I have a crown aruba boiler and after experiencing gurgling for a few days, the hot water is not making its way to the second floor baseboards. I feel hot water on both sides of the solenoid valve for this zone, but a few inches further up the pip it gets cold. I am thinking I may have a air pocket or simply not enough water in the system. Is draining and re-filling the system pretty simple? How do I know when the system is full. I do not see any level gauges anywhere? Do i need to turn the boiler entirely off to do this or just turn the thermostats down? Thanks!