A desiccant air conditioning system for treating an air stream entering a building space, the desiccant air conditioning system being switchable between operating in a warm weather operation mode and in a cold weather operation mode, comprising: a conditioner configured to expose the air stream to a liquid desiccant such that the liquid desiccant dehumidifies the air stream in the warm weather operation mode and humidifies the air stream in the cold weather operation mode, the conditioner including a plurality of plate structures arranged in a vertical orientation and spaced apart to permit the air stream to flow between the plate structures, each plate structure including a passage through which a heat transfer fluid can flow, each plate structure also having at least one surface across which the liquid desiccant can flow, each plate structure further comprising a membrane positioned proximate the at least one surface of the plate structure between the liquid desiccant and the air stream; a fan positioned at an outlet of the conditioner for applying negative pressure to the conditioner to draw the air stream through the conditioner; a regenerator connected to the conditioner for receiving the liquid desiccant from the conditioner, said regenerator causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode from a return air stream; a liquid desiccant loop for circulating the liquid desiccant between the conditioner and the regenerator; a reservoir coupled to the liquid desiccant loop for collecting liquid desiccant flowing from the conditioner; a vertical tube proximate a desiccant entry port at a plate structure in the conditioner coupled to the liquid desiccant loop to detect flow of liquid desiccant to the conditioner based on the height of the liquid desiccant in the vertical tube; an overflow tube coupling an upper end of the vertical tube to the reservoir to inhibit application of excessive pressure by the liquid desiccant on the membranes in the conditioner; a heat source or cold source system (32, 12) for transferring heat to the heat transfer fluid used in the conditioner in the cold weather operation mode, for receiving heat from the heat transfer fluid used in the conditioner in the warm weather operation mode, for transferring heat to the heat transfer fluid used in the regenerator in the warm weather operation mode, or for receiving heat from the heat transfer fluid used in the regenerator in the cold weather operation mode; a conditioner heat transfer fluid loop for circulating heat transfer fluid through the conditioner and exchanging heat with the heat source or cold source system (32, 12); and a regenerator heat transfer fluid loop for circulating heat transfer fluid through the regenerator and exchanging heat with the heat source or cold source system (32, 12). a conditioner configured to expose the air stream to a liquid desiccant such that the liquid desiccant dehumidifies the air stream in the warm weather operation mode and humidifies the air stream in the cold weather operation mode, the conditioner including a plurality of plate structures arranged in a vertical orientation and spaced apart to permit the air stream to flow between the plate structures, each plate structure including a passage through which a heat transfer fluid can flow, each plate structure also having at least one surface across which the liquid desiccant can flow, each plate structure further comprising a membrane positioned proximate the at least one surface of the plate structure between the liquid desiccant and the air stream; a fan positioned at an outlet of the conditioner for applying negative pressure to the conditioner to draw the air stream through the conditioner; a regenerator connected to the conditioner for receiving the liquid desiccant from the conditioner, said regenerator causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode from a return air stream; a liquid desiccant loop for circulating the liquid desiccant between the conditioner and the regenerator; a reservoir coupled to the liquid desiccant loop for collecting liquid desiccant flowing from the conditioner; a vertical tube proximate a desiccant entry port at a plate structure in the conditioner coupled to the liquid desiccant loop to detect flow of liquid desiccant to the conditioner based on the height of the liquid desiccant in the vertical tube; an overflow tube coupling an upper end of the vertical tube to the reservoir to inhibit application of excessive pressure by the liquid desiccant on the membranes in the conditioner; a heat source or cold source system (32, 12) for transferring heat to the heat transfer fluid used in the conditioner in the cold weather operation mode, for receiving heat from the heat transfer fluid used in the conditioner in the warm weather operation mode, for transferring heat to the heat transfer fluid used in the regenerator in the warm weather operation mode, or for receiving heat from the heat transfer fluid used in the regenerator in the cold weather operation mode; a conditioner heat transfer fluid loop for circulating heat transfer fluid through the conditioner and exchanging heat with the heat source or cold source system (32, 12); and a regenerator heat transfer fluid loop for circulating heat transfer fluid through the regenerator and exchanging heat with the heat source or cold source system (32, 12).