The MXZ-2B20NA1 will heat down to +5 degrees F, though its output will decrease as temperatures reach freezing or below. If you need something that heats down to a lower outdoor temperature, Mitsubishi offers hyper-heating units that work down to -13 degrees F.
The Mitsubishi MXZ-2C20NAHZ is the equivalent 20K hyper-heating condenser.
You can also view all Mitsubishi Hyper-Heating Multi-Zone Systems