Energy and Cost Analysis of Commercial HVAC Equipment: Offices and Hospitals

A study was made to determine annual energy use in several typical heating, ventilating, and air conditioning (HVAC) systems for two commercial building types: offices and hospitals. The study was part of a program at Oak Ridge National Laboratory to develop a model to predict energy use in the commercial sector of the United States. This study's objective is to define relationships between energy use and capital cost for commercial HVAC systems.

The NECAP computer program was used to analyze the HVAC systems' energy use. Both pre-embargo and current energy efficient HVAC system designs were modeled with several energy conservation options considered for each. These options included economizer cycles, exhaust air heat recovery, improved controls, double bundle exchangers, improved equipment efficiencies, partial solar heating, an Annual Cycle Energy System (ACES) heat storage system and a total energy system (onsite electricity generation with waste heat recovery). Building models used correspond to national averages for floor space and construction.

Results show that use of exhaust air heat recovery in the original design of a building can provide savings in both first cost and energy use. Savings depend upon weather (predominant savings are in heating energy) and ventilation rates. Results also show that use of electric resistance heat approximately doubles primary energy use when compared to a similar system using gas or oil heat. The use of gas for cooling can increase primary energy use by 40% over a similar system using electricity for cooling.

The "base" office building had a dual duct system with gas heat and electric cooling. Annual HVAC energy use was 326,000 Btu per square foot (primary, or as mined, energy) with approximately 2.3 Btu required to meet each Btu of space conditioning demand. Analysis showed that heating consumed 53% of the energy, cooling 37%, and fans 10%. By retrofitting this system with exhaust air heat recovery, improved controls and a double bundle exchanger, a 49% decrease in energy use could be obtained with a payout of less than three years. If an energy efficient variable volume ducting system was used in the original construction, a 44% reduction could be achieved with a payout of less than one year.

Similar results were found for the "base" hospital with a single zone/fan coil system using gas heat and electric cooling. Annual HVAC energy use was 423,000 Btu per square foot with approximately 1.8 Btu required to meet each Btu of demand. Analysis showed that heating consumed 57%, cooling 36%, and fans 7%. By retrofitting with exhaust air heat recovery, a 14% decrease in energy use could be achieved with a payout of less than four years.

Energy use estimates developed in this study are consistent with national energy use averages. The "base" office energy use was 32% higher than the national average and the best energy conservation case was 55% lower. The "base" hospital energy use was 12% higher than the national average and the best energy conservation case was 47% lower. One reason for the higher energy use for the base cases in this study was the exclusion of variations in operating strategies (such as night setback and reduced ventilation during non-occupied hours).