I have a 5gpm tankless coil in my oil fired boiler;
I've come to the conclusion that it's clogged with minerals
from the hard water out of my well.
The cold water pressure is fine - the hot out of the coil is low.

I found a guy to clean the coil for $225 (I can do it myself with a small transfer pump and a couple of hoses and some CLR, but my wife has no confidence in my abilities as a handyman Isolation valves are in place - but the only thing I'm worried about is a clog when flushing then I'll have no hot water.

Question is:
Should the coil be flushed from the cold water side out the hot side - or should the acid be circulated in reverse, from hot side to cold?
I've heard it has less of a tendency to clog when flushed in reverse, but I don't see how.

Thanks - any suggestions appreciated.