The assembly includes a boil-off gas line that carries storage tank boil-off gas to a condenser that uses condensing liquid from the liquid send-out line to condense the gas. A level control valve on the condensing liquid line actively controls the flow of condensing liquid based on the liquid level in the condenser. A check valve prevents liquid from the send-out line from flowing into the condenser through the condensate line that discharges condensate from the condenser to the send-out line.