This type of heat recovery coil uses a perforated wheel made of special material that can transmit temperature and humidity between the two airstreams. These devices are most beneficial during extreme temperature conditions, since this is when there is the maximum temperature difference between the exhaust and fresh air streams. During very cold conditions, it is possible to cool the heat wheel to the point that ice can form when exposed to the warm moist exhaust air. To prevent this, the heat wheel changes to LL-T Control mode if there is a chance of forming ice. During LL-T Control mode, the heat transfer is limited to allow a minimum Heat Recovery Exhaust Air Temperature of 39 °F.