Hi, I am currently using CIBSE Gain to predict the cooling load in a single large east facing canteen room. The room will be cooled by fan coil units with tempered air supplied into the room at a minimum temperature of 21 deg C at a rate of 10l/sec/person. The tempered air will be heated to a constant 21 deg C unless the external air temperature is higher than 21 deg C, if it is higher then the tempered air will be supplied at the external air condition as the fresh air will not be cooled at the AHU.
I have used the "Fixed temperature air" box in the "design data" tab to fix the air temperature at 23.8 deg C as this is the maximum external design temperature that the air will be supplied to the rooms at. I have then obtained the results which show a Sensible heat gain of 25 kw and Latent gain of 9 kw.
As I am not sure if this is the way to input the fresh air requirement into CIBSE Gain, I have then undertaken a separate calculation by removing the "fixed temperature air" requirement and in the "Fresh air loads" box, I have ticked "fresh air introduced locally to rooms" and ticked fresh air type as 10l/sec/person, the results have now changed showing a Sensible heat gain of 81 kw and a Latent gain of 7.2kw.
Is there a reason that the Sensible Heat gain has shot up so significantly?
Thanks
Duncan Brown
Shane Regan | Principle Building Performance Support EngineerHevacomp to OpenBuildings - A complete guide for every user | Bentley Communities
Hi Kenny,
The reason for the large difference in results is that room 1-01 consists of two walls and two windows only. CIBSE Gain is an Admittance calculation so all surfaces and U-values will impact on the result, not only room volume. The following formulae from the manual should help to visualise this:
On the other hand, the fixed temperature air calculation is a stand-alone calculation that is subsequently added to overall loads.
To resolve the issue simply enter the remaining floor, ceiling, and partition surfaces into Room 1-01.
Answer Verified By: Kelly Chinappi
No problem Kenny. This article may also be of interest.
Please let us know should you need anything else.