Chavez is a LOT smarter than me, so listen to him
But I'd NUKE it with muratic acid from the garden dept at Home Depot (12-13 ?? bukz a gal) and be done in a half hour
My recipe that you'll use at your own risk;
5 gal bucket w/4gal clean water
I by-pass my water box and draw directly from the 5 gal bucket
Turn off all soap dispensers ...
Plug in open end solution line ...like a short open hose you use to mix chems in sprayers ..that's what i use...you DO have one, right??

Start it up, turn on the pump and draw a couple gal of clean water from bucket .
Let the open flow end run to a drain or another bucket
(this purges any alkaline detergent in the system and coils)
turn off pump
add another gal or two of fresh water to the bucket and add a 1/2 gal of Muratic ...WEAR GLOVES AND
EYE PROTECTION
turn on the pump , turn on the heater and recirculate outflow back into bucket
Circulate til temp is appx 120
Shut it down for 5 minutes
Pull the outflow hose out of the bucket, start up and blow the the first round of gunk out in an environmentally responsible manner.
Suck appx a gal of bucket solution out before you put the outflow hose back in and recirculate to temp again.
Shut down and dwell another 5 min
Repeat as needed
you may be able to lengthen dwell time a bit, but when it's real bad, you STILL have to do it in steps like that
cause if you leave the acid dwell too long, it will make such thick goo it can clog the QD on your TM..ask me how I know
Once done, get rid of the left over acid solution ...in an environmentally responsible manner
(you should see how it reacts when dumped on a limestone drive)
purge with a
strong mixed alkaline solution
Purge thoroughly
For the next couple weeks, purge your sol line at job start for 20-30 seconds before hooking it to tool/wand
(if you don't already do that)
That will prevent jets from clogging and strainer screens from packing with bits of loose chunks /debris
..L.T.A.