It depends on the model. The Portable models, i12, FVI12, & iE Series do NOT come with any anti-freezing technology to help prevent freeze damage. This means that even if your i12 for example is installed indoors, it is not protected from freeze damage. The freeze damage can occur from cold air entering the internal chambers of the water heater via the venting. The 45H/HI, EL22/EL22i, 20H/20HI, and Builder Series all come with a built-in anti-freeze technology in the form of heating blocks. In order to ensure these blocks are doing their job, you must keep your water heater plugged in at all times. These blocks help prevent freeze damage to the heat exchanger, but is NOT a guarantee. Proper installation and maintenance must be followed to ensure that your water heater will last as long as possible.