It may be necessary when using the PartL program to enter 'non-standard' HVAC systems. BRE have released the following information as part of their FAQ.
Not all HVAC system types are available through the SBEM calculation engine. All of the systems that are currently considered are available in the Bentley Hevacomp PartL program. When more complicated - or systems that are just not available in the SBEM calculation engine - are required it is possible to enter a 'workaround' that will satisfy your and Building Controls requirements.
"If your system is not available within SBEM it would be acceptable to use a 'workaround' in order to produce results using the existing systems in SBEM. However, decisions made in this process must be justified to Building Control.
When using a 'workaround' in SBEM, the aim is to calculate the CO² output of this system in your building. To do this, it is acceptable, where a system is not included in SBEM, to substitute it with another system. If a heating and cooling system exists in the building, a heating and cooling system must be chosen in order to include cooling in the calculations.
Generally a system with the same fuel type should be chosen, if this is not possible, it may be acceptable to adjust the system seasonal efficiency in order to achieve the same carbon intensity as in the actual system. For example, where a system uses gas for FCU heating coils and an electric heat pump for an associated warm air ventilation system, it could be acceptable to select a FCU system with an electric heat pump and then increase the system COP in order to reduce the carbon intensity of the system to account for some of the heating being provided by gas."
(from the BRE website)
Bentley Hevacomp technical support will not be able to assist with defining 'workarounds'. Please contact your accreditation scheme provider for assistance with these techniques.