For air-to-air heat exchangers, you will likely want to use one of two objects in EnergyPlus:

• HeatExchanger:AirToAir:FlatPlate - has input field that sets ratio of primary air to secondary air (h * A)
• HeatExchanger:AirToAir:SensibleAndLatent - has input field that sets sensible effectiveness and latent effectiveness at different heating and cooling air flows

You can read more about these two AAHX objects in this post.