###### Abstract
Rainwater is used for maize cultivation [13], lettuce and tomato production [14], olive gardens irrigation [15], lawn irrigation [12; 16], and various other crops and horticulture cultivation [15; 17]. Besides the urban application, rainwater harvesting is applied in rural areas. [PERSON] et al. [18] suggested that the use of reclaimed water might decrease domestic water consumption in the UK, and thus lower the demand for potable water. The review concludes that the main trends in this field of research are: water savings, water quality, stormwater control, and recently sustainable resource use. In rural areas rainwater might be re-used for toilet flushing or garden irrigation [18]. Conserving water is necessary due to it being constantly limited. Additionally, there is a growing popularity for indoor crops, which require the adaptation of various systems (water recovery systems, HVAC systems) depending on the type of crop and its location. The numerous analyses of rainwater recovery systems for various locations around the world show the importance of this topic. It is important to conduct this type of analysis because water recovery in various regions of the world is becoming more and more popular and it encourages entrepreneurs to implement new technologies. The performance and design of RWH systems have been investigated using different approaches, including water balance simulation analyses and mass curve analyses, probabilistic methods, and economic optimization [19]. The results indicate that the storage capacity of tanks cannot be standardized but is considerably influenced by local rainfall, catchment surface characteristics, and type of harvested rainwater management. Calculation modules make use of the various mass balance simulation schemes. Basic modeling approaches of \"yield after spillage\" (YAS) and \"yield before spillage\" (YBS) have been used in many models in the context of RWH. Research studies have been devoted to the selection of the appropriate simulation time step to enable a realistic representation of the results of mass balances. Several authors analyzed water-saving efficiency by modeling the RWH system at a range of time intervals (hourly, daily, and monthly) with the YAS and YBS operating rules tested. General conclusions support the use of the YAS operating algorithm for design purposes as it results in a more conservative estimate of water-saving efficiency [20]. Water efficiency modeling approaches within RWH tools have been widely shown to give accurate representations when daily time-step intervals are used [21; 22]. The aim of this study is to determine if the rainwater harvesting system itself, and in combination with water recovery from the exhaust air by a cross-flow exchanger operating in the air-conditioning system is sufficient to cover the water demand for hydroponic lettuce cultivation located in Wroclaw (Poland). The analysis was performed on the basis of the recorded rainfall in Wroclaw in 2000-2019, and the hourly meteorological data (the temperature, the humidity, and the atmospheric pressure) recorded in Wroclaw in 2012-2019. The calculations of the rainwater recovery system were carried out, taking into account the selection of the reservoir for the collected water, as well as, the cost-benefit analysis for other alternative tank capacities. This study determines the share of water which can be covered by the rainwater harvesting system, and in association with the exhaust air water vapor recovery system over the years, in addressing the demand for irrigation of lettuce. The proposed system is an auxiliary system that relieves the water supply network or other water recovery systems.