Leak at T & P: pressure tank or T & P, how to diagnose?

[doughpat]

I've had this happen before: I notice water leaking from the T & P valve. I try "flushing" the TP valve a few times thinking maybe it's just some mineral buildup (and I do know they do 'wear out' after time). Doesn't help, so I replace the TP valve - no big deal.

Except....it doesn't fix the problem.

So, next step is to replace the pressure expansion tank. That does fix the problem.

The question is: how do I distinguish/diagnose which is the problem easily? (ie. without just "guessing" and hoping)

One thought was to measure the pressure of of the expansion tank....not entirely sure of the details of how to do this (i.e. shutting off water first? relieving pressure of the tank? what tool to use? what range of pressure is acceptable?)

FWIW its a bog-standard AO Smith 50 gal heater with a small vertically oriented pressure tank (original install, probably about 4-5 years old)

Edit: Also, if relevant: the nature of the leak is intermittent...i.e. I'm assuming it happens when the tank is recovering, because I never actually observe the leak (if I were patient, I'm guessing I could see it leaks as the temperature tops out). I just diagnose by putting a container under the output of the TP valve. Probably collects 1/2 cup or so of water in a matter of days.

 

[Fitter30]

Need a gauge that will attach to either to a outdoor hose bib or at the clothes washer valve to measure the water pressure. They open at 150 psi or 210°. They might start dripping at 140 lbs.

https://www.amazon.com/CARBO-Instruments-Pressure-Female-Pointer/dp/B07PGXSWF5/ref=mp_s_a_1_3?crid=2UKAXVY79H565&dib=eyJ2IjoiMSJ9.PNNkSdROPToTEBNotYe3qUFgrnnw3d8-_El1Wb54s8uu7QRVgua0TZae8nBMpyaeklbMVF-VkoTFEBQHMc2MTY1P6M9X2piMpqOUxZf06lOF2Bl1RlCkoG-WCy6h957rS34dy5VaqtzqSXTnVP0qJsSRiUP6FyqY3_0Wpp2InrY2za6cEyJ33LJbIy_Vu3-bJla2txZUacfto_j5qNC_BQ.4FieafDje4s_H3nXQiWM6X30cENsozVGYzxWubRN3w4&dib_tag=se&keywords=water+gauge&qid=1713794462&sprefix=water+gauge+%2Caps%2C198&sr=8-3

 

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[Michael Young]

I've had this happen before: I notice water leaking from the T & P valve. I try "flushing" the TP valve a few times thinking maybe it's just some mineral buildup (and I do know they do 'wear out' after time). Doesn't help, so I replace the TP valve - no big deal.

Except....it doesn't fix the problem.

So, next step is to replace the pressure expansion tank. That does fix the problem.

The question is: how do I distinguish/diagnose which is the problem easily? (ie. without just "guessing" and hoping)

One thought was to measure the pressure of of the expansion tank....not entirely sure of the details of how to do this (i.e. shutting off water first? relieving pressure of the tank? what tool to use? what range of pressure is acceptable?)

FWIW its a bog-standard AO Smith 50 gal heater with a small vertically oriented pressure tank (original install, probably about 4-5 years old)

Edit: Also, if relevant: the nature of the leak is intermittent...i.e. I'm assuming it happens when the tank is recovering, because I never actually observe the leak (if I were patient, I'm guessing I could see it leaks as the temperature tops out). I just diagnose by putting a container under the output of the TP valve. Probably collects 1/2 cup or so of water in a matter of days.

Based on what you're saying, you're likely going to need a new PRV and a new Expansion tank. Its leaking at night (when none of your neighbors are showering or using water). That's when you're going to get the highest city pressure. So very likely you need a PRV.

#1: Get a gauge from home depot. put it on your hose bibb. If your pressure exceeds 80-psi, replace the pressure regulating valve.
#2: measure your t&P. determine if you need a short shank or long shank. Now that's its discharged, REPLACE IT.
#3: press the schrader valve on your expansion tank. if you get water the bladder is shot. Replace it.
#4: check the label on your water heater. If it's more than 5 years old, so is the expansion tank. Over 5 years old. Replace expansion tank.
#5: Water heater has an expected lifespan of 8-12 years. If you're pushing into that age-out territory. Replace the heater.

 

[doughpat]

If I'm understanding correctly, a really simple first test might be to just see if the Schrader valve releases air or water. If it's air, it's likely ok. If it's water, then I can skip the PRV valve.

 

[doughpat]

Another quick question: if I release air form the expansion tank (to test it and see if I'm getting air vs. water), in theory I should replace that air with more (compressed) air.

To what PSI should I re-inflate that expansion tank to? I suppose measuring its current pressure and using that as a target would be a good place to start.

 

[Jeff H Young]

You have turn off water and bleed off water pressure to zero and pump air in to match your incoming water pressure EX PRV set at 60 then x tank gets 60 psi of air. with a bad prv you would need to determine how high the water pressure goes as the bad regulator allows pressure to creep up and perhaps go higher but a bad PRV needs replaced when did you install that ? And you gotta check water pressure you buy the test gauge yet ?