[Doug Musick]

So my hot water flow and pressure left something to be desired and was far worse than what I got from the cold water valves.  I disassembled the shower valve and cleaned it with no noticable improvement.  I replaced the vanity fixture (wouldn't shut off and couldn't get parts) but no improvement in flow.

So the likely culprit was calcification of the hot water coils in the Aquahot, or possibly a blockage somewhere else in the main hot water feeder line.  I've read somewhere this can be an issue with the Aqauhots and sometimes results in replacement of the Aquahot (ouch).  Household tankless hot water heaters have the same issue and manufacturers recommend routine acid flushes (usually annually).  In reality your water quality is a big factor in how fast the problem develops if at all.

Anyway I cut my water lines going in and out of the Aquahot, after first shutting off the Aquahot, they looked good, no calcification.  I then put 6 gallons of clear vinegar in a cooler.  I disconnected one of my 12v water pumps.  I attached a suction line to it and dropped the end in the vinegar.  I then attached a line from the Aquahot, hot water (out) line long enough to flow into the cooler.  I then plumbed the pump out (pressure) line to the Aguahot in (cold) line.  I then turned on the pump, checked for leaks, and drank a couple of beers while I let the vinegar circulate for a few hours.

I then drained the vinegar and flushed the loop with clean water a couple of times.  Then disconnected the pump and reattached the cut water lines.

I now have double the flow and better pressure.

I will continue doing this as preventative maintenance once every 1 to 3 years.