According to a decision of the European Commission, measures are to be taken to reduce the use of energy in the EU. The goal is to reduce it by 20 % compared to the current use. This shall be done to the year 2020 (European Commission, 2011). One industry that use large amounts of energy is the construction of buildings which account for almost a third of the energy use (Brogren, 2012). The major part of the energy that is used in the construction industry is not used when the buildings are built, but rather during the rest of their subsequent lifetime. There is a great potential to save energy by reducing the energy that is used to maintain a satisfactory indoor climate. Recovery of excess heat and excess cold is a solution that the European Commission think has the biggest potential to reduce the total energy consumption. The most common system used for energy recovery is air to air heat exchangers connected with the supply air and the exhaust air. For different reasons it is not possible to use this kind of system in several buildings. If that is the case there is a possibility to use a liquid coupled recovery system instead. If an additional source of excess heat or excess cooling exist within the building, or nearby, it is also possible to connect this to the system which would increase the ability to save energy even more. The purpose of this thesis has been to develop a tool for energy calculations in liquid-coupled recovery systems. This tool has been developed in the program IDA ICE (used for energy calculations) and has made it possible to perform dynamic simulations in this kind of system over the timeframe of a whole year and with a very short calculation time. The tool is flexible in terms of its components and system design so it can be used for several different types of projects. Everything from simple systems with fixed brine flow with only one supply air and exhaust air unit to systems with several units, various types of control possibilities and an addition of excess heat from, for example, a room containing computer servers. The tool that has been developed has been verified and used to calculate the potential to save energy in a system that is installed at the Ångström laboratory in Uppsala. The tool has shown that with the kind of control and the conditions that currently exist at the laboratory the energy consumption could be reduced by 444 MWh which in this case almost is 50 % of the current energy consumption. Besides the recovery system in Ångström two more systems have been investigated, a server room for The Royal Institute of Technology and the server halls that Facebook is building near Luleå town. The investigation shows that there exist very large amounts of heat that is possible to recover in buildings that include server rooms and that the installed recovery systems, if there are any, in many cases could be improved. Besides constructing recovery systems that recover heat or cold in buildings it is also possible to build this kind of system that recover heat or cold between buildings in the same area. The tool can also be used to investigate how such a system should work in order to minimize the use of energy as much as possible. A solution where heat and cold is recovered between multiple buildings is a solution that probably will be very interesting in the future, which means that this tool could come in handy.