The concern for energy conservation has led to the development and use of heat recovery systems which reclaim the building internal heat before it is discarded in the exhaust air. On the other hand, economizer cycles have been widely used for many years in a variety of types of HVAC systems. Economizer cycles are widely accepted as a means to reduce operating time for chilling equipment when cool outside air is available. It has been suggested that heat reclaim systems should not be used in conjunction with an HVAC system which incorporates an economizer cycle because the economizer operation would result in heat being exhausted which might have been recovered. Others suggest that the economizer cycle can be used economically in a heat recovery system if properly controlled to maintain an overall building heat balance. This study looks at potential energy savings of such combined systems with particular emphasis on the effects of the solar load (amount of glass) and the internal load level (lights, people, appliances, etc.). For systems without thermal storage, annual energy savings of up to 60 percent are predicted with the use of heat reclaim systems in conjunction with economizers when the heat reclaim has priority. These results strongly demonstrate the necessity of complete engineering evaluations if proper selection and operation of combined heat recovery and economizer cycles are to be obtained. This paper includes the basic methodology for making such evaluations.

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