A method of controlling an economizer in a ventilation system, comprising: measuring a moisture related property of a first input air source to determine if conditioning of the first air source to a desired output air temperature results in a wet-coil or a dry-coil condition; measuring a moisture related property of a second input air source to determine if conditioning of the second air source to the desired output air temperature results in a wet-coil or a dry-coil condition; determining a measure indicative of energy associated with the conditioning of the first input air source for the wet-coil condition and for the dry-coil condition; determining a measure indicative of energy associated with the conditioning of the second input air source for the wet-coil condition and for the dry-coil condition, wherein the measure indicative of energy for the first and second input air sources includes sensible energy change associated with the conditioning of the first and second air sources when the dry-coil condition is determined and, includes sensible and latent energy change associated with the conditioning of the first and second air sources when the wet-coil condition is determined, wherein the measure indicative of energy for the first and second air sources has the same units for both the wet-coil condition and the dry-coil condition; comparing the measure indicative of energy associated with the conditioning of the first air source and the measure indicative of energy associated with the conditioning of the second air source to determine which of the first and second input air sources applied to an air handler requires the least amount of energy for the conditioning; and selectively enabling the economizer based upon the comparison of which of the first and second input air sources applied to an air handler requires the least amount of energy for the conditioning. measuring a moisture related property of a first input air source to determine if conditioning of the first air source to a desired output air temperature results in a wet-coil or a dry-coil condition; measuring a moisture related property of a second input air source to determine if conditioning of the second air source to the desired output air temperature results in a wet-coil or a dry-coil condition; determining a measure indicative of energy associated with the conditioning of the first input air source for the wet-coil condition and for the dry-coil condition; determining a measure indicative of energy associated with the conditioning of the second input air source for the wet-coil condition and for the dry-coil condition, wherein the measure indicative of energy for the first and second input air sources includes sensible energy change associated with the conditioning of the first and second air sources when the dry-coil condition is determined and, includes sensible and latent energy change associated with the conditioning of the first and second air sources when the wet-coil condition is determined, wherein the measure indicative of energy for the first and second air sources has the same units for both the wet-coil condition and the dry-coil condition; comparing the measure indicative of energy associated with the conditioning of the first air source and the measure indicative of energy associated with the conditioning of the second air source to determine which of the first and second input air sources applied to an air handler requires the least amount of energy for the conditioning; and selectively enabling the economizer based upon the comparison of which of the first and second input air sources applied to an air handler requires the least amount of energy for the conditioning.