Diagnosing problems with NPE 240A recirculation

[seatan]

Hi - first of all thanks for everybody that contributes to these forums, I've learned so much here from the experts willing to help.

My situation:
NPE-240A-NG installed when the home was built (2019)
Dedicated recirculation return line, uses the water heater's internal recirculation pump

For the last few months, we'd been having an issue where filling the bath start hot but eventually spewed cold water. Used this forum to diagnose a possible problem with our recirc check valve. Removing it did in fact show that the o-ring had become dislodged and valve was stuck open. So I ordered a new one and installed that a few days ago.

Since then I've noticed that it takes a long time to get hot water from any faucet, (90 seconds or more to get hot, used to be just a few seconds) and I'm now wondering if our internal recirculation pump might be bad. The pump does turn on when the unit starts up, and does turn on every half hour for a few minutes, but I'm worried its not actually moving water.

I called Navien and they (reluctantly, it seems like they REALLY hate talking to homeowners) gave me some information about some relevant diagnostic readings (hit the wrench button and then +/- to cycle through readings).

A: Flow rate for the whole unit (gpm)
B: Water temperature leaving unit (fahrenheit)
G: Flow rate for the heat exchanger (gpm)
R: Water temp leaving the heat exchanger (fahrenheit)

So when I can hear the recirc pump turn on every half hour or so, I would expect the G reading to be non-zero (since it's sending water through the hot water lines), but it stays at 0.0. When I just turn a hot faucet on however, I see a non-zero G reading (~1 gpm).

I'm also a bit worried that the new check valve might be the problem, since (a) this started happening just after installing it, and (b) if it was failing to allow water through it seems like that would be hard to differentiate from a failing pump.

It looks like these pumps are reasonably easy to replace, but I'm curious to know if there are other things I should check or tests I could perform to prevent unnecessary work/costs. Thanks!

 

[Breplum]

Pull the check valve and see if it is stuck open (the little plunger rod would be stuck out the bottom and not springing back)
a zero reading when pump has power can be a non op pump OR a bad flow sensor, but bad flow sensor won't show flow even if you trip the T&P.

 

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[Lifespeed]

Sure am glad I didn't get the recirc model, Navien appears to have cheaped out on some important internal parts. My external recirc is on 12 years without failure. Cheap to replace or not, the PITA factor of tearing into the water heater is so not worth it.

 

[John Gayewski]

Are you sure the recirc line itself has been fully bled out? It can take a surprisingly large amount of water to fully burp the air from the line and if there's a bubble in the pipe it won't circulate.

Hopefully your system is setup to give a good bleed with a garden hose attached and ran to a drain. If it is you should open it fully and see if it started spitting air.

 

[Lifespeed]

Isn't the recirc line subject to water pressure on the inlet side, and as such not require bleeding? I'll defer to the experts, but my external recirc has been drained several times when plumbing service was required elsewhere in the house. It spits some air out the hot water side of the faucet then functions normally.

 

[John Gayewski]

Isn't the recirc line subject to water pressure on the inlet side, and as such not require bleeding? I'll defer to the experts, but my external recirc has been drained several times when plumbing service was required elsewhere in the house. It spits some air out the hot water side of the faucet then functions normally.

No. Water takes the path of least resistance. It's easier for water to travel from the supply to the outlet. There's no reason for it to go around a loop first.

 

[Lifespeed]

Hmmm, maybe Navien doesn't use a self-priming pump and this is specific to Navien? Specifics matter, if you say they need bleeding I believe you. With the O-ring failures and bleeding requirement, it would appear they could have done better.

Taco pump doesn't need bleeding. No doubt it's 50X the Bill of Materials (BOM) cost of a Navien pump.

 

[John Gayewski]

Hmmm, maybe Navien doesn't use a self-priming pump and this is specific to Navien? Specifics matter, if you say they need bleeding I believe you. With the O-ring failures and bleeding requirement, it would appear they could have done better.

Taco pump doesn't need bleeding. No doubt it's 50X the Bill of Materials (BOM) cost of a Navien pump.

All pumps (centrifugal) need water to pump. If there's no water in the line it can't pump. Some system configurations are more apt to need more bleeding than others. But in general and most of the time if a system is drained the recirc line needs refilled with water. Most configurations of piping need valves to facilitate bleeding air.

Not sure why we're talking about a silly thing such as this. It's pretty basic and common. Your not knowing about it doesn't make it special. Navien, taco, grundfos, and any other pump needs water to pump. Some have different ways to bleed air and there are many many different methods with different mechanisms to bleed air from systems but it's basic and required.

 

[Lifespeed]

It's also pretty basic to place the pump near the high point in the system so it self-bleeds. Problem solved. Can't happen in every case, but is completely practical for many.

 

[GReynolds929]

High, low, upside down; it doesn't matter if there's nowhere for the air to escape. Pumps don't bleed air out of the system. That's why there are specific devices designed to remove air, or as was stated a drain valve to flush the air out. Glad you are happy with your water heater setup, but unless you have a small buffer tank it's not better than the "A" model with everything you have built in, maybe more if no external buffer tank.

 

[Lifespeed]

Open the hot faucet, air is lighter than water. Water flow blows it out. There's your bleeder. You've never seen this?

Needing a bleeder means the plumbing was done in a way that traps air. It's usually avoidable, but not always.

 

[John Gayewski]

Open the hot faucet, air is lighter than water. Water flow blows it out. There's your bleeder. You've never seen this?

Needing a bleeder means the plumbing was done in a way that traps air. It's usually avoidable, but not always.

The water doesn't move reverse through the system. Your not fully grasping the concept here. Water has to replace the air. It can't move backward through the check valve which leaves a large void where water cannot travel.

The more you comment the more it's obvious you're not grasping here. Sometimes a system can bleed itself but most will not. Most times the water will not push the air out from the return side because it is rushing through the faucet. It's common and basic.