There are 2 ways of modelling HVAC systems in DesignBuilder:

1. Simple in which the system is defined using basic HVAC data descriptions and heating and cooling loads are calculated using the idealised EnergyPlus "Ideal Loads" system. Heating and cooling energy consumption is calculated within DesignBuilder from loads reported by EnergyPlus using system efficiency factors.

2. Detailed where the individual components are defined and connected together. See:

In both HVAC modes, generic default HVAC data can be loaded from template.  DesignBuilder calculates energy consumption and CO2 emissions in a post process using seasonal boiler efficiency and chiller COP data. You can select the HVAC mode from the Model Options dialog:

http://www.designbuilder.co.uk/helpv6.0/#_HVAC_model_detail.htm