CAUTION: For your safety, DO NOT attempt repair of electrical wiring, controls, heating elements, or other safety devices. Refer repairs to qualified service personnel.
Possible Cause: Cold Water supply is off.
What To Do: Check all valves to ensure the system can be filled with water.
Possible Cause: Combined non-return/safety valve.
What To Do: The combined non-return/safety valve has a check valve integrated. If installed backward, flow can be prevented from filling the water heater.
This answer is applicable for the following models: GT50C10BAM, GT40C10BAM, GT25C10BAM.