Our Noritz tankless water heater seems to be clogged.
I usually flush it with vinegar annually but I'm getting lazier and I haven't done it in four years. The unit is about 12 years old.
This morning the hot water cut off while my wife was showering. This is a real life emergency!

I hooked up my pump to a bucket of vinegar and to the heater, as I do when I flush it, but nothing will pump through. The pump will whirl but nothing goes through the heater.

I cleaned the filter before trying to pump, and I've checked it since, and it's still clean.

I switched the hoses around so that the vinegar would be pumped in the reverse way, but still no joy.

I'm guessing that a bunch of scale broke off and clogged up the work well enough that I'm now stuck.

Have any of you any insights or experience with this problem?

I called Noritz and I'm waiting on a call back. Thanks.