Home Hydronic System - How do I Flush it Out?

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Hi everyone. I'm not a professional plumber, but I do a lot of my own work. I have a roughly 22-year-old Apollo home hydronic system. There are two lines that come off the side of my water heater that supply/return water to a radiator in my attic. I'm planning to turn on my heater in November. I'm being told that water in these lines becomes stagnant when it's not being used in the summer, and I need to flush the lines before I use it to prevent contamination of my water heater. The house came with the system and local plumbers don't know much about maintaining the system, because its rare in my area to have this type of heating system. I've read that you need to be careful about getting air in the lines. Does anyone out there have knowledge on the proper way to flush the lines? I've attached the pictures of the hydronic lines going into the water heater. It might be hard to see, but both the supply and return lines have a gate valve. The return line at the bottom of the tank also has a hose spigot (hidden in pipe insulation) that could be used to release water. I would greatly appreciate any help that I can get!


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