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Hello, my name is ***** ***** I'm here to help you out with your problem. If your pilot is lighting but the gas valve is not opening, we need to check to ensure there are no tripped safety switches. If all saftey switches are okay, first would be to replace the thermocouple. We can verify the thermocouple is producing Millivolts first with a Millivolt meter if you have one available - if not, thermocouples are inexpensive. Follow the installation instructions for the thermocouple as to not tighten the nut at the gas valve too tight.
I'm having trouble locating documentation on your heater - is there a specific model number or just RC90. Also, if you could provide pictures that will be a great help as well.