This report examines how the preheating, bypass and imbalance frost prevention methods impact the energy consumption and indoor environmental quality and aims to identify the most optimal one. Moreover, analysis on the influence of certain outdoor and indoor conditions on frost formation are also provided.

To accomplish this goal, the dynamic simulation software, BSim, has been used to find out the critical outdoor and corresponding indoor conditions for frost formation. The analysed geographical locations for outdoor climate are Southern Scandinavia, Central Europe, Southern Germany and Austria, and Scotland. As for indoor climate, building usage types for classrooms and offices are considered. The findings are that an indoor relative humidity range of 20-40 % with 22 ◦C extraction temperature corresponds to outdoor temperatures causing frost.

To determine the energy consumption of the frost prevention strategies, the frost limit temperature for the above mentioned indoor conditions has been identified by conducting experimental tests on a counterflow plate heat exchanger. Climatic chambers have been used to maintain the defined operation conditions. The outcome is that frost appears on the exhaust port at 0 ◦C exhaust temperature. The calculations revealed that there is no difference in the energy required to maintain the temperature above this limit among the investigated frost prevention methods.

In terms of impact on indoor environmental quality, indoor air quality and thermal comfort have been criteria for assessment. For the analysed climatic conditions, the indoor air quality is not compromised with any of the methods. However, imbalance would jeopardize the thermal climate due to risk of draught because of the higher infiltration.

Finally, it has been observed that indoor relative humidity affects the area where frost starts forming on the exhaust port and that with low humidity levels, where the dew point is below 0 ◦C, frost forms without any condensation.