Help, water is hot then cold then hot....and then e003

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unagi

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I hope someone can help with real advice. Tech support at Navien just wants me to call my plumber so he can call them. For the last 3 years, the shower has been hot, then cold then hot again. It happens after about 10 minutes into the shower. Navien says it's the mixing valve in the shower or somewhere in the house that's making this happen. I can't replace every one, but have some and this does not fix it. In the last 2 weeks, I have had cold water to start, sometimes the e003 error. After a power shutdown and wait 5 minutes, it's fine, still get the hot and cold things happening. Its a combi, NG and we have 4 zones of baseboard hot water heat. The run is not that far from the boiler and the house is only 1600 sq. ft. Any ideas? Pressure valve? Control board?
 

automaton

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I hope someone can help with real advice. Tech support at Navien just wants me to call my plumber so he can call them. For the last 3 years, the shower has been hot, then cold then hot again. It happens after about 10 minutes into the shower. Navien says it's the mixing valve in the shower or somewhere in the house that's making this happen. I cant replace every one, but have some and this does not fix it. In the last 2 weeks, I have had cold water to start, sometimes the e003 error. After a power shutdown and wait 5 minutes, it's fine, still get the hot and cold things happening. Its a combi, NG and we have 4 zones of baseboard hot water heat. The run is not that far from the boiler and the house is only 1600 sq. ft. Any ideas? Pressure valve? Control board?

How old is it? Ever had the ignition rods/element replaced? If e003 happens often, could be that - about a $15 part. Otherwise, could be the infamous 3 way valve that redirects heating water to the DHW plate exchanger.
How I would troubleshoot the valve: open a high flow hot faucet and watch the boiler. Does it stay lit? Does it keep firing at a constant rate or does it sound like it's ramping up then down? Does the space heating temp go way up, then down when the boiler backs off? Is the space heating loop warm/hot to the touch (shouldn't be if heat is off). If the valve is stuck, not enough heating water is making it to the exchanger so the boiler would be compensating by firing up.
My Navien (which needs a good flushing) needs space heating temp of 135F to make 120F temp for my wife's showers.
 
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unagi

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It's a few years old, maybe 5. NCB 240 NG. I believe it has recirculation. I did the troubleshooting again today, open DHW, flame icon stays lit, psi goes up and down 14.9-16. Temp stayed at 149 f. This was for the kitchen sink. Next test where the cold fluctuates, is the shower, I will record on my phone for 15 minutes and report. As far as I can tell (its hard) there is no fluctuations of temp for heating. This morning we had the e003 error when I woke up and went to grab a shower. This is getting silly if we have cold water to start every 2 days. The loop is supposed to keep it warm so it doesn't take too long to warm up, that seems to be taking cold for longer now.
 

Fitter30

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Screenshot_2021-04-01-19-00-13.png Have you ever cleaned the domestic water side with a pump and vinegar? Air filter, water filters?
https://www.manualslib.com/manual/801298/Navien-Ncb-180.html?page=3#manual
 
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