A ventilation system for a group of cabins of a ship, comprising a group of fan coil systems each suitable for a cabin of the group of cabins, each fan coil system comprising a fan coil device adapted to draw cabin air of the cabin into the fan coil device, cool the cabin air, mix the cabin air with fresh air to be received from outside the cabin and conduct the cooled and mixed cabin air back into the cabin, and a first control valve capable of moving to a first position and a second position, the first and second positions being for regulating inflow of fresh air into the fan coil device, wherein each first control valve is configured to receive a first control signal and capable to move to the first and second positions in accordance with the first control signal, wherein the ventilation system further comprises an air handling unit comprising a supply duct system for supplying the fresh air to the group of fan coil systems, characterized in that the ventilation system further comprises a second control valve capable of moving to varying positions for regulating the supply of the fresh air through the supply duct system, wherein the second control valve is configured to receive a second control signal and capable to move to a predetermined position in accordance with the second control signal, and an automation system, being connected to the first control valve of each fan coil system and the second control valve and adapted to generate the first and second control signals and to transmit the first control signal to the first control valve of each fan coil system and the second control signal to the second control valve, wherein the ventilation system, the automation system or the group of the fan coil systems is capable to generate indications representing whether each cabin of the group of cabins is occupied or unoccupied, wherein the indication is the first control signal or the automation system is adapted to generate the first control signal based on the indication, and wherein the automation system has means to form data representing occupancy rate of the group of cabins based on a group of the indications representing whether each cabin of the group of cabins is occupied or unoccupied. a fan coil device adapted to draw cabin air of the cabin into the fan coil device, cool the cabin air, mix the cabin air with fresh air to be received from outside the cabin and conduct the cooled and mixed cabin air back into the cabin, and a first control valve capable of moving to a first position and a second position, the first and second positions being for regulating inflow of fresh air into the fan coil device, wherein each first control valve is configured to receive a first control signal and capable to move to the first and second positions in accordance with the first control signal, wherein the ventilation system further comprises an air handling unit comprising a supply duct system for supplying the fresh air to the group of fan coil systems, characterized in that the ventilation system further comprises a second control valve capable of moving to varying positions for regulating the supply of the fresh air through the supply duct system, wherein the second control valve is configured to receive a second control signal and capable to move to a predetermined position in accordance with the second control signal, and an automation system, being connected to the first control valve of each fan coil system and the second control valve and adapted to generate the first and second control signals and to transmit the first control signal to the first control valve of each fan coil system and the second control signal to the second control valve, wherein the ventilation system, the automation system or the group of the fan coil systems is capable to generate indications representing whether each cabin of the group of cabins is occupied or unoccupied, wherein the indication is the first control signal or the automation system is adapted to generate the first control signal based on the indication, and wherein the automation system has means to form data representing occupancy rate of the group of cabins based on a group of the indications representing whether each cabin of the group of cabins is occupied or unoccupied.