Use Case - Sizing Cooling Loop and Coils (Space Loads Vs. Coil Loads Vs. Loop Loads)

Why are numbers in the spreadsheet and web app different?


Peak Zone Cooling Load is the total amount of heat, both sensible (1) and latent (2), that needs to be removed from the zone to maintain the temperature setpoint. This is the space load on the peak cooling design day and hour. These can be different days and hours for every zone. For example, in this list, some are June and some are September.


The All-Zones Peak Coil Load (3) is the total cooling load seen by all the cooling coils in the fan coil system, at its peak condition. If you have air being supplied to the space (which you do in this case) then you will see some of the cooling done by the ventilation air and some of the cooling done by the fan coils.

note: these images are indicative only and from different projects so numbers will not relate to each other.


This means that your All-Zones Peak Coil Load will inevitably be lower than the sum of Peak Zone Cooling Loads (when there is also ventilation in the zones and the heat transfer of this ventilation is negative in relation to the zones).


How does ventilation affect the coil sizing?


In effect:

Peak Coil Cooling Load + Peak Cooling Contribution from Ventilation air = Peak Space Cooling Load.

For example, if you have 3 spaces with a peak space cooling load of 3kW and your ventilation was providing 0.5kW of cooling, the All Zones Peak Coil Load result in Sefaira would show 2.5kW and this would be the required cooling loop sizing for the FCU system.


If you had no ventilation air, then your Peak Cooling Coil Load should equal the Peak Space Cooling Load. Sometimes these will again be slightly different because EnergyPlus considers a range of dynamic factors in both calculations that may not result in them being exactly the same but they should be very close if not the same.


So how do I size my coils? 


You will see in your spreadsheet that you have in Column W - Coil Loads (4).

This is the load on the cooling coil in each zone (e.g. Fan Coil Unit). When added up, these probably won't sum to be the All-Zones Peak Coil Load because they are probably not being seen simultaneously.


In summary, what the All Zone Peak Coil Load is useful for is sizing the cooling loop for the fan coil system when ventilation is taken into account and what the spreadsheet is useful for is seeing the peak space load and sizing the FCU in each zone.


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