Once I have asked to a plumber and he told me they never need to be serviced...

Perhaps its time to invest in a transfer pump and make the proper connectors yourself. 

I think you could buy the pump make the connector hoses for $100 maybe a lot less depending on your tank. Remember the hoses do not need to handle any more pressure than what the pump can create with the discharge valve open.

Harbor Freight is a good place to start.

ciao
Santana

You need to do it if you have hard water / mineral deposits (sounds like you do); otherwise you're doing more harm than good.

Yep!  I am late to this game.  I have had a tank-less water heater since 2012, and the house has had NE Charlotte city water since it was built in 1995.  Like with anywhere, the water matters.  It is my understanding that these things only need to be flushed on a situation appropriate schedule.  I have done this once and probably should now do it again.  This is done with off-the-shelf white vinegar.  I used a pond pump, a 5-gallon bucket, two 6-foot garden hoses, and fittings for the pump.  My heater installation plumbing includes shut-off valves and hookups for this.  The vinegar is pumped from the bucket in reverse through the heater and back to the bucket.  It then cycles for however long the process is allowed to execute.  I don't remember the recommendation for this.  I believe this is followed by a rinse cycle using water.  Most of the mineral buildup is pulled out with the vinegar.  Find the manual for your heater.  My unit is branded "GE Profile", and I am certain that it is manufactured for GE by Rinnai.  Rinnai is or was the premier manufacturer for this tech internationally, and it supposedly is widely used in Europe.

Robert C. wrote on 10/13/18 at 9:49pm:


A sump pump likely is more expensive than a simpler and less powerful submersible pond pump.  The pump only needs to have enough power to lift the water vertically 4-6 feet.  The distance is based on the bucket's location relative to the heater hookups and hoses.  I placed my bucket on the garage floor and certainly could have positioned it closer by using a stool or some other makeshift setup to lower the vertical lift distance.  My pump is in the shed at home, and I am not at home.  This one at Lowe's likely is sufficient.

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Be sure to buy a pump that either has fittings for the supply hose to be used or support for using stuff from the PVC Plumbing section for adapting.  You can have a standard threaded hose connection at the heater and push-on friction connection at the pump.  My preference is to use a threaded connection at both ends.  The pump I have included a variety of fittings that screw onto the pump, and I am certain that one of these mates with 3/4" NPT.  Good luck.