When I woke up this morning and find that we have no hot water.  I have a Noritz NRC111-SV Natural Gas tankless, installed in Dec 2010.  It is located in the basement on an outside wall and it has a dedicated black gas pipe coming in.

I take a look at it, it is turned on with no error codes.  But when I run water it does not fire up, and the "burner" light does not come on.  This happened a few years back and I cleaned the inlet filter, problem was solved immediately.  So again I cleaned the inlet filter, no joy.  Then I pulled out the flow turbine, took it apart and cleaned it.  Put it back together, everything spins real nice.  Reinstall--no joy.  Assuming the main board is good (thinking failed main board would pop an error code), the only other thing I can think of is the hall effect sensor and electronics (probably a simple op-amp and pull-up resistor).  So I order a new flow sensor turbine and take a cold shower ;-)

Tonight around 5PM I am cooking and I am running the hot water (I am an optimist).  And I notice it is no longer ice cold.  In a few minutes it is hot.

Any ideas on the cause?  I think these things are made for outside use, I can't imagine anything getting cold in there that would prevent operation?  Last time this happened was also a cold, winter morning.  Anyone have this experience with a Noritz unit?

Thanks in advance

I have a rinnai -- When Mine quit I pulled the flame sensor -- Cleaned the crud off it with some emery cloth and it fired right up -- Ordered new sensor and installed -- been good for a couple years now -- My rinnai is probably 12 to 15 years old now

 

 

I had a Bosch for 10 years and now I have a jacuzzi. I had a flow switch go bad on the Bosch that gave similar symptoms to yours. That was 15 years ago and now they may have some sensor thingy instead. But it’s easy to find and bypass to test if it does have one. 

So I am still thinking the flow sensor is bad. Even though it comes on now, it does not seem to heat the water enough at lower flow rate (like the bathroom sink). Kitchen sink and shower seem ok, but sometimes inconsistent.

I believe it uses the flow sensor to control BTU based on flow rate, so maybe it is generating inconsistent it erratic readings.

I have one one order, will see if it costs the issue