An earlier study advanced the concept of energy delivery efficiency (EDE) of HVAC systems by which energy mixing in large commercial buildings can be evaluated against an absolute energy standard, namely, the energy needs of the building if operated as a one-zone building. The study also pointed out that this EDE(1-zone) is an upper limit, and that the efficiency of the two-zone building, which we term EDE(2-zone), would constitute a more realistic energy standard in order to evaluate the efficiency of actual HVAC systems. The mathematical treatment, previously limited to dealing with sensible heat flows, has been expanded in this study to cover supply air latent effects as well as the influence of economizer operation. Appropriate equations are presented for the minimum heating and cooling energy use for ideal one-zone and two-zone buildings with and without economizer operation.

An energy standard based on a two-zone model may still be an unrealistic standard when comparing the energy efficiency of actual HVAC systems. This argument has a parallel in thermodynamics where the efficiency of an actual steam power plant is more realistically compared with the Rankine efficiency rather than with the Carnot efficiency. An energy standard based on ideal HVAC system performance would then be the logical basis of comparing actual system performance. Consequently, we have suggested an HVAC configuration that ensures the stipulated ventilation airflow rates, necessary for satisfactory IAQ, to each of the multiple zones of the building. Subsequently, we have defined the operation and control of such an ideal HVAC system, which will minimize thermal energy use while being subject to some of the fundamental restrictions of system operation and human comfort under which an actual HVAC system operates. Simulations, assuming typical two-zone building and system parameters, are used to illustrate the extent to which EDE(2-zone) and EDE of ideal HVAC systems differ from EDE(1-zone). Other studies have applied the concepts developed in this paper to year-long monitored data from several buildings and illustrated the usefulness of the EDE approach as a diagnostic tool to evaluate HVAC retrofit performance and Operation & Maintenance measures.

Units: SI