This master thesis examines the energy efficiency of preheating outdoor air in ventilation systems by using geothermal heat from boreholes. Energy efficiency in this context only relates to minimise the energy consumption and to minimise the peak power consumption during cold days. Preheating the outdoor air before it enters the heat exchanger is a techinque that is used to avoid frost formation in the heat exhanger. The aim of preheating the outdoor air is to reduce the energy consumption in the buidling and to reduce the peak power consumption during cold days. In this project a buliding in Töfsingdalen, Stockholm, is investigated. In one part of the building the outdoor air is preheated while the air in another part of the building is not. The investigation is partly based on an analysis of measured temperaures in the building’s ventilation system during 2017. The results of the analysis show that the heat exchanger in the air-handling unit with preheating has not been defrosted during the investigated period. In this thesis, models are also constructed to simulate the system. The models are created in the simulation programme IDA ICE, wich is a simulation tool for bulidings. The simulations show that the total energy consumption is 1.2 kWh/m2Atemp, year higher and the maximal power consumption is 18 kW lower for the system that preheats the outdoor air compared to the system without preheating. In addition to this, the conclusion of this report is that the energy efficiency of preheating the outdoor air depends on the type of heat exchanger. A heat exchanger with high efficiency results in a lower peak power consumption than a heat exchanger wth a lower efficiency. Futhermore, the results show that geothermal preheating is better from an energy perspective when heat exchangers with moderate as opposed to high efficiencies are used. The energy efficiency of preheating outdoor air in ventilation systems also depends on the outdoor climate and the control strategy of the preheating. In this report, the control strategy for the preheating system in Töfsingdalen has been optimised in order to reduce the energy consumption in the building.

En jämförelsestudie har genomförts mellan två värmesystem. Analysen har genomförts med ett flerbostadshus som referensfasighet. Fastigheten är lokaliserad i Södertälje kommun. Analysen går ut på att bestämma vilket värmesystem som är fördelaktigt vid renovering av äldre flerbostadshus med avseende på energi och kostnad. De systemen som har behandlats är ett centralt värmesystem med motströmsvärmeväxlare och ett centralt värmesystem med frånluftsvärmepump. Det centrala värmesystemet (Eq aggregat) är ett centralt från- och tilluftssystem med återvinning (FTX system). Systemet använder en motströmsvärmeväxlare för överföring av värmeenergi mellan från- och tilluften. Det centrala värmesystemet (Energi well) är ett frånluftssystem (FX system) med en ny teknisk lösning. Systemet återvinner värme ur frånluften med hjälp av kondenserande frånluftsvärmpumpar. Värmepumparna finns i en frånluftskammare på vindsvåningen. Den återvunna värmen förs sedan vidare till undercentralen för att värma upp varmvattnet. Analysen har genomförts med hjälp av teoretiska energi- och kostnadsberäkningar, intervjuer och faktainsamlingar.  Energiberäkningarna har bestått av en energibalansberäkning för att få fram den köpta energiförbrukningen. Kostnadsberäkningar har bestått av en livscykelkostnads kalkyl och en kostnads beräkning per producerad värmeenergi. De resulterande värdena för båda värmesystemen har sedan jämförts med varandra. Resultatet visade att värmesystemet Energy well var mest fördelaktig ur både energi och kostnad perspektiv. Den årliga köpta energiförbrukningen var                          lägre än värmesystemet (Eq aggregat) med en motströmsvärmeväxlare. Driftkostnaden var  lägre än Eq aggregat, installations-kostnaderna var  lägre än Eq aggregat och slutlig var livscykelkostnaden  lägre än värme-systemet (Eq aggregat) med motströmsvärmeväxlare. Men underhållskostnaden var  högre än värmesystemet Eq aggregat. Slutsatserna för de teoretiska undersökningarna gav bättre värden för värmesystemet Energy well. Vilket innebär att Energy well är mest fördelaktig utifrån energi- och kostnads perspektiv. Däremot ger värmesystemet med motströmsvärmeväxlare en lägre risk att ett driftfel inträffar i verkligheten. / A comparing study between two different heating systems has been accomplished. The analysis has been conducted with an apartments building as a reference project. The building is located in the municipality of Södertälje. The goal of the analysis is to determine which of the heating system is beneficial for older apartment buildings in terms of energy and cost. The systems that have been treated are a central heating system with a counterflow heat exchanger and a central heating system with an exhaust air heat pump. The central heating system (Eq unit) is a central exhaust- and supply air system with heat recovery. The system uses a counterflow heat exchanger to transfer the heat energy between the exhaust- and supply air. The central heating system (Energy well) is the latest technical solution of a central exhaust air system. The system recovers heat from exhaust air by condensing exhaust air heat pumps. The heat pumps are in an exhaust air chamber and it´s placed on the attic floor. The recovered heat energy sends then to the mechanical room to heat up the water system. The analysis has been accomplished by using theoretical energy- and cost calculations, interviews and data collection. The energy calculations are based on an energy balance equation to determine the bought energy consumption. The cost calculation is based on a life cycle cost equation and a cost equation per produced heat energy. The results of both heating systems have been compared with each other. The results showed that the heating system Energy well was most beneficial in both energy and cost perspective. The annual consumption of bought energy was                          lower each year than the heating system (Eq unit) with a counterflow heat exchanger. The operating costs of the system were  lower than Eq unit, the installation costs were  lower than Eq unit and final was the life cycle cost  less than the heating system (Eq unit) with counterflow heat exchanger. But the service cost was  higher than the heating system Eq unit. The conclusion of the theoretical investigations gave better values ​​for the heating system Energy well. This means than Energy well is most beneficial from the energy and cost perspectives. Contrariwise has the heating system with counterflow heat exchanger a lower risk of an operational failure to occur in reality.