I have a 20ft 3/8th inch copper coil that had served as my chiller for 8 years, until I upgraded to a bigger coil.
I have been wondering what is going on inside the coil and if I could use it to pass wort through for use as a HERMS as is. I figured if I recirculated some PBW or TSP through it for awhile if I could clean out any hidden corrosion in there.
Any thoughts?
If all that has been going through is water, there is likely no corrosion.If it makes you more comfortable to run some cleaner sanitizer through it, then by all means do it. Obviously, the greater issue will be to clean it out after each brew session when used as a HERMS coil.
Happy HERMing! 
I just did my first batch with a HERMS coil; because of circumstances surrounding my basement’s water evacuation, my coolant line into my counterflow chiller was run out through the coil and then into a garden hose. The following day I blasted it with home pressure water after soaking it in bleach. Should I run some star san in there too or will what I’ve done be ok?
That’s more than fine. (that’s my opinion) After rinsing like that, there is gonna be far less (or zero) contaminants in the HERMS coil than in the grains in the mash tun!. Remember, it will all get boiled in the end!
Is 20ft of 3/8th too long (volume-wise)? What dimensions do others use for a HERMS coil?
When I made my first CFC I used 25’ of 3/8 and it was more than adequate. For HERMing you should have no problem at all with 20’ of 3/8. Remember that transferring heat through the copper is easier than trying to cool it.
my HERMS coil is 25ft of 1/2 copper, just be careful with the 3/8 and leaf hops you may plug it.
If you just want to maintain the temp in Mash tun, 3/8 will do just fine.
If you want to raise mash temp with it, I would say that 1/2" would be a better choice.
20 feet is long enough.
It is all about how much volume you can pump in certain amount of time.
If you want raise temp fast, you also need to have appropriate heater in your HLT.