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Laboratory experiments are performed to determine the frosting conditions for an enthalpy wheel in simulated extreme cold weather. Frosting of enthalpy wheels in energy recovery units is common when operating at very cold outdoor conditions. Energy transfer effectiveness decreases and pressure-drop across the wheel increases significantly as the frost layer grows in the wheel channels. Determining the frosting conditions of the enthalpy wheel thus helps the designers and manufacturers to implement defrosting or frost prevention measures in the wheel during operation. The laboratory test conditions for the wheel provide supply-inlet (outdoor) air temperature between -20 °C (-4 °F) and -38 °C (-36.4 °F) and relative humidity between 45% and 100%. The exhaust-inlet (indoor) air is delivered to the wheel at about 22 °C (71.6 °F) and either 30% or 40% relative humidity. The supply and exhaust airstreams flow in a counter-flow configuration. An air flow rate between 80 cfm (40 L/s) and 98 cfm (50 L/s) is maintained during the tests to provide almost equal mass flow rates (0.05 and 0.06 kg/s) on the supply and exhaust sides. Measurements during the tests include instantaneous air temperature, dew point temperature, relative humidity, flow rate, and pressure-drop across the wheel. Test results of air temperature and relative humidity on all sides of the wheel, sensible effectiveness, and pressure-drop between the supply-inlet and supply-outlet are then presented. The frosting initiation and frost growth in the wheel pores are determined based on the instantaneous rise of pressure-drop values between the supply-inlet and supply-outlet side airflow. The results indicate the limit of supply-inlet (outdoor) temperature and relative humidity for the no-frost operating condition of the wheel. To avoid frosting during an operation at an extreme cold outdoor condition, the supplyinlet air flow to the wheel may be pre-heated above the frosting temperature or the wheel speed decreased to reduce the effectiveness.