Something else no one appears to have pointed out. You problem is
bigger than just making sure that you have a 20amp outlet on a 20amp
breaker with 12 gauge wiring. Since this one appliance requires
almost all the capacity of that circuit, it means you really can't have
any other loads on other outlets on that circuit. In other words, it
will likely need it's own circuit.

However, from the description, it appears that this model can be used
in a mode where only one heater is active at a time, meaning it will
work on a 15 amp circuit. If you can live with it in that mode, it
could funciton on a 20amp circuit with some room for other loads.
They also have a 15 amp model, which I'm guessing is basicly the same
thing, but hard wired wo that it only runs in that one heater at a time
mode.