I'd like to flush out my water heaters and since the return line is actually lower than the WH, I think I should try and flush that as well. The sticky about flushing didn't have any details about WH with return lines. Here's what I'm working with.

Hey! First post. I'm having a problem with my 2004 American gas water heater. The original issue is the pilot light would not stay lit. I had this issue previously with an identical water heater and I was able to repair it by grabbing a burner assembly kit from Lowes and replacing it. So I tried that this time and it worked for about 2 days. My next step was draining the water heater and replacing the thermostat, which I did last night. The water heater was fine until I came home from work today and the pilot light was off again. I was able to get it back on but when I adjusted the temperature the flame inside became violent and I saw flames coming out of the back of the water heater (or, more likely, out of the bottom of the water heater and around the back). This only lasted for like a second, but I have noticed that every time I adjust the temperature, the fire starts a little more aggressively. At this point I am afraid to touch it because I am afraid it will become even worse next time, maybe exploding. I will add in two perhaps key pieces of information. 1. When I mess around with the water heater, there is a light gas smell. This goes away after the heater has been put back together and turned on for an hour, but it still bothers me. 2. The past few times I have turned on the water heater, I have had to reset the thermal switch (on the outside of the water heater, on the heat assembly. I pushed the little black button). I would like if at all possible to repair the water heater myself. I have had terrible luck with contractors and, especially, local plumbers (I have called over 20 this week). The vast majority will only replace the water heater with a new unit, and the two I plumbers who have been willing to repair instead of replace have quoted me $700-1000, and at that point I might as well replace the unit. Any ideas as to what might be causing this? Do you think there is any way to proceed without buying a whole new water heater? I know it is old but money is tight. Any help would be greatly appreciated :)