Wood stoves- how much of a rise do I need before going horizontal?

I want to use a wood stove instead of my fireplace insert (which doesn't put out any heat when the power is out). A wood stove sales person said I needed 3 feet of vertical rise before going horizontally into my chimney with the pipe. I have the feeling she just wanted to sell me a fan-less insert for an extra $2500 over the price of a stove. Any chance of running the pipe at an angle until it's in the fireplace chimney instead of straight up, then horizontal?