Fresh Air Having Large Effect on CIBSE Gain Results

Hi, I am carrying out cooling calculations for an open plan office with FCU's. When using CIBSE Gain to calculate the room cooling loads I am selecting 'conditioned primary fresh air' as I have air at 18deg C from a central AHU with no humidification.

When I compare the results for one of the internal zones to my manual calculations the loads are significantly lower. 

I have checked the model and all input parameters are as expected, the CIBSE Gain result is just lower than expected.

I would like to know how HevaComp treats fresh air introduced with this option.

The CIBSE Gain results for the perimeter zones with solar gain are close to the manual calcs

Any assistance would be much appreciated. 

  • Hi Simon,

    You can find information on the calculations in the Help for the CIBSE Gain program. This explains how the Fresh Air is treated.
    It's difficult to give a specific reason without seeing both hand calc and your project but my initial thought would be to check your Plant On and Off Time in the Building Data section. A Plant On and Off of 1-24 will give a lower result than if you run your plant during the occupied period only.

    Best Regards

    Steve Brown | Technical Support Manager