You MUST vent this to the outside, especially if, as your screen name indicates, you are in a northern climate. I wold not necessarily call an HVAC contractor for this, but rather a good carpenter. Between floors, you will need to fish a flexible duct through the joist space and penetrate through the rim joist. Code requires that you put this on it's own circuit breaker, and frankly, it draws enough current that you really DO need that second circuit.
Initially, in old construction, I installed mine on the same breaker as the bathroom convenience outlet. It worked fine in the summer, but in winter with the heater turned on, the breaker tripped the first time someone plugged in the hair dryer and curling iron. I ran a new circuit the very next weekend. Not sure, but I think code requires a 20 Amp circuit.
This heater does a fine job of heating, and evacuating humidity from an otherwise unheated, 6x8 bathroom with a tile floor and wainscot finish, even when the outdoor temperature is well below zero (we live in the mountains of Colorado), but be aware that it is somewhat noisy.