A heat kit is an auxiliary electric heater installed inside an air handler to provide extra warmth during extreme cold. The MRCOOL Signature Series Heat Kits are essential for northern climates. They ensure that your home stays warm even when the heat pump efficiency drops in sub-zero temperatures. You can find these in the Accessories section for various air handlers. They are easy to install and provide instant emergency heat. This is a great add-on for the MRCOOL Signature Series systems.