Navien NPE-240a - No heat with Low Water Flow but Works fine with High water Flow

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rstarr

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I've had my Navien for 6 years (almost exactly) with zero problems. This past week we noticed the problem and with this site and troubleshooting I determined the problem is just associated with low water flow. When I turn on multiple faucets or both showers it works fine with high heat. I descale and clean it every year religiously. I have no error codes either. I did replace the Flow Sensor and the problem persists. Another thing to note, whenever I power it off and back on it works fine for about 5 minutes. I welcome any input or guidance.
 

Bannerman

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A common issue with Navien 240A models, is a defective internal check valve.

When the check valve does not close fully, cold incoming water can bypass the flow sensor and heat exchanger, and allow cold water to flow directly to the hot water outlet connection. This not only will reduce the hot water temperature flowing to fixtures, but the flow bypassing the flow meter will not be recognized so the WH heat output will be reduced, further compounding the low temperature issue.

Because the hot water to your fixtures is actually hot when the flow rate is high, it's likely that your check valve remains stuck open only slightly, so the majority of high flow is passing through the flow meter and heat exchanger, but all cold water is being permitted to flow directly to the hot water outlet connection when the flow rate is low.

Replacement of the internal check valve is relatively cheap, and easily performed by a DIYer. Suggest obtaining a 2nd check valve to have on hand for the future. Some do remove to dismantle and clean the defective check valve, to reuse that again in the future.
 
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