Navien NPE-240A Water Temperature Problem

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Mike_90503

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Hello,
I checked through this forum, but couldn't find anything that seemed to address my problem. Here are the details of my issue, what I have done to address it, and finally, my questions.

Water heater details:
Navien NPE-240A
Installed December 2013
External re-circulation loop installed
Scale filter installed at cold water inlet

Repair history:
March 2016: PC Board & Gas Valve failure. Both replaced.
September 2020: Re-circulation pump stopped working. Determined to be due to air in the lines caused by recent installation of whole house water filter. Air purged from system and has worked fine since.

Maintenance:
Scale filter replaced annually.
Descaled 2015 & 2017. Annually since.
Inlet water filter, re-circulation water filter and air filter cleaning irregular.
Last descaling was in October 2021 (this year). Scale filter also replaced.
Note: whole house pre-filter replaced every 6 months, last time in September 2021.

Problem:
Water temperature fluctuations in shower every day over several days. Continuous temperature cycling from hot to warm to hot to warm. Verified that no other water fixtures (cold or hot) were being used at the time. Recent problem; never seen before.

Troubleshooting:
First, to eliminate the shower head and shower proportioning/anti-scald valve, measured hot water temperature at sink faucet in same bathroom as the shower. Used a thermocouple attached to a DVM, calibrated at 32 F and 212 F. Sink faucet is a widespread design with completely separate cold and hot cartridge valves. Water heater is set to 120 F. Hot water temperature at sink continuously cycled from about 112~113 F down to about 105~106 F. Did not actually measure the cycle time, but it was on the order of 15~20 seconds to drop to minimum temperature then another 15~20 seconds to climb back to the maximum temperature. Watched this for a few minutes with no change to the cycle.

Second, repeated this test at the kitchen faucet, which is the farthest fixture from the hot water heater and also the far end of the re-circulation loop. Same result seen, though the highest and lowest temperatures were a degree or two lower (attributed to the extra run length and probably poor pipe insulation).

Third, turned the shower on with the temperature set to high and went out to the water heater (located in an insulated closet under the house roof-line, but only accessible from outside). There were no error codes shown, the water flow through the heater was indicated to be ~2.1 gpm, the outlet water temperature was shown as 127 F and the inlet water temperature was shown as ~71 F. The burner indicator was cycling on and off, with a sound like a turbine engine spooling up when it came back on. The cycle time was about the same as noticed before: 15 or 20 seconds on followed by 15 or 20 seconds off and repeating. Watched this for a few minutes.

Steps taken:
  • Turned off the water heater at the switch inside the water heater cover. Turned off the water at the inlet. Turned off the gas.
  • Drained the water heater.
  • Cleaned the water inlet filter, the re-circulation loop water filter, and the air filter. There was only very slight contamination seen in the water filters (estimate less than 5% filter blockage), and almost nothing seen on the air filter.
  • Put everything back in, turned on water, turned on gas and turned on power switch.
Result:
Problem no longer present after one day. Water temperature remains constant in shower. Burner indicator on water heater remains on as long as water flow is present. Error code indicator says "F.0" I think (it's hard to read).

My questions:
Do I write this off as a one-time occurrence and assume/hope it wont happen again? Should I treat the Navien like a windows computer that requires an occasional power cycle to continue working? Or is this indicative of an underlying problem that I should get fixed before it happens again, or causes a more severe problem?

Thank you!
 

Terry

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Have you looked at the incoming air filter?

navien-filter-2.jpg
 

Bannerman

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A common issue with that model is failure of the internal check valve from closing fully. When the check valve is not fully closed, incoming cooler water can flow in reverse through the internal circulation pump so some of the flow to faucets will flow backwards from the recirculation return line, thereby reducing the water temperature to faucets. The intermitant nature of the problem maybe due to the internal circulation pump becoming activated, thereby restoring the correct flow direction until warm water is again sensed entering the WH's recirculation water supply connection whereby the pump will be shut off again.

While replacing the internal check valve is relatively easy and low cost, suggest also installing an inline check valve at the far end where the hot supply line intersects with the return line. The additional check valve will provide additional protection to prevent the cooler water within the return line from flowing in reverse to faucets.

npe-recirc-flow-external.jpg
 

Mike_90503

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Fitter30: I watched the flow indicated on the water heater's display, and it was, for the most part, constant. I saw only a 0.1 gpm fluctuation (2.1 to 2.0 gpm)

Bannerman: Thank you for the suggestion. Would an issue with the check valve cause the burner to cycle on and off in a 15 to 20 second cycle? I believe that was the source of my temperature fluctuation: the burner turning off and on. The timing of the temperature swing and the burner on/off were the same.

John: None of the documentation I have explains how to clean the condensate system. Is this something an owner, who happens to be a mechanical engineer by trade, can tackle? Or is it best left to professionals? I will look online for instructions.

Thank you all for responding!
 

Bannerman

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Would an issue with the check valve cause the burner to cycle on and off in a 15 to 20 second cycle?
That will depend on the amount of reverse flow to your faucets.

Because the water flowing in reverse through the check valve will not be flowing through the flow sensor and heat exchangers, the WH will sense less or no flow and so the flame will be reduced or even shut down completely. Once the return temperature sensor senses cool water, that can then cause the circulation pump to become reactivated so as to restore flow through the flow sensor, causing the burner to re-ignite so as to heat the water flowing through the heat exchanger.

Edit to add: Ensure also the 2-way valve is fully in the External circulation position. When there is reverse flow through the check valve while the 2-way is in the Internal position, cool water flowing in reverse through the check valve and pump, will mix with the hot water exiting the heat exchangers and buffer tank, lowering the temperature of the water exiting the WH while also reducing the flow rate through the flow sensor and heat exchangers.
 
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