Navien Tankless Flush Problem: No Flow Out Hot Service Port

[schalliol]

Hi all — hoping for ideas from anyone who has seen this specific flush issue on Navien tankless.

System: Two Navien NPE-240 units (one NPE-A2 and one NPE-S2) installed back-to-back on a rack and configured as a digital cascade (NaviLink). Each unit has its own hot/cold isolation valves and service ports. Units heat and supply domestic hot water normally.

Goal: Annual descaling/flush on one unit at a time (cascade keeps the house running).

The problem: On one unit, I cannot get flow out of the hot (red) service port during flushing — essentially zero flow / a tiny trickle.

• I verified the valve handle turns and the ball valve visibly opens/closes (I’ll attach close-up pics of open/closed). The valve looks clean and unobstructed.
• The unit otherwise heats water normally at fixtures.

What I tried (confirmed multiple times):

1. Standard flush setup (per Navien style):
   • Big cold/hot isolation valves closed
   • Blue service valve open, red service valve open
   • Pump into cold service, return from hot service into bucket
     Result: pump can’t circulate; only drips/trickle at hot service port.
2. House pressure test:
   • Big cold isolation OPEN (city water feeding unit)
   • Big hot isolation CLOSED (prevent sending anything to house)
   • Blue service CLOSED
   • Red service OPEN
     Result: still only a tiny trickle out the hot service port even with hose removed.
3. Verified cold side works:
   • With house pressure I can get strong flow out the cold (blue) service port.
4. Cleaned the Navien cold inlet filter screen — it was basically clean.
5. Tried reverse-flow pump test (pump into hot service, return out cold service) — also no circulation.

Why this seems odd: Unit is only ~1 year old, on city water, whole-house extensive filtration/conditioning, and DHW flow/heat is normal. So a heat exchanger “blocked solid” seems unlikely. It feels like either:

• the hot service port is on the wrong side of the isolation/check arrangement, or
• there is an internal restriction in the service valve body/path that isn’t visible looking straight at the ball.

Questions for the forum:

• Has anyone seen a Navien (or common tankless valve kit) where the hot service port has essentially no flow even though the ball opens?
• Is there a known check valve/backflow arrangement in some installs that can make the hot service port effectively “dead” for flushing?
• Any recommended diagnostic steps to confirm whether the hot service port is on the heater side vs house side of the isolation/check?
• If it’s a valve/port issue, is the right fix to replace the hot-side service valve assembly/kit?

BTW, there weren’t condensation filters, so I’m installing those now.

 

[GReynolds929]

Sounds like the water adjustment valve is stuck and most likely needs replaced. It is odd that it is happening with it being only a year old. But parts are still under warranty and you may be able to fight for labor coverage depending on how close to the year you are. You can go into the advanced menu and test the water adjustment valve to see if it is working or not.

 

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[schalliol]

Sounds like the water adjustment valve is stuck and most likely needs replaced. It is odd that it is happening with it being only a year old. But parts are still under warranty and you may be able to fight for labor coverage depending on how close to the year you are. You can go into the advanced menu and test the water adjustment valve to see if it is working or not.

Thanks for your comment! I did the same test on the other unit in the cascade with the same result (again the screen is perfectly clear). The water adjustment valve says 100% in the menu.

Is there something I should do other than power the unit off, close the house valves and open the service valves?

 

[GReynolds929]

How are you powering it off? I use the power button on the front panel, wait about 15 seconds, you should hear the WAV opening. Then I unplug the unit and flush. 100% means the WAV is closed if I remember correctly, you wand it to be at zero. You could try running hot water, make sure the unit you want to flush is running and providing hot water and pull the power cord while the unit is running.

 

[schalliol]

I have them each on a HUGO V2 tankless UPS and just turned the power off by front panel but didn’t unplug. Maybe I need to unplug? It does sound like the valve is the issue not opening. Interesting.

 

[schalliol]

Does anyone have any more ideas? When I turn them off, there’s no valve sound, and these seem essentially brand new. How do I get the adjustment valve to allow the water through?