A simple life-cycle cost model can be analyzed with a computer spreadsheet. The model can account for the marginal construction cost differences for Variable Air Volume (VAV) systems and other EEMs in comparison to a base Constant Volume (CV) system. Typically, an HVAC system is assumed to condition 100 percent outside air to a supply temperature between 55°F (13°C) and 65°F (18.3°C), depending upon climate.
According to Lacey (1993), each laboratory system needs to be evaluated individually. However, it is instructive to review some typical differences between VAV and CV systems, which Lacey describes as follows:
VAV systems are most economical when operated at low diversity factors. At a diversity factor of 75%, the fixed-cost of a VAV system usually surpasses the cost of a CV system with or without heat recovery.
The VAV system operational cost is generally insensitive to heating degree-days. The annual energy cost of a CV system increases rapidly as the climate becomes more severe. In mild climates, the CV system can offer lower life-cycle cost than a CV system with heat recovery.
The life-cycle cost of a CV system is more sensitive to heating fuel cost; only peak heat gain and hood airflow volume are more important factors. CV systems that employ heat recovery are less sensitive to heating fuel costs than CV systems without heat recovery. VAV systems are less sensitive to both heating and cooling costs than CV systems.
Generally, a CV system has the best life-cycle cost during five years. The VAV equipment usually produces the lowest life-cycle cost during fifteen years and less especially in new installations.
The first-cost of control systems has a relatively minor impact on the life-cycle cost of a VAV system.
Life-cycle operating costs of CV and VAV systems are not highly sensitive to the average heat gain that is part of operating laboratories. [Lacey, 1993]
Life-cycle cost model study—VAV vs. CV
