Navien NR-240 Cold Sandwich

Discussion in 'Tankless Water Heater Forum' started by ROBERT CARTIER, Sep 6, 2020.



    Sep 6, 2020
    We have a older Navien NR-240 NG. This is a non "240A". Just coming up on 10 years and up to this point it has been very good. Over the last month the family has been experiencing the "cold sandwich" effect. Initially thinking it was the mixer in the Moen shower we replaced the cartridges. I did go and make sure the vent and cold water inlet filters were clean. No error codes are displaying.

    Searching I saw several post on 240A models for the Check Valve cleaning/replacement but I don't see that on the standard 240 system. We are due this fall to standard servicing/de-scale. Any ideas?
  2. breplum

    breplum Member

    Oct 2, 2004
    Plumbing and heating contractor
    San Francisco Bay Area
    I am a plumbing contractor with over 12 years experience with Navien and level 3 tech training from Navien.
    Assuming you have been descaling on an appropriate schedule, then get the next descaling done and report back.
    If you don't have an "A" model with a pump, then there is no check valve.

    I definitely have experienced problems with the modern NPE A series that has the pump, and some problems of temperature control were definitely solved by replacing the check valve (even when Navien tech support was skeptical because the units were under two years old. )
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  4. Bannerman

    Bannerman Well-Known Member

    Mar 19, 2014
    Ontario, Canada
    As you initially thought the shower faucet mixer was the cause, it would seem the 'cold sandwich' effect is not ocuring when other hot faucets throughout the home are flowing.

    As there are no error codes, it would appear the NPE-240 is not recognizing an irregular situation such as insufficient combustion air or blocked exhaust vent, either of which would cause the burner to be shut down to ensure safety.

    I suspect the Navien is detecting a non unusual condition such as insufficient water flow which is then causing the burner to shut down just as would occur when no hot water is being consumed.

    Although a typical shower head will flow more than the 0.5 GPM needed to activate and maintain burner operation, have you removed and inspected the shower head to ensure the water inlet and internal passage(s) is not partially plugged and to ensure each water jet is not blocked with scale?

    Alternately, there could be scale buildup in the NPE's water flow sensot or it could be defective so that it is not reliably sensing sufficient water flow.
    Last edited: Sep 7, 2020
  5. fitter30

    fitter30 Active Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    With two people one at the kitchen sink the other at the heater could measure the flow and check where the heater turns on.
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