Navien NPE-240A - Temperature Fluctuations

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WorldPeace

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I recently installed a Navien NPE-240A. The water temperature fluctuates 20 degrees when I take a shower. Navien currently does not have tech support for homeowners but I was able to troubleshoot and determine the source of the problem.

When I have the tankless water heater set to 125 degrees and I turn on the shower, the burner will kick in for 1 minute and then turn off for about 18 seconds. It will continually cycle like this. I determined the gallons per minute is 1.0.

When I turn on an adjacent hot faucet, the gallons per second will increase to 1.4 GPM and the problem disappears. The burner will stay on the entire time.

So, I've determined that the burner will turn off when the GPM dips below a certain point. I've read that it shouldn't turn off until the flow rate dips below .4 GPM.

Does anyone know how to fix this problem - decrease the flow rate activation rate?

*By the way, I scoured this site for an answer to this. I noticed others had the same problem as mine but I couldn't find an answer. Thanks for any help!
 
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LLigetfa

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I'm surprised you can take a comfortable shower at 1 gal per min.
I assume that is just the hot portion of the mix, so if mixed 50/50, that would be 2 GPM. Mind you that's still nothing to write home about. I think 2.5 GPM showerheads are standard. The first thing I do is to locate and remove the flow limiter. That might be an easy fix for the OP.
 

WorldPeace

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I assume that is just the hot portion of the mix, so if mixed 50/50, that would be 2 GPM. Mind you that's still nothing to write home about. I think 2.5 GPM showerheads are standard. The first thing I do is to locate and remove the flow limiter. That might be an easy fix for the OP.

Thanks for the advice. I guess there is no way to fix it at the water heater? Someone on another forum mentioned that the unit probably doesn't have a way to modulate the burner's intensity. And, since the flow rate is too low, when the burner turns on with a low flow rate, the internal temperature eventually gets too hot so the burner must turn off temporarily until the temperature decreases sufficiently.

I wish I could confirm this with Navien but they now have a policy of talking only to licensed plumbers.
 

John Gayewski

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I assume that is just the hot portion of the mix, so if mixed 50/50, that would be 2 GPM. Mind you that's still nothing to write home about. I think 2.5 GPM showerheads are standard. The first thing I do is to locate and remove the flow limiter. That might be an easy fix for the OP.
A shower is about 80 percent hot and 20 percent cold. The heater should kick on with less than 1gpm, but 1 gpm of hot isn't enough for a comfortable shower. It should be 2 gpm. There are some shower heads that are designed for 1.6 or 1.7 I think. Maybe some designed for less. But not what I'd call a good shower.

I don't Google people's devices when they post them. If they provide a link and I have the time I'll look. If this is a non condensing heater it'll cycle, but that shouldn't effect the output temp. I don't have a specific familiarity with this heater or this symptom so all I could think of was a better showerhead.
 

Bannerman

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There is a common frequent issue with the NPE-240A internal check valve not closing completely. With the check valve remaining partially open, cold incoming water can flow backward through the circulation pump and 2-way valve, allowing cold water to mix with the heated water exiting the WH, resulting in low water temperature or temperature fluctuation to the home's hot faucets.

Because the water flowing in reverse through the check valve will be bypassing the flow sensor, the sensor will often recognize insufficient flow, particularly when the flow rate exiting the WH is only marginally greater than the rate needed to maintain constant burner operation. When the sensor senses insufficient flow, it will shut down the burner.

Since all of the issues you describe are often a result of a defective check valve, suggest replacing the check valve. The CV is a low cost component which maybe easily and quickly replaced by a home owner with even modest DIY skills.

npe-recirc-flow-internal.jpg
 
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Bingow

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@WorldPeace I'm following faults like these "just in case" our newish NPE-240A ever develops similar. Did your system throw any codes at all? Reading @Bannerman above and the user's manual, I'd expect E439 or E785 (flow switch/ sensor) to show.
 

Bannerman

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I'd expect E439 or E785 (flow switch/ sensor) to show.
When there is a check valve issue, there will be no error codes to indicate a flow switch/sensor problem since there is no actual problem with those components.

Because the water flowing backward through the CV will be bypassing the flow sensor and heat exchangers, the flow rate through the flow sensor may be less than required to maintain constant burner operation, similar to closing a single hot water faucet to only a trickle so the flow rate becomes less than needed to continue to operate the WH burner.

Also, although the NPE-240A controller can modulate the flame size between 19,900 - 199,000 BTUs, since a portion of water flow is bypassing the heat exchangers, the temperature of any water that is actually being heated, may rapidly exceed the thermostat's 125°F temperature setting even while the BTU input is reduced to the minimum possible. If so, the thermostat is likely shutting down the burner to prevent the heated water from exceeding the temperature setting.
 
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Bingow

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Thanks, Bannerman. Interesting that the manual's troubleshooting guide lists: no water, cold water, slow water, and too hot water, but not fluctuating temp water. I've added that to the page. Also, the installation manual lists the valve's part # as 30008359B, which seems to be replaced everywhere online as 30008360A. Tempting to buy one to have on hand.
 

WorldPeace

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@WorldPeace I'm following faults like these "just in case" our newish NPE-240A ever develops similar. Did your system throw any codes at all? Reading @Bannerman above and the user's manual, I'd expect E439 or E785 (flow switch/ sensor) to show.

@WorldPeace I'm following faults like these "just in case" our newish NPE-240A ever develops similar. Did your system throw any codes at all? Reading @Bannerman above and the user's manual, I'd expect E439 or E785 (flow switch/ sensor) to show.

Hey, sorry for the late reply. My system didn't throw any codes at all. The burner just shuts off.

I didn't feel like it was worth the time and money to figure out what was wrong. I just ended up lowering the temperature to 120 degrees. Now, the issue has disappeared since the showerhead draws more hot water. It's a little troubling that the unit doesn't work properly when it's drawing about 1.0 gallons but is it really worth it?
 

WorldPeace

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There is a common frequent issue with the NPE-240A internal check valve not closing completely. With the check valve remaining partially open, cold incoming water can flow backward through the circulation pump and 2-way valve, allowing cold water to mix with the heated water exiting the WH, resulting in low water temperature or temperature fluctuation to the home's hot faucets.

Because the water flowing in reverse through the check valve will be bypassing the flow sensor, the sensor will often recognize insufficient flow, particularly when the flow rate exiting the WH is only marginally greater than the rate needed to maintain constant burner operation. When the sensor senses insufficient flow, it will shut down the burner.

Since all of the issues you describe are often a result of a defective check valve, suggest replacing the check valve. The CV is a low cost component which maybe easily and quickly replaced by a home owner with even modest DIY skills.

npe-recirc-flow-internal.jpg

Bannerman, thanks. I'll actually call Navien and see if they'll send a replacement check valve. I've checked out videos on YouTube like this one and it seems really easy to replace.

On the other hand, Navien are really hardcore about not talking to anyone except a licensed plumber, but I'll give it a try. Thanks!
 

Wasillaguy

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I think Navien is making a mistake only talking to plumbers. Their product is half electronics, and if they talked to an electronics tech, they might run across someone who figured out the actual cause of hot/cold cycling that plagues just about every brand.
Because it's likely there's an electronics guy out there somewhere who gave up on plumbers fixing his problem, and dug in and solved it. Cheap.
 

Fitter30

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I think Navien is making a mistake only talking to plumbers. Their product is half electronics, and if they talked to an electronics tech, they might run across someone who figured out the actual cause of hot/cold cycling that plagues just about every brand.
Because it's likely there's an electronics guy out there somewhere who gave up on plumbers fixing his problem, and dug in and solved it. Cheap.
The small heat exchanger on any combi heaters are looking at two temps water in and out with such a low flow and volume my guess is that the algorithms of the program can't be fast enough to modulate the gas valve before hitting hitting the limit temp. By lowering the set point the heater is starting from a lower temp with more flow.
 

Bannerman

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I've checked out videos on YouTube like this one and it seems really easy to replace.
In the video you posted, the new check-valve does not appear identical to the exsisting one that was removed. I suspect the new part number you specified in post #11, may relate to a new valve design to address the frequent failure of the original valve.
 

Gefoster

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I have a two year old NPE 240A that just started the water temp fluctuating and have had two service techs out to inspect and their conclusion was to replace the check valve. Factory disagreed, but sent a new one, which was replaced by the last tech. The new check valve did not fix the problem. It was also suggested that the shower cartridge be replaced, which was, and still no change in temp fluctuation. Filters were inspected and the tank descaled and still no changes. Any other possible fixes?
 

Fitter30

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Have u run a bath tub or sink with a thermometer checking temperature? That takes the shower cartridge out of the picture.
 

Gefoster

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I have not, but the techs both did and the water temp did not vary for them, 120 factory number. Water temp has never varied for them.
 
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