Water adjustment knob appears to be stuck

[bajaman]

My Bosch tank less flash heater water adjustment knob is frozen it will not adjust. How can I fix that?
Now that summer is here the water is too hot and I can not adjust the flow.

 

[Master Plumber Mark]

spray it down with some wd40
....wait about 30 minutes then smack it with some channel locks
it will probably break free...