Kruse (1991) discusses the combination of recirculation and make-up air units.
Some projects may require an air handler to be able to function as a recirculation unit while treating a certain portion of outside air, essentially combining a recirculation unit with a make-up air unit. The common way to do this is to add a return air opening next to the outside air inlet of a make-up air unit. Add some dampers for air balancing and you are in business. The problem with this from an energy standpoint stems from two sources. First, this arrangement forces all of the recirculation air through all of those coils and filters, which caused the fan horsepower to be so high. It would be more energy efficient to direct this air through only those components necessary for its proper conditioning. Second, during humid weather the outside air is being dehumidified by cooling and re-heated to the required design condition. The recirculation air doesn't need all of that. An arrangement of a relatively energy efficient dual purpose air handler...for a specific project will depend, of course, on the ratio of outside air to return air as well as all of the usual design requirements.
[The energy consumption of each component should follow the rules described elsewhere is this guide.]
