What is failing? TRP valve or expansion tank or both?

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Neil.Steiner

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[Disclaimer: I posted a very similar question at Stack Exchange Home Improvement, but then wondered why I didn't go straight to the experts on this site.]

Do water heater TPR valves fail gradually or catastrophically? A day or two ago, I noticed about a capful of water in the pail below my PTR valve outlet. The only recent change to my system is that I used to have my washing machine hot water faucet connected to a pressure gauge instead of the washing machine, and I instead removed the gauge and reconnected the washing machine hose. And now the pipes give a conspicuous knock when any valve is opened or closed too quickly—the washing machine being the prime culprit.

I opened the TPR valve to test it and wasn't entirely happy with the way it closed, so I opened and closed it again and found that it stopped dripping much more quickly the second time.

I then shut off the water supply to the hot water system (my water heater and my expansion tank), opened all hot water faucets, and checked the air pressure on my expansion tank. The pressure was 28 PSI instead of the original and expected 60 PSI, so I'll have to address that. And the expansion tank is now six years old, so it's probably time for preventative replacement. (Never mind the fact that the water heater must be 25 years old and still kicking, perhaps thanks in part to annual flushes and anode replacements as needed.)

Then I reopened the valve to the hot water system and closed each of the faucets, and the TPR valve spit up some water when I closed the last faucet. I'm not sure what the pressure was the moment the system became closed, but I'm sure it wasn't the 150 PSI that the TRP valve is rated for, nor probably much higher than the 60 PSI coming off the PRV from the water main. But all of the sudden, I'm not sure whether I should suspect the TPR valve or the water expansion tank or both.

I don't know why the washing machine causes the loud noise when its hot water valve opens and closes, nor how the pressure gauge that used to be connected to the system would mitigate that same effect from the cold water valve. Can any of you offer me any insight into what is likely happening?
 

Reach4

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Do water heater TPR valves fail gradually or catastrophically? A day or two ago, I noticed about a capful of water in the pail below my PTR valve outlet.
The T+P valve opening is usually due to thermal expansion. After you use hot water, and stop using water, the water expands. If there is a one-way valve on your water, and there is in many case, you need a thermal expansion tank. Those should usually be empty of water due to an air precharge as high as, or a little higher than, the normal water pressure. Those can fail, get waterlogged, and need replacing.

The valve should close on its own after the pressure has fallen back below 150 psi. If they do not close on their own due to deposits or whatever, they need replacing. The test lever mainly tests for whether the valve closes back up when opened.

I don't know why the washing machine causes the loud noise when its hot water valve opens and closes, nor how the pressure gauge that used to be connected to the system would mitigate that same effect from the cold water valve. Can any of you offer me any insight into what is likely happening?
The sound when the valve at the WM shuts off sounds like water hammer. It is moving, and then suddenly stopped. The momentum of the water produces a bang. A water hammer arrestor at the WM should solve the bang when the valve closes. I don't know why you get the bang when the valve opens.
wh_660hb.jpg
 

Neil.Steiner

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Reach4, I may have misspoken when I said that I get a bang both when the washing machine closes and opens its hot water valve. I haven't actually paid close enough attention to know if it happens when the valve opens.

But as to the TPR valve, is it really likely that the pressure could have exceeded 150 PSI when I was refilling the hot water system and closed the last faucet? With the pressure gauge that was previously in place, the resettable maximum pressure needle never got as high as 100 PSI over years of use. Thermal expansion wouldn't have been a factor at that time since I had temporarily turned off the circuit breaker for the water heater. The only source of pressure would have been the PRV-regulated 60 PSI coming into the hot water pipes and tanks (WH and ET).
 

Reach4

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But as to the TPR valve, is it really likely that the pressure could have exceeded 150 PSI when I was refilling the hot water system and closed the last faucet?
The pressure would not rise as you were refilling, but after all water use stopped, and the water heater is heating the water. Then pressures at 150 are expected. The only reason it does not go much higher is the release of water by the T+P valve.
 

jadnashua

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Two things here...you may not normally be using lots of hot water such as when you opened all of your hot valves around the house. This would end up using a lot of the water in the tank. Second, unless you pumped up the pressure in the ET (and it held), just the normal water pressure would conpress most of the air behind the bladder in the ET, and there would be minimal space for thermal expansion. So, yes, during the reheating of the water with little room in the ET, you could quickly peak the pressure high enough to cause your T&P valve to open to relieve it.

While installing a hammer arrestor on the WM shutoff usually works, ideally, they are installed as close as possible to the offending valve in the WM. IF that doesn't dampen the hit enough, install it on the WM, and run the hose to it verses installing it on the valve and then the hose to the WM.
 

Neil.Steiner

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Reach4 and jadnashua, thank you for your explanations. Let me carefully explain what happened after I checked the ET air pressure, because it probably tells me what is failing in my system.

At that time, all of the hot water faucets in the house were open, except for two of three on the first floor (a.k.a. basement) that were lower than the top of the WH tank. (I wanted to let the pressure out of the how water system without forming air bubbles.) I opened the ball valve leading into the WH and ET, and water began coming out of the open faucets. Silly me, I think I began closing all the hot water faucets in reverse order from what I needed to do, starting at the third floor and working my way back down to the first floor. Because of that, the last faucet I closed was in the same room as the WH, and when I closed it, the TPR valve spit up some water.

I positively know this was not a thermal expansion effect, because I had turned off the circuit breaker for the WH. Cold water from the water main was coming into the WH, but the heating elements weren't powered so they couldn't have been causing any water expansion. In other words, the only things (to my non-plumber mind) that could have affected the TPR valve when I closed the last faucet are the 60 PSI average pressure coming from the PRV, and any transient overpressure from the flow of water suddenly stopping. I wouldn't imagine the transient pressure could hit 150 PSI, and that leads me to suspect that the TRP valve must be faulty.
 

jadnashua

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The spring on a T&P valve can wear out if it is exercised a lot. The more common problem is mineral deposits on the stem and it either wont' open at all, or won't close fully.

Unless your PRV failed, and the incoming water pressure is way high, I don't think the pulse of shutting a quick acting valve would cause the T&P valve to spike high enough to open up. The ET will act like a buffer, too, limiting the effect. Sounds like it's time to dig out the gauge and check your pressure. If you didn't recharge the ET, so that, too. Be ready to replace it, as if your static pressure is 60, and the precharge is less than half of that, you've stretched that bladder quite a bit, and that will shorten its life. Ideally, the precharge is essentially your static pressure or a little less so that the bladder is normally in its 'neutral' position, and won't be stretched all that much if it is sized properly due to thermal expansion.

Note, to check your water pressure, you can do that on your ET with your tire pressure gauge...once the system is pressurized, the air pressure will be the same as your water pressure. If you didn't get water out of the ET when you checked the pressure, it has not failed yet. They don't last forever, though.
 
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