A general reduction in total energy consumption is needed, due to the increasing concerns about climate change caused by CO2-emmissions from fossil fuels. In 2004, the building sector accounted for 40% of the total energy consumption in the EU and the US and therefore must play a crucial role in reducing CO2-emmissions. Over the last decade, initiatives have been taken to reduce its energy consumption e.g. by the European Union, national governments or NGOs. The initiatives have mostly focused on improving the thermal properties of the building envelope to reduce heat losses. Building services, including ventilation, therefore now represent a larger part of the total energy consumption. Mechanical ventilation has been the most widely used principle of ventilation over the last 50 years, but the conventional system design needs revising to meet future energy requirements. The increase in the use of natural and hybrid ventilation systems is intended to reduce the energy consumption for ventilation, specifically the power consumption of fans in mechanical systems, but these alternative systems have other flaws, e.g. higher ventilation heat loss. Meanwhile, little has been done to improve the performance of mechanical ventilation systems. The power consumption of mechanical ventilation depends on the flow rate, fan efficiency and pressure loss in the system. This thesis examines the options and develops a concept and components for the design of low-pressure mechanical ventilation. The hypothesis is that A new type of low-pressure mechanical ventilation with improved indoor environment and energy performance can be developed, by optimizing and redesigning each constituent element of conventional mechanical ventilation systems with respect to pressure and the development of new low-pressure components. The goal was to develop a mechanical system with an SFP-value of 0.5 kJ/m3 and a heat recovery efficiency of 85% that can meet current indoor environment requirements without discomfort in terms of thermal, acoustic and draught issues. The concept was developed for a temperate climate, such as Denmark’s, and the objective was to provide comfort ventilation all year round and avoid overheating through increased ventilation and night cooling. This would mean that only one system needs to be installed and mechanical cooling is unnecessary. The potential to reduce pressure losses was examined for the main constituting parts of a mechanical ventilation system and the parts that are critical for the hypothesis were identified. The system proposed consists of electrostatic precipitators for filtration, an “oversized” heat exchanger to reduce pressure loss and improve heat recovery efficiency, diffuse ceiling ventilation for air distribution, and a static pressure reset control system to control the airflow to the individual rooms. The investigation of the hypothesis is reported in four papers appended to the thesis, and the thesis summarises the results and adds further discussion and an extensive study of the literature. Paper I introduces the concept and its performance is evaluated through simulations of a system designed for a test-case building. All the components were designed to minimize pressure losses and therefore the fan power needed to operate the system. The total pressure loss was 30-75 Pa depending on the operating conditions. The annual average specific fan power was 0.33 kJ/m3 of airflow rate. This corresponds to 10-15% of the power consumption for conventional mechanical ventilation systems, enabling the system to help meet future energy requirements in buildings. Paper II describes the development of a static pressure reset control system using a new type of flow control damper. The performance of the control system was examined using a test set-up duct system. Measurements showed that the developed control algorithm and the flow dampers were able to regulate the airflow accurately down to 5 Pa. Paper III reports on an investigation into the performance of diffuse ceiling ventilation in a school classroom. The investigation included tracer gas, air velocity and temperature measurements and showed perfect mixing of the air in the room without any discomfort issues. The diffuse ceiling ventilation was part of a low-pressure mechanical system that included an “oversized” air handling unit and duct system and a new type of flow damper to regulate the demand-controlled airflow. The performance of the system in terms of indoor environment, pressure loss, energy consumption and life cycle cost are reported in Paper IV. The system was able to provide an acceptable indoor environment and the annual average SFP-value of the system was 0.61 J/m3. The life-cycle cost investigation showed that some components (measures) were cost-effective but the total cost of the system as a whole was higher than the reference system. In theory, it is possible to fulfil the claims of the hypothesis and the goals stated, but it was not possible to reach that level in practice mainly due to limitations in the conventional solutions used in the pilot systems. However, the concept and the solutions developed are believed to be a contribution to making the design of low-pressure mechanical ventilation systems realistic in the future.