Navien 240A2 low heat capacity problem

[nw1997]

So we had this unit installed about a year ago (I forget exactly when, because the installer has gone AWOL and won't return our calls). It worked well until a few weeks ago. So here is the problem.

When I first turn on a hot water, the water comes out lukewarm even though the unit is set at 130 degrees. I don't believe it ever changes because I've waited for 15 minutes before and it's still at that level. I go downstairs to the unit, and I see maybe 0.6gpm flow and the "heat capacity" is running at maybe 15%. We're in Boston so the incoming water is pretty cold, and the lukewarm "hot" water is around 70 degrees. [I'm getting all these measurements from Status Information on the unit itself]. To fix this problem, I'd have to turn on more faucets, and maybe play around with the flow (turning faucets on/off or up/down). Eventually, the unit sudden comes alive, flow per faucet jumps to maybe 2gpm and the heat capacity jumps to the 30's-60's and now we get actual hot water. I feel the hot water pipe coming out of the unit, and sure enough it becomes too hot to touch. For the next few minutes, all the faucets/showers have hot water. But after we turn all the faucets off for a while, we're back to square 1 of this problem.

In summary, when we get lukewarm water, we're seeing maybe 0.6gpm/faucet and around 15% heat capacity per faucet. When I turn on/off a few faucets, the flow jumps to around 2gpm/faucet and heat capacity goes to maybe 30%/faucet.

I called Navien and they told me to go get a plumber. We got a plumber here yesterday, but all he did was clean the unit and change some filters. He said the problem was fixed, but it wasn't. I think he had enough faucets running, so the unit was already "hot". When I came home to take a shower, the unit was cold again and it was back to lukewarm water until I get enough faucets going. Anyone with any ideas?

P.S. I have no idea if we have recirculation installed. Definitely nothing external.

 

[Fitter30]

Measure flow rate at a bath tub compare that reading with the units then the kitchen faucet. Possible that the flow device isn't reading correctly.

 

[Sponsor]

 

[nw1997]

Measure flow rate at a bath tub compare that reading with the units then the kitchen faucet. Possible that the flow device isn't reading correctly.

Sorry for the dumb question, but how do I measure the flow rate? If I just go by the reading on the unit's LCD, it's definitely wrong. When the water is hot (and heat capacity high), the flow rate is around 2gpm. When the water is lukewarm (and heat capacity around 15%), the "flow rate" being displayed on the unit is around 0.6-0.8gpm. Looking at the water actually coming out of the faucet, it appears to be the same rate (I assume 2gpm).

 

[Reach4]

1. To measure flow rate in a tub, fill a 5 gallon bucket and time the fill. The GPMs = 300/seconds. For the kitchen, use a smaller container and compute appropriately. If measuring water flow into a container in the kitchen, a kitchen scale can be used where a gallon weighs 8.35 lb or
133.5 ounces. A gallon is 128 fluid ounces, but a fluid ounce of water weighs more than an ounce.


2. See the thread https://terrylove.com/forums/index....ess-be-misreading-the-flow-rate.100953/page-2 including post #30.

 

[nw1997]

1. To measure flow rate in a tub, fill a 5 gallon bucket and time the fill. The GPMs = 300/seconds. For the kitchen, use a smaller container and compute appropriately. If measuring water flow into a container in the kitchen, a kitchen scale can be used where a gallon weighs 8.35 lb or
133.5 ounces. A gallon is 128 fluid ounces, but a fluid ounce of water weighs more than an ounce.


2. See the thread https://terrylove.com/forums/index....ess-be-misreading-the-flow-rate.100953/page-2 including post #30.

Very interesting. Thank you. The plumbing company is sending someone out here again today, and I will make sure I mention this to him. Will report back on what he finds.

 

[Zl700]

A small internal waterway check valve has failed and it’s mixing the inlet cold water that is bypassing heat exchanger

 

[Breplum]

The new A2 series Navien are suffering defective/sticky internal check valves way-prematurely. The old A series did also and still do. In my mind, the engineers never solved the problem from the A series. They never admit failed product design!
Navien is aware of this and fyi, they pay licensed plumbers up to two hours for service in the first year in addition to all parts.

 

[GrumpyPlumber]

Hi, also from MA.

Had this exact problem once with a Bosch tankless after a year (last one I'll ever install), Homeowner couldn't get a hot shower without turning on other faucets.

Engaged in an argument with tech support for two hours, told them it wasn't a flow issue with the piping, but an internal issue with flow detection, and that was my final experience with Bosch, they literally suggested I should repipe the water system at one point, rather than admit a defect.

Later that night, the homeowner tried twice, somehow managed to call and get through to a tech that agreed with my conclusion, they shipped a flow detector, once replaced, the unit was fine.

I'm inclined to think zl700 & breplum are right about the check valve, but thought I'd throw in the flow detector for good measure, very similar.

 

[nw1997]

Quick update for all. The plumber came over a week ago, agreed with the assessment that the flow sensor may be faulty (although I don't know if he checked it himself or was just happy to get the job done). Navien paid for parts/labor and the issue was fixed for about a week. Now it's back to the same!
For example, this morning I turned on the kitchen faucet's hot (good flow). Ran to the water heater and it only detected 0.5gpm. I looked at the "system information" and saw "heat capacity" running at maybe 10%. Hot water coming out of the heater was also 0.5gpm. Incoming water temperature around 50 degrees and out water around 60 degrees.
My wife called the plumbing company to complain. Not sure what is going on here.

 

[GrumpyPlumber]

Sounds to me like zl700 & breplum are right and your plumber made the same assumption I did about the flow detector.

breplum specifically says it's a known problem with Navien tankless.

.

 

[nw1997]

Sounds to me like zl700 & breplum are right and your plumber made the same assumption I did about the flow detector.

breplum specifically says it's a known problem with Navien tankless.

.

But why is the problem back again, when they just replaced the flow detector last week?

 

[GrumpyPlumber]

Flow detector isn't the check valve.

The first (and seemingly obvious) assumption is the flow sensor, but breplum makes a strong argument that it's a failing check valve, a known issue with Navien.

 

[nw1997]

Flow detector isn't the check valve.

The first (and seemingly obvious) assumption is the flow sensor, but breplum makes a strong argument that it's a failing check valve, a known issue with Navien.

Ahh I see, sorry I didn't go back and read their comments. I will mention the check valve to the plumber when he comes back.

Just curious though, what does the check valve do? I'm trying to understand how a sticky check valve would lead to my situation: drawing too little hot water when I first turn on hot, but somehow normalizes when I open 2-3 faucets or jig around my hot water handle (to vary the flow rate)?

 

[GrumpyPlumber]

Check valve is a one way valve, like a diode in electronics, it serves to allow flow in one direction only.

In this case it may be in place to allow cold water to dilute hot when it overheats, it might be a combo solenoid & check valve that allows cold water in when a temp senor trips, I'd have to dig up design specs to know.

It sounds like it might be stuck partially open, affecting the flow sensor....where you've now eliminated the flow sensor as the problem.

Another possible problem, if there's obstruction, calcium buildup around the flow sensor on the piping, you might try to flush it for an hour with white vinegar, Home Depot has small pumps that can be used with a cordless drill, and two washing machine hoses.

I'm familiar with Boston, inside the 128 area you "shouldn't" have that much mineral in the water to be affected this badly in just a year, though you should flush it each year, but outside the 128 metro belt some area's are bad.