The installation of technical solutions for heat recovery from the ventilation air is a very important step, both with respect to improving the indoor climate and as an energy-saving measure. Ventilation without heat recovery use a substantial part of an entire building's heating use, especially if the building is a low energy building. To realize a large implementation of heat recovery property owners must increase their knowledge about how these systems work and they must be able to rely on them functioning properly. The function and performance of a ventilation system when it comes to heat recovery can be a problem as soon as temperatures drop far below zero. When freezing occurs some form of defrosting is required and this can lead to indoor climate problems and to little or no heat recovery being obtained, which will reduce the energy efficiency and, at the same time, may compromise moisture safety. The goal of this project is to increase knowledge about how heat recovery systems for residential buildings with respect to defrosting and frost protection measures in heat exchangers. The knowledge can thereafter be used to develop effective defrosting strategies and frost protection measures so that the energy use and power requirements are minimized. Measurements were performed for heat recovery systems placed in dwellings in north Sweden to investigate occurrence of frost formation. The paper will present these measurements and some comparing simulations which can serve as a basis for further development of good defrosting strategies.