Sorry for the late response here. The full answer to this question is very long, so much so that there are going to be a full 3 hours of video tutorial content uploaded to our ondemand video platform in the next few weeks that are devoted primarily to this issue. Specifically, the workshop recording #4 Energy Modeling for Heating/Cooling Systems with Honeybee will be posted in the next few weeks and contain a lot information that helps explain the difference in energy use.
I’ll give a few brief bullet points that explain this now:
Separation of Ventilation and Loads - Probably the biggest reason why the FCUs save energy over VAV is the fact that FCU systems have a separation between the heating/cooling needs of the ventilation air and the heating/cooling needs of the zones. VAV systems, on the other hand, try to satisfy both the need for ventilation and the need to heat and cool the space entirely with the supply air. Naturally, some zones will have more of a cooling demand than others and so this leads to wasteful practices like cooling down all of the supply air of the VAV system to 12C centrally to meet the high cooling demand of a few zones and then re-heating up the air for all of the zones that don’t need such cold air (hence, the name “VAV with Reheat”). A lot of energy is wasted in the process of cooling down only to reheat it.
Method of Humidity Control - Even when there isn’t a zone with high cooling demand that drives down the central supply air temperature of the VAV, the VAV template that is built into Honeybee by default will still centrally cool down the air to 12C anyway because of concerns about high humidity in the supply air if the air is not cooled to a 12C dew point. This cooling down to wring out humidity also often happens in DOAS systems that come with FCU systems. However, because this DOAS air is only the minimum that is needed for ventilation (not the amount of hot/cold air needed to heat/cool the whole space), the energy spent in this respect is a lot less.
Energy Use of Fans Compared to Pumps - The cooling/heating delivered by VAV systems is done though air that is blown through ducts by fans. Conversely, most of the cooling/heating delivered by FCUs is done through water that is pumped to the FCUs though pipes. Water has a much higher heat capacity than air and, as a result, the energy needed to pump water around is a lot less than the energy needed to blow air.
Where the Templates Come From (and the default inclusion of heat recovery) - The default FCU template in Honeybee comes from the ASHRAE Advanced Energy Design Guides (AEDG) while the VAV template comes from ASHRAE Appendix G. The AEDG has a few other things built into it that give the FCU template in Honeybee a bit of an edge over the VAV one. Notably, the default FCU template in Honeybee has a heat recovery system that comes by default because this is a part of the AEDG by not necessarily a part of the ASHRAE Appendix G. So, if you want a more fair comparison between just the HVAC types, it might make more sense to add heat recovery into both Honeybee models using the AirDetails. With this said, I have never seen anyone put a heat recovery system on a VAV HVAC unless they were forced to do it by code and I think a lot of this has to do with the high expense of such a heat recovery system when you have the high air flow rates of a VAV.