Sounds like you had bad wiring in your house, and once you put a constant 80%+ amp load on the circuit the wiring heated up and eventually caught.
Just because it ran for a week doesn't mean your OK, you need to make sure you don't exceed 70-80% of the circuit amperage.
I've seen many houses in my area catch on fire from electric heaters. Those big ones pull 8A - 15A alone from the socket and easily overloads it.