Assuming all is connected properly and your water heater is also heated by your engine, let the engine heat warm the water while leaving the electric power turned off if your concerned about the heater element. This should allow for you to test for water in the tank prior to turning on the electric. Remembering my recent tank installation last year, the most important consideration is to make sure the in / out connection and tank orientation is correct so the air that will be at the top of the tank will be forced out during normal operation