Tankless Hot Water Heater, Noritz N0751M Will Not Heat Water

[iceundersiege]

MAKE: Noritz
MODEL: N0751M
PROBLEM: Will not heat water.
AGE OF UNIT: 2 Years
BACKGROUND:
Unit has run perfectly fine for the past 2 years with temperature set at 120'F.
Last six months, on occasion, the water would be nice and hot and then only cold water would come out for a handful of seconds and then it would be hot again.
All of a sudden, this past week, no hot water will come out of the unit.
The remote has not ever and does not depict any error codes.

RECENT SERVICE:
I cleaned the filter (unscrewed it from the unit - it was full of debris - I put the filter in hot water heated off of the stove and washed it out). I reinserted the filter.
I turned the unit off and on.
I reset the unit, turning the circuit breaker off. Let it set for a few minutes and turned it back on.
There were and still are no error codes.

UPDATE:
The burner has kicked on with some not bitter cold, but lukish warm water coming out but nothing not. Then just cold water.

I've read horror stories about calling plumbers out and the problem not being solved time and time again.

Seeking other possibilities on what could possibly be wrong and what I can do to fix the heater.

Appreciate your help.

Thanks!
CJ

 

[havasu]

You obviously have a problem but I'm wondering why you are not receiving any error codes. I have a different brand of tankless W/H and when I developed an error code, I called the company tech support and they solved all my problems. I would recommend this as a first plan of attack. Please keep us informed!

 

[LiQuId]

if you have not properly maintained the unit then it is likely that your heat exchanger wall is covered in scale, these units need to be flushed at least once a year and more often is recomended. do you use softened water ?

 

[iceundersiege]

LiQuid, Per your inquiry, I do not use softened water - just local tap water. How will softened vs. tap water effect tankless units?

Thanks!

 

[johnjh2o]

LiQuid, Per your inquiry, I do not use softened water - just local tap water. How will softened vs. tap water effect tankless units?

Thanks!

You would have less chance of scaling with softened water.

 

[FlushMasta]

my first suggestion would be to rip it off the wall, throw it out back and give it a few blasts with a shotgun...

second, as havasu mentioned, call tech support.

i've had to diagnose a couple noritz units, their tech support was very helpful... and again as havasu said its very odd its not displaying an error code or locking out. i know its possible to pull up the past error codes the unit has had, but i can't remember how to do it... when you do call tech support, have a volt meter on and and a manometer on hand with a few hand tools to help diagnose... if you do call a professional, don't be surprised if multiple trips have to be made... one part may have gone bad which caused another part to go bad, but you won't know until you get that first part replaced

 

[LiQuId]

You would have less chance of scaling with softened water.

most manufacturers instruct that the units must be supplied with softened water to retain manufacturers warranty for the reasons that john cites here and as per my original question

Edit: meant to quote the OP's response.