Tankless not flushed for 10+ years!! Best way to flush?

[g20zoom]

So my buddy has a place with a tankless...but no flush valves installed. This unit is 10+ years old, natural gas on a 1600 sf home. I'm not sure of the brand...but I can dig up a so so photo.

I'm going to help him get some flush valves installed...but was wondering the best solution to flush this one?
It's been on San Diego CA hardwater all these years. I was thinking of putting about 3 gallons water with say the 28oz bottle of CLR...run for 1-2 hours? Then repeat with a new bottle of CLR?

Other suggestions? Will the flush water be obvious when done? meaning...does it go from murky to clear eventually when sufficient?

Thanks!

 

[Dana]

If it's 10+ years old and still flowing the water can't be very hard or it would have limed up to a trickle.

I'm not sure CLR is really the right stuff. Most people use a few gallons of white vinegar, per some manufacturers' instructions.

 

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[Leon82]

I use a flow aid kit but you can make one with a 5 gal bucket a bottom suction pump and 2 washing machine hoses.

I use vinagar per Bosch instructions. But flow aid makes a product to descale also

 

[Leon82]

I do mine twice a year