This type of heat recovery coil uses two coils, a circulation pump, and, optionally, a valve to move heat from the exhaust duct to the fresh air duct or vice versa. 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 glycol to the point that ice can form on the coil in the exhaust air duct. To prevent this, the glycol loop 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 Glycol Temperature of 39 °F.