only No Latent Load. By default, the option is Sensible Load Only No Latent Load, which disregards zone latent loads during sizing. For other options, the zone humidity level is controlled, and the method uses either the zone sensible or latent air flow rate for component sizing or the larger of the two.

Zone Latent Cooling Design Supply Air Humidity Ratio Input Method: Specifies the method for determining the supply air humidity ratio used for latent sizing. The choices are SupplyAirHumidityRatio and HumidityRatioDifference (default). SupplyAirHumidityRatio is used when zone dehumidification is required. In this case, the supply air humidity ratio should be less than the zone humidity ratio at the set point condition. HumidityRatioDifference is used to enter the difference in humidity ratio from the set point condition.

Zone Latent Heating Design Supply Air Humidity Ratio Input Method: Specifies the method for determining the supply air humidity ratio used for latent sizing. The choices are Supply Air Humidity Ratio and Humidity Ratio Difference (default). Supply Air Humidity Ratio is used when zone humidification is required, in which case the supply air humidity ratio should be greater than the zone humidity ratio at the set point condition. Humidity Ratio Difference is used to enter the difference in humidity ratio from the set point condition.

Zone Heating Design Supply Air Humidity Ratio Difference: This field is used only when the Zone Latent Heating Design Supply Air Humidity Ratio Input Method is set to Humidity Ratio Difference. Specifies the difference between the zone humidity ratio at the set point condition and the supply air humidity ratio entering the zone.

DOAS

A Dedicated Outdoor Air System (DOAS) is an HVAC system that conditions outdoor air separately before delivering it to conditioned spaces. This system can improve energy efficiency, reduce ductwork size, and better control humidity and temperature.

DOAS primarily controls humidity and supplies fresh air for indoor air quality, and it can also handle part of the sensible heat load. It can work independently or together with other HVAC systems like radiant panels or VAV systems.

Account for Dedicated Outdoor Air System: This is a choice field with options Yes or No. By default, the option is No. If Yes is selected, the zone sizing calculation will consider the heat gain or loss imposed on the zone by a Dedicated Outdoor Air System (DOAS). The heat gain is then added to the zone design heat gain, and the zone design air flow rate is adjusted accordingly.

Dedicated Outdoor Air System Control Strategy: Specifies three control strategies for the DOA system: Neutral Supply Air, Neutral Dehumidified Supply Air, or Cold Supply Air. By default, the option is Neutral Supply Air.

1. Neutral Supply Air means that the ventilation air supplied to the zone will cause minimal heating or cooling. The air will be heated or cooled to keep it between the low and high-temperature setpoints specified in the subsequent fields.

2. Neutral Dehumidified Supply Air means that the ventilation air will be cooled and dehumidified and then reheated to a neutral temperature. The ventilation air is cooled to the lower setpoint temperature and reheated to the upper setpoint temperature.

3. Cold Supply Air means that the ventilation air will be used to supply cooling to the zone. Cold outside air is heated to the upper setpoint; warm outside air is cooled to the lower setpoint.

Dedicated Outdoor Air Low-Temperature Setpoint for Design: Specifies the lower setpoint temperature for with the DOAS design control strategy. The units are degrees C. The default is auto-sized to the values given above for the three design control strategies.

Dedicated Outdoor Air High-Temperature Setpoint for Design: Specifies the higher setpoint temperature for the DOAS design control strategy. The units are degrees C. The default is auto-sized to the values given above for the three design control strategies.