Hello everyone,
I’m a beginner in Ladybug Tools, so please excuse me if I’m asking a very obvious question, but I’m having trouble interpreting the usage of Ideal Air. What would be the actual capacity of the air conditioning equipment? I simulated an Ideal Air, and I noticed there’s a table called “Zone Sizing Information” which, for example, gives a value of “User Des Load” of 9983.67 W.
While in the “Component Sizing Information” table, the value for the same space in “Design Size Maximum Total Cooling Capacity [W]” is 17978.45 W, which is twice as much as the first case.
I hear you that sizing calculations are confusing, partly because there are all of these conventions about them that have evolved over time. You can always check the EnergyPlus Engineering Reference for these but I’ll try to define them quickly here:
Zone Sizing Information - Calc Des Load: This is this peak heating or cooling energy to be added or removed from the Zone on the design day to meet the zone thermostat setpoint. It DOES NOT account for any outdoor air brought in through a mechnical system to ventilate the space. It’s only looking at what is needed to meet the setpoint in the absence of any mechanical ventilation.
Zone Sizing Information - User Des Load: This is this the “Calc Des Load” multiplied by a user-defined “safety factor” to account for unexpected situations (eg. actual weather worse than the design day, higher internal gain from equipment than expected). ASHRAE recommends a factor of 1.15 for cooling and 1.25 for heating. You can change these factors on the HB Sizing Parameter component.
Component Sizing Information - Design Size Maximum Total Capacity: This is the final size of the system which is designed to meet both the thermostat setpoint AND to condition any outdoor air brought into the space for ventilation purposes. Note that, if you think your zone does not require mechanical ventilation (most Residences do not have mechanical ventilation because infiltration is already high enough), you can always set the ventilation to zero using the HB Apply Load Values component and you’ll find that this value here becomes equal to the “Zone Sizing Information - User Des Load”.
So, if you ware asking which one of these an MEP engineer would use to select an air conditioner, it depends. If it’s just a window or wall air conditioner that recirculates air and doesn’t do any mechanical ventilation, then it’s the “Zone Sizing Information - User Des Load”. If this is a system that is also going to bring in fresh outdoor air to ventilate the space at your specified ventilation requirements, then it is the “Component Sizing Information - Design Size Maximum Total Capacity”.
