Navien NCB240-E , Taco fill valve

Nutjob

My Navien has been trouble free for 6yrs until losing dhw. I performed yearly flushing with vinegar without issues. After flushing the dhw side, error code E.001 appeared, called plumbing company that did the installation, plumber came and said fill valve faulty, flushed heating system, bypassing normal flushing by shunting dmw manifold inlet water to heating system inlet valve and opening each zone valve outlet (3-zones/2-family home) then checked system operation. No call back to schedule fill valve replacement. The next morning, error code E.777 appeared. I managed to top off system water so the LWCO switch reset to allow normal operation (dhw). The Taco 329 fill valve appears to be in the FAST FILL position since installation (2017) and stuck/locked. About a year after Navien installation, the NYC water department dug up my block to replace main water and sewer lines. I checked Navien filters for rust and debris and found little. Navien system pressure displays 23.9 psi. Here are my questions.

1-If the Navien fill valve failed, is it preventing heating system water from filling? Did this cause the E.001 code?
2-Is the Taco 329 jammed in FAST FILL position stuck from rust/corrosion? Is the built-in screen blocking water flow that caused the heating system to lose water since last the heating season several months ago, cause my Navien to trigger E.001? Flushing the heating system allowed my Navien to reset the error code, allow heating and dhw back to normal operation.

GGross

E.001 Seems to be lack of heat transfer, I would be expecting something like air, failed pump, etc, something that is stopping flow (sounds like you purged the system and it ran, possibly it was air locked). A failed fill valve could stop the system from re-filling, but the cause would be the initial leak that is allowing pressure to drop, leaks need to be fixed, as they let out system fluid, and let in air.

E.777 seems to be associated with a tripped LWCO, again pointing to leaks in the system. You need to have the filling units fixed or replaced, the system needs to be purged of air, but above all else the leaks that are causing the drop in pressure need to be fixed as well.

23.9 PSI is too high in my opinion for a cold static fill pressure. The relief valve is probably set to blow off at 30 PSI, usually static fill pressure for residential is somewhere between 12-18 PSI for water, and you need to have the filling units set to that pressure, as well as the expansion tank needs to be set for the static fill pressure as well.

Nutjob

Plumbing company called today and said the fill valve and O-rings arrived and scheduled a second visit for Monday 9/25/23.

Does this solenoid operated fill valve operate only when heat is called for? An inoperative/dead solenoid may explain the main heat exchanger overheating and shutting down when dhw was needed. Observed dhw temps, preset, are at max - 120°F. When E.001 occurred, I noticed temps around 237°F and a long time to drop after the unit was shut down. I think the main heat exchanger was gurgling and over temp condition is the main heat exchanger partially empty.

My understanding of Navien is the main heat exchanger is in a closed pumped loop when dhw is needed, the cold water circulates thru a second heat exchanger to transfer main heat exchanger heated water to the second exchanger for heating dhw.

GGross

Yes the way a combi boiler works is that you have a standard closed loop boiler system for space heating, and generally speaking there is an internal diverting valve that diverts the flow of the closed loop through one side of a flat plate heat exchanger while the DHW flows through the other side of that heat exchanger when there is call for DHW. Generally a flow sensor will trigger that diverting valve and fire the boiler when enough flow is detected through the domestic side. I don't have much first hand experience with specifically Navien brand equipment, but this is the way modern wall mount combi boilers generally function.

As for the internal fill valve I am not familiar with how they function (seems to be a Navien thing?), but if I had to wager a guess I would say it has nothing to do with the DHW flow, and more than likely operates based on a pressure sensor inside the boiler (a built in LWCO of sorts) While this is very convenient I am not a fan of fill valves that are left open, because I want to know when a leak has occurred so I know to fix that leak. Another far more simple explanation to your boiler overheating and gurgling is that it was low on pressure, and partially filled with air, thus no flow through the heat exchanger, no heat transfer, which equals the boiler shutting down on its fixed high limit safety

Is there an external low water cut off safety device on this boiler? can you post a pic showing as much of the install as possible?

Nutjob

These are snapshots during installation, 2017. The Taco pressure regulator was left in the FAST FILL (O) position since.

Nutjob

Following up on final repairs after loss of dhw, then subsequently leading to loss of heating. The auto fill valve was replaced, system turned on. Below are images of the original afv assembly, partially disassembled - separating brass fitting from inlet valve, and view of the fine mesh screen (7/16" ID) before the inlet valve. The plastic valve inlet is sized down from 1/2" to 7/16" where the fine mesh screen was found almost completely covered in rust sediment. Unfortunately, I didn't take snapshots when I disassembled it to show the amount of debris blocking water flow into the dmh heat exchanger. This explains the quick rise above dhw temps above the 120F setting (200+F in a few seconds) before automatic shutdown. Once the afv was replaced, the unit was turned on and a how water faucet was opened. Water temperature (displayed) rose gradually from around 75F to 130F quickly settling back to 120F without ever going above 135F. When heating was asked, the display showed boiler temps rise to 180F, as set and remained as long as zone heating was demanded.

In the second or third year of operation, my street was opened to replace water and storm sewer lines as part of long term city maintenance plans. If I'm not mistaken, original water lines are cast iron, steel or a combination (cast iron valves, steel piping?). Black plastic piping replaced old lines (10" ID?). I worried about precipitates flowing into my new combi-boiler and saw very little debris in the main cold water inlet mesh filter. I suspect the auto fill valve fine mesh filter clogged up since then.

I separated the plastic afv from brass adapter and found the inlet screen almost blocked with rust sediment. While one plumber replaced the afv with new one, he gave me the old one. I measured the original coil resistance; it measured 90 ohms. The replacement measured the same 90 ohms. The close up image of the fine mesh screen was taken after I cleaned it of sediment. I can assure anyone that this screen was almost completely clogged.



There was a decision to either make this repair longer to remove the entire afv or, as is readily seen, separate original plastic valve assembly from the brass adapter by pulling off the stainless steel clamp/clip, remove and insert the replacement into the existing brass adapter. The snapshots shows an unused new brass adapter fitting with the original afv assembly on it. Rust stains covered most of the interior of the old valve assembly.