Expansion Tank vs Static Pressure

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Kistle

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Hello, I am replacing a failed expansion tank on my water heater. I checked static pressure at an outside hose bib and found 115 p.s.i.

The maximum charge pressure of the Watts tank I bought is 80 p.si., and maximum working pressure is 150 p.s.i.

Seems like 115 p.s.i. is a high static pressure, which would cause the capacity of the expansion tank to be overwhelmed from the cold supply side, (as opposed to the hot side where the expansion capacity is needed.

Any thoughts? Should I just fill the expansion tank to ~75 p.s.i. and go with it, or should I check static pressure somewhere else?

Thanks in advance,

John
 

John Gayewski

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You need to have a pressure reducing valve installed before you replace the expansion tank. Your last one likley failed early and this one will too, due to the high pressure.


main-shutoff-01.jpg
 
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Kistle

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Interesting. That makes sense. Would there be an easy way to see if I already have a pressure reducing valve installed? I will Google some pics and see if I can find something like that already installed.

But I am guessing you are right, that that is the reason for the initial failure.
 

Reach4

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eems like 115 p.s.i. is a high static pressure, which would cause the capacity of the expansion tank to be overwhelmed from the cold supply side, (as opposed to the hot side where the expansion capacity is needed.

Any thoughts? Should I just fill the expansion tank to ~75 p.s.i. and go with it, or should I check static pressure somewhere else?
115 is not necessarily worrisome IF you recently took a shower and then stopped using water. When you use water, the pressure rise due to thermal expansion goes away.

Measure the water pressure while dribbling water. Use that number to do your planning.

It is also possible that you have a working PRV, but the outside spigot tees in before the PRV.

So dribble water, and measure pressures.

So after you study this, I would set the air precharge in the thermal expansion tank to the incoming pressure when the water use in the city is low and the incoming pressure would be near max. But dribble water while testing to be sure that you are not measuring the effect of the WH warming and expanding the water.
 
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Kistle

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The plot thickens. I found a Watts 25-75 PRV just about 6" downstream of the main shutoff ball valve for the house. I assume that the outside spigot would surely be "after" that, since that is the main source line for the whole house? Obviously I can test that, but seems odd that I have a 25-75 PRV, and am seeing 115 at the outside spigot?
 

John Gayewski

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If your hot water is expanding because the expansion tank has failed your pressure will rise. Pressure reducing valves (generally) only work one way.

Like reach says. Dribble some water to relieve the pressure and measure.
 

Kistle

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Dumb question, but do I dribble hot water, or cold water, or does it matter?
 

WorthFlorida

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Open a faucet with both hot and cold water dribbling. PRV's do fail and for many rebuild kits are available.
 

John Gayewski

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Shouldn't matter they are connected at the water heater. Cold water would be better. You really would just need to let some cold out then measure. You might see it spit with more pressure at first and then slow down. You could then just shut it and measure.

If you let more cold water onto the water heater it would then start expanding and be less of a reliable measurement.
 

Kistle

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Ok, so I dribbled both hot and cold water and measured the spigot outside. About 105 p.s.i. Here is a pic.

Water Pressure.jpeg
 
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Kistle

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I will say, my son had just taken a shower moments before I started dribbling, so not sure how much effect that has on this measurement, but it is consistent with what I measured the other day when no one had taken a shower.
 

John Gayewski

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Just measure it when the water heater isn't heating. If your burner is running then your not likely to get a good measurement.

Still sounds high. You want about 60 to protect your fixtures from undo wear.

Start looking for a prv rebuild kit or a new prv.
 

Terry

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The plot thickens. I found a Watts 25-75 PRV just about 6" downstream of the main shutoff ball valve for the house. I assume that the outside spigot would surely be "after" that, since that is the main source line for the whole house? Obviously I can test that, but seems odd that I have a 25-75 PRV, and am seeing 115 at the outside spigot?

Sometimes the outside faucet is before the PRV.
I would check at a washer shutoff inside the home or on the water heater drain. All of those have hose threading.
 

jadnashua

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If the ET is sized properly, even with expansion, it shouldn't rise to 115psi.

IMHO, it's cheap insurance to bump up the size of the ET from what some of the calculators say to minimize the pressure rise, and thus, extend it's life since it won't have to flex as much.

Getting to 115psi either means the PRV is bad, or the adjustment has moved. If you don't tighten the locknut properly, it can slowly walk out, allowing the pressure to rise over time.
 

Kistle

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Certainly Terry's suggestion of confirming the pressure measurement "inside" the house makes sense before starting to replace the PRV. Washer tap is not easy access. Any special tricks for checking the water heater drain? Assume leave everything on? I.e. the ball valve that supplies the water heater? Do I need to turn the heat to "pilot only" setting? Do I need to dribble a faucet anywhere?

Sorry for all the questions, but I am thankful to be amongst many experts here.
 

Reach4

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Any special tricks for checking the water heater drain? Assume leave everything on? I.e. the ball valve that supplies the water heater?
Attach the gauge before opening the drain valve. Close the drain valve before detaching the gauge.

Do I need to turn the heat to "pilot only" setting? Do I need to dribble a faucet anywhere?
If the WH has been heating, I would dribble a faucet. Dribbling a faucet won't hurt, even if the WH has not been heating.

Putting the WH into pilot only is not necessary, but won't hurt.
 
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