Tankless Navien NPE-240A - Temperature Fluctuations

BasicSulfite

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 GPS 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!

Jamie Hall

GPM I hope. The problem almost certainly is not actually a problem, but a characteristic -- and may not be solvable. What is happening is that there isn't enough water running through it to absorb all the heat which it is producing without hitting a high temperature limit. If it can't modulate down enough -- and I'm honestly not sure that it modulates at all -- the only way it can avoid exceeding the high limit is to turn off for a few seconds until it hits a low limit, then turn back on -- and so on.

I'd have to take a fairly deep dive into the Navien control system to see if there is a way to reduce the problem -- but I rather doubt it.

BasicSulfite

Jamie Hall said:

GPM I hope. The problem almost certainly is not actually a problem, but a characteristic -- and may not be solvable. What is happening is that there isn't enough water running through it to absorb all the heat which it is producing without hitting a high temperature limit. If it can't modulate down enough -- and I'm honestly not sure that it modulates at all -- the only way it can avoid exceeding the high limit is to turn off for a few seconds until it hits a low limit, then turn back on -- and so on.

I'd have to take a fairly deep dive into the Navien control system to see if there is a way to reduce the problem -- but I rather doubt it.

What you said definitely makes sense. (Very intelligent deduction.)

I'm speculating that the Navien unit that it replaced didn't have this problem because it didn't work as efficiently so there was a higher flow of colder water.

Unfortunately, there is no way to confirm this with Navien Tech Support since they now have an established policy of not talking to anyone except licensed plumbers. But, thanks for the insight. What you said definitely makes sense. I guess I have to dial down the temperature. ☹

*Yep, it's GPM. I corrected my initial post.

BasicSulfite

Jamie Hall said:

GPM I hope. The problem almost certainly is not actually a problem, but a characteristic -- and may not be solvable. What is happening is that there isn't enough water running through it to absorb all the heat which it is producing without hitting a high temperature limit. If it can't modulate down enough -- and I'm honestly not sure that it modulates at all -- the only way it can avoid exceeding the high limit is to turn off for a few seconds until it hits a low limit, then turn back on -- and so on.

I'd have to take a fairly deep dive into the Navien control system to see if there is a way to reduce the problem -- but I rather doubt it.

Jamie,

I actually was able to talk to a Tech guy. (Took a lot of persuading.)

He said that the burners are actually modulating. The burners modulate from .5 to 1.5 GPM.

The Tech also said that it's not normal that the unit's burner turns off at 1.0 GPM. He suggested that the problem might be the gas pressure (which I'll check) but I don't see how low gas pressure would lead to the burner cutting out. I guess I have to do more research.

Thanks.

neilc

try a lower domestic setpoint,
you're mixing cold water in at the shower, right?
if you run a 115, or 110 setpoint then you mix less cold at the shower,
and get more flow thru the domestic heater, keeping it on,
try it

ChasePowers

Is there a flow restriction device in the shower head you can remove?

emnamay

Navien wouldn't talk to me, so I had to call a plumber. The plumber sat on HOLD with Navien for more than an hour and a half and went from number 39 in the queue to number 20 in the queue. I made them leave before they could charge me for another hour. After watching a YouTube video, I took out the check valve, cleaned it, and smeared a light coating of silicone grease on the inside O-ring, and it's working beautifully again!

emnamay

It has nothing to do with the flow rate of the shower.