System Data (Load Calc)
On the System Data page you can define information about the air handlers in the project, such as the indoor temperatures for summer and winter, relative humidity, infiltration, ventilation, as well as the actual equipment that will be installed. If the building you are calculating has more than one air handler unit, then you should divide the building up into Systems, with one system per air handler.
If you are doing duct design in your project, there is more information that you can enter for the same system on the System Data - Duct Design page.
The System menu gets inserted into the main menu while you are on the System Data - Load Calculation page.
Add System: Adds a new system to your project, then sets the page to be in editing mode for your new system.
Delete System: Deletes the current system. Asks you to confirm deleting the system first.
Copy System: Copies the current system to the clipboard. The system will remain in the clipboard until you copy another system to the clipboard or log out.
Paste System: Pastes the system in the clipboard into the current system. This item is only available if you have copied a system to the clipboard since logging in.
Current System: Specifies the system that you want to edit.
Name: Specifies the name you want for the system.
Indoor Temperature Winter: If not set by local codes, this is the inside temperature that the client wishes to maintain during the heating season. A typical value is 72 degrees. Valid range for this input is 45 to 85 degrees Fahrenheit.
Indoor Temperature Summer: If not set by local codes, this is the inside temperature that the client wishes to maintain during the cooling season. A typical value is 75 degrees. Valid range for this input is 55 to 90 degrees Fahrenheit.
Relative Humidity Winter: If there is going to be winter humidification and the humidification device that will be used does not have its own source of heat, enter a desired indoor winter relative humidity and set the Do Winter Humidification input to Yes. The program will then calculate an additional heating load due to the humidification.
Relative Humidity Summer: Enter the Summer indoor relative humidity as a percentage (i.e. for 50%, enter 50). The term "grains difference" refers to the difference between the outdoor grains of moisture (grains is a unit of absolute humidity) and the indoor grains of moisture.
Manual J recommends the following values under the following conditions:
45% - If grains difference at 45% indoor RH is less than or equal to zero
55% - If grains difference at 55% indoor RH is greater than or equal to 50
50% - If neither of the above conditions is true
Winter Leaving System Coil to Room Temperature Change (Lvg. Coil-rm DT): This value represents the difference in degrees Fahrenheit between the air entering the rooms which are in this system and the inside design temperature of the system. For example, if 140 degree air enters a room that is in a system with a 70 degree indoor temperature, the Lvg. Coil-Room Delta T would be 70 degrees. Valid range for these fields is 1 to 85 degrees Fahrenheit.
Summer Leaving System Coil to Room Temperature Change (Lvg. Coil-rm DT): This value represents the difference in degrees Fahrenheit between the air entering the rooms which are in this system and the inside design temperature of the system. For example, if 55 degree air enters a room that is in a system with a 75 degree indoor temperature, the Lvg. Coil-Room Delta T would be 20 degrees. Valid range for these fields is 1 to 55 degrees Fahrenheit.
Infiltration Winter and Summer: Shows the current amount of infiltration specified for the current system. To change the infiltration rate click either Select button to open the Infiltration page.
Ventilation, Winter and Summer: Specifies the amount of outside ("fresh") air that is mechanically introduced into the conditioned space by the HVAC equipment. This value can be entered either as CFM or as air changes per hour. To change the ventilation rate click either Select button.
The addition of outside air to the HVAC equipment imposes a load that must be accounted for. The additional heating and sensible cooling loads are determined using the design temperature differences, while the latent cooling load is determined using the wet bulb temperature. Typically, less ventilation air is needed in the winter than in the summer.
If a sensible heat recovery device is going to be used to process the outdoor air before it is introduced into the system, then enter the appropriate values in the Heat Recovery inputs.
To determine the amount of ventilation air recommended by Manual J, click the Select button beside either Ventilation input to open the Ventilation Rate page. The Ventilation Rate page calculates the amount of ventilation that may be necessary in order to provide enough outdoor air if the amount of infiltration in this system is insufficient (the cooling infiltration is used to determine the requirement).
Exhaust, Winter and Summer: Specifies the total amount of air being exhausted to the outside from rooms in the current system. The only affect that exhaust will have on your load calculation is to adjust the load from infiltration, described below.
Note that the amounts you enter for ventilation and exhaust will affect the amount of infiltration assigned to this system based on these formulas. If ventilation is greater than exhaust, the system will be under positive pressure and the infiltration will be reduced from the amount you asked for. If exhaust is greater than infiltration, the system will have negative pressure and the amount of infiltration will be increased above the amount that you specified.
System Duct Load Factors: Shows "(None)" if you have not yet specified duct load factor data, or "(Data)" if you have specified such data. To edit duct load factors click the "Set" button.
Do Heat Recovery, Winter and Summer: Determines whether or not a heat recovery device will be used in the winter and summer. Select Yes if a heat recovery device is going to be used to process the outdoor ventilation air before it enters the system. A heat recovery device will reduce any sensible loss or sensible gain due to ventilation.
Heat Recovery SER, Winter and Summer: Specifies the Sensible Effectiveness Rating (SER) of the device, in percent, if a heat recovery device is going to be used to process the ventilation outdoor air.
Blower Power: Specifies the input wattage of the cooling equipment installed in this system. Only enter anything here if the ratings for the cooling equipment to be installed in this system do not include an adjustment for blower heat. Note that residential units usually do include an allowance for blower heat.
Miscellaneous Supply and Return Gains: If there are any miscellaneous supply or return sensible or latent gains at the system level, enter them in these inputs. You should usually enter supply side gains in the Equipment Sensible and Equipment Latent inputs on the Room Data page, but if there are gains that are not specific to any particular room you can enter them here in the supply miscellaneous gain inputs.
If you enter any return side miscellaneous gains, they will increase the load on the system, but will not affect the amount of supply air needed.
Water Delta T: If a hot/chilled water system is to be used for this project, Rhvac Online can calculate the required water flow rate through the heating/cooling coils if it knows what the expected temperature difference (delta T) of the water entering and leaving the coil is. The calculated water flow rates are shown on the Miscellaneous report.
Do Winter Humid: If there is going to be winter humidification and the humidification device that will be used does not have its own source of heat, select "Yes" to have the program calculate a heat loss associated with the humidification. Note that you will also need to enter a desired indoor winter relative humidity.
Radiator Btuh/ft.: For wet-heat applications such as radiators, enter the Btuh per linear foot of radiation. The valid range of values for this field is 0.000 to 999.999 Btuh/ft.
System air type: Enter "Fixed" (just type "F") if you want to enter the actual CFM of the unit that will be installed for this system. Enter "Auto" (just type "A") if you want Rhvac Online to instead calculate the minimum required CFM for the system. If you enter "Fixed", you will also need to enter a CFM value in the System CFM input (see below).
System CFM: If you have selected 'Fixed' for the 'System air type' input, enter the actual flow of air here. If you select 'Auto,' this input will be grayed out and unavailable.
This value will be used for both heating and cooling unless you enter a value in the "System CFM Htg." input as well. In that case the value you enter into this input will be the cooling CFM.
Normally, you will only need to enter one system CFM value, but the "System CFM Htg." input is provided for situations where you have an air handler that will operate at a different speed for the heating season.
System CFM Heating (Optional): Normally, you will only need to enter one system CFM value, but the "System CFM Htg." input is provided for situations where you have an air handler that will operate at a different speed for the heating season. If you do want the system CFM for heating to be different from that for cooling, enter a value here. If you leave this input at zero the program will use the value you entered for System CFM for both heating and cooling. If you enter a value here, the program will use the value you entered in the "System CFM" input for cooling.
Heating Duct Loads: If you are using some sort of radiant heating rather than heated supply air to meet the heating load for the rooms in this system, set this input to No to make it so the program will not add any sensible loss due to ductwork for the system.
Use Constant Volume (CV) Even if System is Multizone: If you put the rooms within this system into more than one zone, Rhvac Online will normally switch to the Peak Fenestration Gain (PFG) procedure in order to calculate the cooling load and supply air requirements for the rooms and zones in this system. The PFG procedure is based on the assumption that automatic VAV boxes will be installed in order to divert more air to zones during their peak load times, while delivering less air to the other zones.
However, if you are going to be using a constant volume system instead of a VAV system but you would still like to put rooms into multiple zones so you can see subtotals at the zone level, enter Yes for this input. That will make it so Rhvac Online will not switch to the PFG procedure for this system's rooms and zones, while still letting you see the load and supply airflow subtotals at the zone level.
Room and Zone Airflow Option: Select whether you want the airflow values assigned to rooms and zones in this system to be based on the airflow (heating or cooling) that is larger for the system, or for the airflow that is larger for that room or zone.