I am going to rework most of my plumbing because there are a few leaks where they used non-pressure fittings when the heater was installed. On both the supply and return lines to and from the heater, they have "cleanout" fittings and I'm wondering if those are necessary? I'm guessing they concept was to drain the lines after using the heater each time but we didn't do that last Fall as they are difficult to access and I didn't know if it was necessary or not. Now that I'm re-plumbing I would like to know whether to include those or not. The heater lines are lower than the rest of the plumbing so there would be water sitting in them after using the heater. I just didn't know if that's a bad thing or not?
The picture is the return line and the supply is just below it.