CIBSE Gain - Fresh Air Loads

Question

Good Afternoon, 
 I have a query relating to the fresh air loads input data for heat gain calculations. 
 Where there is fresh air introduced to a room unconditioned from outside, but with heating/cooling provided through local fan coil units, from Hevacomp Wiki https://communities.bentley.com/products/building/performance/w/building_performance__wiki/2345/calculations-using-fresh-air-loads-in-design-database , the correct option would appear to be 'Fresh air introduced locally to rooms'. When this method is selected however and the relevant air volume entered, the results show '0' in both sensible and latent columns under the fresh air loads. 
 Also at the bottom, the fresh air flow rate is shown as 0m3/s. 
 If I use the same figures but select 'Fresh air from central plant with conditioned air', then the gains (or losses) are shown in the results under latent and sensible fresh air loads. The results given would appear accurate for the situation mentioned when compared to hand calculations. So whilst this method gives me the results I'm looking for, it feels as though I'm not using the correct method? 
 
 Kind Regards, 
 Will

Shane Regan · Answer

Hi Will, 
 Fresh air introduced locally into the rooms is a room load and not a central plant air load. It is added to the reported room load, not the central plant as it would be the case with an all air system. 
 With this in mind you should see an increase/decrease in room load when comparing the two methods (convective and latent). 
 The following from the CIBSE Gain section of the Hevacomp manual should help to clarify: 
 &ldquo;If fresh air from central plant, with conditioned air is specified, there will be no additional load on the room. 
 When room loads are computed, building fresh air loads both sensible and latent are computed. If no additional fresh air is specified, building fresh air loads will be zero. If fresh air is introduced locally into room, building fresh air loads will be zero.&rdquo;