sorry, I have no experience with that particular heater. 7500 watts = 25,500 BTUH so if it's only putting out 17,000 BTUH then it's wired as a 5400 watt heater.
1 watt = 3.412 BTUH
sounds like you'll need to contact the manufacturer or scour the 'net for a wiring diagram. Pretty cheezy if...