If you have a fireplace or wood burning stove you should have it cleaned by a competent professional every year. Soot and other combustible materials can build up in your chimney over time. A fire can burn in your chimney for hours before you can detect it from inside your house. If you need to use a portable heater be very careful not to place anything combustible near it. They should always be kept in plain sight.
Electricity Be careful with electricity.
It is inherently dangerous. Heat is produced every time an electric current flows through a wire. Usually the heat is dissipated properly. But certain uses of electricity can be predicted to cause trouble. Extension cords are not designed for permanent use.