Colgar

Just cuz the reading on the display are saying what they say doesn't mean the thermistor is reading correctly.

Thermistors work on resistance right? (Someone correct me if I'm wrong)

If I'm right, make them give you the ohm values for various different temps and check for closeness in actual reading.

I doubt the thermistors are your problem, but don't let tech support flunkies troubleshoot over the phone- unless you like the runaround.

Good luck.

Tankless

You answered your own question with that last statement. To me, this doesn't sound like a tankless issue at all. Rarley do tankless units run hot than cold....hot than cold without erroring out or shutting down completely.

It sounds like a bad shower valve in or near the trouble areas. This includes above and below, the troubled areas.

Protech is correct. Turn something on and put your hand on tyhe HW pipe. It won't lie. If you get consistant hot water you have now proved it is not the tankless unit. Which is hard to believe because it's a Navien POS.

When you were adjusting the Posi, that would make sense to me that you have different responces. Bad cartridges are very sensitive to how thay malfunction with respect to pressures.

I've seen shower / lavs go hot to cold -- cold to hot just by flushing a toilet.

Thermisters typically don't break, it's just another clue in the puzzle. I would focus on X connection and would be looking at shower valves.

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plumr

did you every solve the problem

robthaplumber

Definately check the gpm on the tankless and the gpm on the fixture being affected. I have seen where the gas line was undersized causing fluctuating temps. too. Starving for gas(btu's).

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Herk

There could be a defective Moen cartridge somewhere else in the house causing the problem. I've seen that happen a number of times. Even the newer plastic ones can fail. allowing a cross connection.

RealLivePlumber

No, I haven't been able to get back yet. Our schedules have been conflicting. I hope to get there this week.

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Tankless

If you want to prove what a great plumber you are, pull off the trim and shut off one of the supplies (doesn't matter which one) and then check to see how hot the water gets. Of course this is assuming the shower valve actually has a little 1/4 turn shut off. If not, ya gotta just replace the cartridge. Pulling it out and cleaning it off and reinstalling it may fix the problem, but that's a hack fix and cartridges are usually pretty cheap.

__________________
"It's now. You just go for it now and you worry about the future and the past later." -Steve Nash

And when I leave come together like butt cheeks
-Grits n Gravy

RealLivePlumber

I don't thinks its a cartridge. It is a brand new Moen faucet. The previous Peerless faucet had the same symptom.

I am leaning towards the clothes washing machine. It is directly under the problem bathroom. I asked the ho to turn off the control valve(s) to the washer, to eliminate it, but, they said they can't.

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RealLivePlumber
You can make it happen, you can watch it happen, or you can wonder wtf happened.
Phil Harris 2009

Redwood

If there are 2 separate valves feeding the washer it's not the problem. The washing machine solenoid valve are pretty much impossible to have cross over...

If they have a Symmons washing machine valve and it will not shut off the water it is very likely the crossover culprit.

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907plumber

If its a crossover problem why wouldnt it affect the other fixtures?