Necessary and sufficient conditions for passifiability (feedback passivity) of infinite-dimensional linear systems are discussed. New sufficient conditions for output G-passifiability of nonsquare infinite-dimensional linear systems are established extending the existing results for finite-dimensional case. Application of the results is illustrated by example of heat conduction system. The proofs are based on Yakubovich-Kalman-Popov (Kalman-Yakubovich) lemma.