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tankless water heater extreme water temp changes hot to cold.

Luis_3 Member Posts: 2
I have a Norwitz tankless water heater that is less than 1 year old. The heater is installed in my garage and it is rated to handle 6 showers.
The problem:
In my master bath I have a Moen single handle water valve, the shower is at the end of line. When I initially turn the shower to heat the water it works fine, but as I turn the handle to cool the water down, the heater turns off and I get cold water.

The slightest adjustment causes this.I receive hot water again only after I turn the handle all of the way back to hot and after several minutes.

I strongly believe that the problem lies with the Moen valve. Maybe it has something to do with "scald guard?"

The Moen handle is about a year old.


  • Tony_23
    Tony_23 Member Posts: 1,033
    Could be

    the faucet. Or it could be the piping. It works off flow rate, so when you turn the temp down at the faucet you're slowing the HW flow, shutting down the Noritz.
    Could be a partially clogged screen or even an abnormal obstruction in the line.
  • Luis_3
    Luis_3 Member Posts: 2

    If the screen was clogged that would mean that i would not get hot water at all, right? Do you think that the faucet may have some kind of setting that may be causing this. This problem has also occurred when this shower was the only fixture that was turned on. So the flow rate should not be a problem. My home is about 1900 sf, single story, water heater in garage and the problem shower is at the end of the line.

    Tony I will check the screen, how do I that?
  • Dave Larsen_5
    Dave Larsen_5 Member Posts: 2
    Flow issues

    You have hit upon the real problem with "on demand" water heaters. They'll give a large flow all day long, but ask for a small amount and you run into the problem you are facing right now. clean the screen in the inlet water line to the heater, clean the aerator of your sink, and take the water saver out of your shower head. The heater isn't seeing enough flow through it to turn the unit on. Chances are you will have the same problem in your kitchen anytime you want to rinse a plate with warm water. I believe the noritz needs 3/4 gpm to turn on. you can also lower the hot water temp. to allow more through the noritz. Good luck. ( I don't care for on demand heaters for this reason)
  • Jack
    Jack Member Posts: 1,047
    Check the flow

    thru the water heater with the shower running. You can read flows on the touch pad on the Rinnai. I would think you can do the same with the Noritz. You should be able to get a number to within .1 gpm. Don't know the key sequence to get the info. Once you set the temp on the shower valve at the desired temp and read the flow you will have a better idea of whether or not you are cycling off on low flow on your unit. How high a output temp are you running on your water heater. If in fact you are having low flow issues, running a high output temp on your system would exacerbate the low flow issue due to the mixing ratio.
  • Rich Kontny_4
    Rich Kontny_4 Member Posts: 73
    Same problem

    I have the same problem with my non pressure balancing Moen faucet that also is furthest away from the Takagi on demand heater I installed.

    I have my own well and I have a 20# differential between cut in and cut off (40#-60#)I also know that the standard Moen cartridge is notorious for crossover. I have changed that and still experience the problem.

    I have learned to adjust as I shower since this is in my bathroom and I intend to upgrade both the bathroom and the pressure system in the near future.

    I have concluded that the on demand is indeed pressure sensitive and when I approach my cut-in point I get less hot water due to the pressure loss that takes place from inlet to outlet of the heater and the additional loss across the tempering valve I have in my system.

    If I was on a municipal water system with constant pressure I would not have this problem.

    Rich K.
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