My water heater is above the engine. I eliminate trapped air by disconnecting the top hose and pouring anti-freeze into the hose until antifreeze comes out of the water heater fitting. When that is happens I put a thumb over the heater fitting leaving a very small gap at the top of the fitting and continue pouring coolant into the hose until I am getting no more air then put the hose on. Never a problem.

An alternative method is to use a vacuum pump and draw a cacuum on the heater then let it draw antifreeze in