Feeling stupid, my tankless water heater turns cold when I'm in the shower

Discussion in 'Tankless Water Heater Forum' started by Terry, Aug 16, 2019.

  1. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    Tankless water heaters are fairly new to us, and we're just starting to find out the funny little tricks they play on us. The favorite one is water turning cold while in the shower. That like never happens with a standard vented tank water heater. The tankless is different though. For starters you plug it into electricity to run controls and a fan. With some you get an air intake, often plumbed to the outside and an exhaust, also plumbed to the outside. And don't even tell me you didn't do that one right.

    So for those wondering if the water turning cold while in the shower is fixable.............you may want to check the filter on the incoming air. It's a simple little thing to try and does need doing often.

    This was performed on a Navien CR-210 from 2009.


    Yep.........turning the unit off is a good idea.


    OMG, did that dirty filter come out of my tankless water heater? Come on, admit it. You've never, ever even looked at the dirty piece of wire screen in your tankless water heater before.

    This filter came from the top left of the tankless.


    And now hopefully you will get that warm fuzzy feeling about your tankless while in the shower again. :)
    Last edited: Mar 10, 2020
    Tuttles Revenge and FinchDaddy like this.
  2. FinchDaddy

    FinchDaddy New Member

    Aug 16, 2019
    South Carolina
    Thank you, Terry. I'm going to actually check this today before I ask anymore stupid questions. :)
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  4. Michael Lance

    Michael Lance New Member

    Oct 8, 2019
    Central Virginia
    Hey - I have a Rinnai R75 from 2008, and it did not have a filter! The first time I cleaned the fan, it was solid bug guts. For some reason I thought it was normal for the hot water heater to sound like the space shuttle taking off! I really should have known better. Now I routinely clean it.
    Terry likes this.
  5. MarcinNJ

    MarcinNJ New Member

    Nov 15, 2019
    I just had a Navien 240S installed to replace another tankless that would rumble loudly and sometime shut off. I began to get cold water sandwiches over and over during a shower after the install. I'd say cold would be gradual descent from hot to warm to cold so we can at least get out of the shower water and ~20 seconds later it goes back up and stays that way for 2-3 minutes then repeats.

    I noted the Navien 240S would turn off the flame (at least on the LCD) for ~20 seconds, then fire back up, over and over.

    So I get that this is enough evidence to show that if water heater's flame is off then on (and off and on again) during a shower, the exact duration of the fire being off is 1:1 with the duration of cold water in the shower.

    Plumber replaced 240S with 240A but even checking the dip switches (#1 on, #5 on) to use internal pump for .5 Gal and well pump (actually a domestic booster we have on the water inlet - city gave us 1.5" instead of 2" so pressure was ~31 and now is closer to ~75psi) are in place.

    Same issue - recirculation helps the initial on cold water sandwich, but then recurring ones happen like the 240S so figured the problem is not the tankless but the system:

    1. Double checked dip switches #1 and #5 are on, all others off
    2. No gas leaks (did the clorox spray - no bubbles, no real smell of gas)
    3. Did note that the fresh air intake has a louvered end cap that is for exhaust types (e.g. ,must have pressure from inside PVC to open) so might be some fresh air limit it's reaching - getting contractor to look at this so that variable will be accounted for soon enough).
    4. Checked for cross contamination of hot cold and noted my kids sinks do run some cold when hot handle turned on and water heater hot out is off. Retried with their cold water supply off and made no difference but this will also be addressed soon.
    5. We have 4 different showers (make/model) and all are affected similarly as our baths. Sometimes even kitchen sink.
    6. All plumbing past the water inlet and booster pump and filter is PEX blue/red lines that look correctly connected (dark metal bands cinched on joins) - no leaks noted anywhere in the house.

    So other than #3 above, I have no working theories but working with visual confirmation that the flame does turn off (again LCD screen of Navien shows this) every few minutes for about 20 seconds then fires up again. Believe it should be constant.

    Looking for any suggestions.
  6. Navien Inc

    Navien Inc New Member

    Sep 3, 2019
    20 Goodyear, Irvine, Ca
    Hi MarcinNJ,

    We recommend that you call into our tech support at 1-800-519-8794 option 2 so that we can have our team diagnose the issue and provide a solution. Our tech support is available Mon-Fri from 8am - 9pm EST and Sat-Sun from 8am - 8pm EST. Thanks!
  7. lifespeed

    lifespeed Member

    Apr 27, 2007
    Is there a similar screen below the air inlet in my Noritz NRC1111-DV tankless? Will give it a cleaning next time I'm in the attic.
  8. DIYorBust

    DIYorBust Member

    Mar 5, 2019
    Long Island, New York
    Good tip, I have actually had this happen. I really wish the navien's could attempt to restart after an error event. Probably a safety feature, but if we turn off the gas or water pressure and then forget to restart it, that's a cold shower.

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