Might my Noritz tankless be misreading the flow rate?

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Nathan Meyers

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I started having a problem recently with my Noritz tankless. Problem is when taking a shower (one knob, Speakman head, 1.5 gpm flow restrictor), it recently stopped keeping the burner on when the shower is set to a comfortable temperature. Plumber insists there is no problem, but I'm suspecting there is a problem with how the Noritz measures flow rate. Here's my story:

I'm measuring flow rate at the fixture using a measuring cup and a stopwatch: computing it from the time it takes to fill a quart measuring cup. Not perfect, but not terrible. Here are a couple of findings:

- At a two-knob sink faucet, I turn on only the hot water, and measure a flow of 1 GPM (it takes ~15 seconds to fill the quart measuring cup). Viewing the flow rate in Noritz maintenance mode, it reports a rate of .6 GPM.
- At the shower, turning the knob to full hot, I measure a flow of 1.5 GPM (it takes ~10 seconds to fill the measuring cup). The Noritz reports .9 - 1 GPM.

The plumber has tried to explain to me that the flow rate measured by Noritz's expensive computer and hardware isn't expected to match that at the fixture, but I'm really having trouble following his reasoning. Is he right? Am I crazy?
 

Terry

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Is the shower a single handle or a two handle?
If it's single, often behind the knob, there is a hot temperature limit setting, not allowing full hot in some cases, but ensuring that there is a mix of hot and cold to prevent scalding.
 

Nathan Meyers

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Thanks for the response, Terry. It's a single-handle Moen Posi-Temp valve, which probably has the behavior you're describing.

So I've taken another reading at the bathtub - a two-knob fixture that delivers 4.25 GPM with the hot fully open. The Noritz read 3.0 GPM (maintenance monitor display #14). Is that expected?

(additional note: I also checked the temp at the bathtub tap. It matches the Noritz's output temp within a degree. No cold water mixed in.)
 

Terry

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Thanks for the response, Terry. It's a single-handle Moen Posi-Temp valve, which probably has the behavior you're describing.

So I've taken another reading at the bathtub - a two-knob fixture that delivers 4.25 GPM with the hot fully open. The Noritz read 3.0 GPM (maintenance monitor display #14). Is that expected?
And that's going to be a question for the Noritz team maybe. Unless someone sees this that has some experience with that.
It's not sounding logical. At least to guy with some experience at installing, but not troubleshooting them.
 

Nathan Meyers

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And that's going to be a question for the Noritz team maybe. Unless someone sees this that has some experience with that.
It's not sounding logical. At least to guy with some experience at installing, but not troubleshooting them.
Thanks, Terry. Is there a way for a consumer to reach the Noritz team?
 

Nathan Meyers

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Unit needs to be cleaned out by pumping vinegar through it then see close it reads
Thanks, Fitter30. I see this is a recommended descaling procedure from Noritz, doable either by techs or by "savvy homeowners". It's been six months since a routine maintenance visit/cleaning from the contractor... does scale seem likely (water is softened)? Does scale seem a likely explanation for inaccurate flow readings?
 

Nathan Meyers

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Timed it by the videos timing line got 10 seconds. Whats the model and serial of heater?
Thanks for the closer look, Fitter30. I'm also using timing on the video, but I know it's hard at that rate to see exactly what's happening. I've made another video that shows results at three different flow rates - consistently reading about 30% low on the Noritz remote.

I suspect the answer to your question lies behind the front cover, and it seems the last tech to screw on the cover was a weightlifter - I'm having trouble getting in. It's a condensing heater, and the owner's manual is for models GQ-C3259WX-FF and GQ-C2859WX-FF. I'll soon be re-contacting the plumbing business I bought it from, and can get that info - but trying to get my ducks in a row before they hear from me yet again.



(If you're having trouble playing it within the forum page (as I am), click the link to watch it on YouTube.)
 
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John Gayewski

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Without an obvious problem like a leaky heat exchanger or some other anomaly it does seem odd for you to get MORE water from the outlet than your heater is metering. I have no idea how the water heater decides how much water is flowing through it. I would guess it's based on time. X amount of water flows (which is controled by the size of the heat exchanger) within certain psi perameters (between 40 to 60 psi normally) so the heater is calculating it that way. Or if there's other logic. For all we know these have a flawed logic or it's wear and tear. Most tankless companies won't talk to homeowners so you need a pro to hunt it down for you. Or be unhappy and try another brand. Navien seems to be the leader.

This is kind of an unsatisfying answer, but the companies who make and sell these really do have the best info on them. There are some really odd problems the can occur which give some odd and unintuitive error codes or backward problems they only a person taking troubleshooting all day long calls can sniff out. Unless there is a plumber who deals in these exclusively and has had the same problem its not really a plumbing problem, more of an appliance problem.
 

Nathan Meyers

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Without an obvious problem like a leaky heat exchanger or some other anomaly it does seem odd for you to get MORE water from the outlet than your heater is metering. I have no idea how the water heater decides how much water is flowing through it. I would guess it's based on time. X amount of water flows (which is controled by the size of the heat exchanger) within certain psi perameters (between 40 to 60 psi normally) so the heater is calculating it that way. Or if there's other logic. For all we know these have a flawed logic or it's wear and tear. Most tankless companies won't talk to homeowners so you need a pro to hunt it down for you. Or be unhappy and try another brand. Navien seems to be the leader.
Thanks, John. It's a year and a half old and still under warranty (the problem started recently), so I'm not in any rush to shop around. But the service tech gave me an unsatisfying response (I need a higher-flow showerhead... which isn't legal in California), and I'm having trouble buying that story. So I'm trying to ascertain if my opinion of what's happening has any merit before I take it back to the contractor. I think the answer is yes.

To answer one of your points: based on a Noritz flow sensor cleaning document (https://www.noritz.com/media-archive/Flow-Sensor-Cleaning-Procedure.pdf), it appears to use an impeller for sensing flow.
 

Nathan Meyers

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This is kind of an unsatisfying answer, but the companies who make and sell these really do have the best info on them. There are some really odd problems the can occur which give some odd and unintuitive error codes or backward problems they only a person taking troubleshooting all day long calls can sniff out. Unless there is a plumber who deals in these exclusively and has had the same problem its not really a plumbing problem, more of an appliance problem.
Interesting insight. In the circumstance, it takes an annoying, assertive customer to get the support promised by the warranty. It's not a role I enjoy, but apparently one I have to play right now.
 

John Gayewski

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No, but the circuit was extended from a box servicing the living area on the other side of the wall. The one big draw is a space heater that is definitely not in use in San Diego at this time.
I had a similar symptom (water went cold during showers) on a Bosch
The owners had a sewing machine plugged into the same outlet. It was messing with the control board. But it was throwing error codes so not really the same. These water heaters are overly complicated for what they do which is simple.
 
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