Rinnai Tankless & Thermal Expansion Tank - unable to read PSI due to sticky/thick substance discharge

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Eldrith

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Background:
My house is a new build, currently two years old. Last year, I measured the PSI from the expansion tank without any issues. Recently, out of curiosity, I tried to get a PSI reading again but this time a small amount of a sticky, clear fluid (possibly sealant) slowly leaked from the valve.

Pressure Regulator Valve (PRV) Status
I haven't experienced any issues with the PRV discharging.

Previous Plumbing Issue & Pressure Readings
During the first year, while the house was still under warranty, a plumber came to investigate a low-pitched screeching sound coming from the shower head. He said it was due to low water pressure and subsequently increased the pressure using the PRV on the main line that's located in the garage. While he believed the issue was resolved, I remained somewhat unconvinced; i was still able to hear the pitched sound.

During his pressure testing we noticed a discrepancy in pressure readings: the pressure gauge on the main water line inside the garage read 100 PSI (static), while the hose bib at the front of the house read 80 PSI. The plumber did not explain this difference, nor why he relied on the hose bib reading for adjustment. Needless to say he adjusted the hose bib to 80 PSI and therefore the gauge on the main water line always read 100 PSI.

Possible Expansion Tank Issue & Closed System
My home likely has a closed water system due to the presence of a backflow preventer valve associated with the in-house sprinkler and irrigation systems. This is likely why the expansion tank was installed with the tankless water heater.

Questions Implied
  • Is the leaking fluid from the Schrader valve a definite sign that the expansion tank has failed? If so, why might have that happened within 2 years?
  • Why the 20 PSI discrepancy in static pressure between the gauge on the main water line and the hose bib?
    • How do i know which pressure setting to trust?
Link of the high pitched sound of the shower head:
 

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Fitter30

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When having a problem i never trust a gage that hasn't been checked out by seeing if it goes to 0 psi and needle doesn't sticks or just use one gage.
Ex tank if the tank was bladder was compromised water would come out. Drain system down to 40 lbs put some air into tank with a hand tire pump and then depress schrader maybe can get so of the slime out. Then drain system down to 0 psi recharge tank. 80 psi water pressure is max for water pressure normally is 60 psi. Ex tank charge 2 lbs less the what your running.
 

Reach4

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During his pressure testing we noticed a discrepancy in pressure readings: the pressure gauge on the main water line inside the garage read 100 PSI (static), while the hose bib at the front of the house read 80 PSI. The plumber did not explain this difference, nor why he relied on the hose bib reading for adjustment. Needless to say he adjusted the hose bib to 80 PSI and therefore the gauge on the main water line always read 100 PSI.
The pressure measured in the garage is likely before the PRV.

While hose bibs are sometimes piped to before the PRV, I presume yours is not. The plumber would be unlikely to adjust the PRV to 80 psi, but instead would use something lower, such as 50 or 60 PSI.

Is the leaking fluid from the Schrader valve a definite sign that the expansion tank has failed? If so, why might have that happened within 2 years?
Failed. Your air precharge may have been low. Air precharge is normally set to the PRV adjustment pressure.

I watched your video of the showerhead for a while. I saw no event, but maybe I missed it. Why did you post that video?
 
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