Scotland PartL Calculation Room Ventilation

Question

Hello, 
 I have a building which has a zone with local split AC unit providing the main heating and cooling with a centralised ducted air handling unit which provides tempered fresh air. The air handling unit has heat recovery and the capacity to heat or cool the air to the room set point. 
 The building regulations state that a central balanced mechanical ventilation system with heating and cooling should achieve a SFP of 1.6W/l/s

But when I try to input the ventilation system into the room ventilation the SBEM calculation assumes that the supply SPF is a zonal supply and extract system and assumes that the exhaust system is a zonal extract system rather than a central balanced mechanical ventilation system.

How should I input a central balanced mechanical ventilation system with heat recovery and heating and cooling (to temper the fresh air) into the SBEM calcualtion when it is not the primary source of heating and cooling for the zone? 
 Do i ignore the room exhaust and input the combined supply and extract SPF into the supply SPF or should I ignor the complaince report where is flags up the exhaust SFP as too high? 
 Many thanks, 
 
 Ross

Shane Regan · Accepted Answer

Hi Ross, 
 I would not encourage ignoring the SFP figures flagged by the Compliance report and you are right to seek clarification about this. The System Type categories you refer to are largely standardised systems as set out in the Non-Domestic Building Services Compliance Guide so it may even be the case that your HVAC system is outside the scope of the SBEM calculation. If so, then the guidance offered by BRE about entering non-standard HVAC systems here may be of assistance. 
 However as the Part L calculation is performed by BRE's SBEM program (further info here ) it can be very difficult for us to interpret results such as this. Often, the best resource available to us is simply the SBEM / iSBEM manuals . With this in mind your accreditation scheme provider would be much better placed to offer guidance on the appropriate design.