Compare constant air volume terminal reheat system with four-pipe fan coil system in terms of their (a)running cost, (b)capital cost and (c)their ability to achieve design conditions of a building.
The comparison in terms currency is dependent on building characteristics and climate.

In general terms parameters to be looked at are:

    Cooling loads (heat gains) and diversity

    Climate

    Outdoor air enthalpy vs. prevailing load conditions

    Energy cost rates

To meet the design conditions, the constant air volume terminal reheat system (CAV_TR) requires significant air flow rates especially on higher levels of cooling loads ( commonly 40...80 W / sqm or even higher). This implies that investment and running cost structures of studied systems differ remarkably from each other especially in high load levels.

Variable air volume system with terminal reheat (VAV_TR) would meet the design conditions and energy efficiencies with smaller margin than CAV_TR, when compared with four pipe fan coil system (4P_FC) especially in buildings with long operation times.

On the the other hand, the running costs are highly dependent on load variations and their diversity in same building sectors (zones). The energy cost of reheating (electricity or LTHW) depends on the load variations (orientation, solar shading, occupancy/load profiles, individual adjustment of set values) by control zone ( room, group of rooms, etc). Whereas the primary cooling/ heating energy consumption of CAV_TR might be reasonably low in mild climates.

The air handling units (AHU) , ductwork and space cost of CAV_TR become significant,whereas the terminal unit & piping costs remain moderate. Looking at running costs, the fan power consumption is high in buildings with high cooling loads.

4P_FC system investment cost structure contains moderate air handling unit & ductwork costs whereas LTHW and chilled water piping, pumps and terminal units represent significant cost levels. If more hygienic operation is desired, condensation should be avoided, which leads to even higher piping and terminal unit costs.

The total fan power consumption (AHU and FC) of non-condensating and even condensating 4P_FC system lowers the performance ratio (internal load / power input) to reasonably low level Reference : Knight & Dunn; A/C Energy Efficiency in UK Office Environments /IEECB 2002

On the contrary, in the same study chilled ceilings (passive & active) were discovered to have significantly higher performance ratios. Chilled ceilings and cooled beams are air- water air conditioning systems with CAV.

The requirements for indoor environment (thermal and acoustic conditions) should certainly be added to the parameter list in this kind of comparison. This kind of multi-parameter comparison is interesting, and reveals differences between air condtioning systems, which should be considered in the triangle investment costs-indoor environment-running costs.