High pressure, failed expansion tank vs pressure reducing valve

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  #1  
Old 10-21-16, 07:30 PM
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High pressure, failed expansion tank vs pressure reducing valve

Hello,
I have a failed thermal expansion tank which I am replacing. My home is 15 years old. Part of the job is charging the tank to the same water pressure of the house. When I measured the home water pressure in 3 places it read at 100 psi, obviously very high. My home does have a pressure reducing valve. Could the high home psi be due to the failed thermal expansion tank or does it also indicate a failed pressure reducing valve? Thank you for your experience and expertise.
Brian
Colorado
 
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Old 10-22-16, 03:57 AM
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I don't think your main line water pressure has anything to do with it. The thermal expansion you have ,Should work just fine.
 
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Old 10-22-16, 04:42 AM
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Is the water presssure 100 PSI much of the time? If you open a faucet and drain a quart of water and close the faucet, does the pressure rise back to 100 PSI within minutes? If so then your pressure reducer or regulator may need adjustment or replacement

After hot water is used and the heater cycles on and then off with no one using water, the expansion of the tankful of water can cause the pressure to rise significantly. A new expansion tank will fix that.

Drain another quart of water. Then with a pressure regulator or reducer on the incoming line the system pressure should be in the 40 to 60 PSI range with no one using water. Turn off the main water valve and open an upstairs cold faucet and then set the new pressure tank to slightly under this pressure. But don't go over the maximum pressure in the expansion tank instructions.

After everything is all set, the system pressure will still rise after a lot of hot water is used and then no one uses water for awhile, but the pressure will probably not get up to 100 PSI. The tankful of water is going to expand when heated no matter what the plumbing system consists of or what the condition of the expansion tank is. With your old failed expansion tank the system may have relied on slight give in the sides of the water heater tank for expansion purposes, which can shorten the life of the heater. If the tankful of water expands enough to occupy half of the air space in the expansion tank just prior to the last water heater cycle, the system pressure will double at least for a moment. After someone uses water, the pressure will drop back to "normal".
 

Last edited by AllanJ; 10-22-16 at 05:17 AM.
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Old 10-22-16, 04:50 AM
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15 yrs on an expansion tank is a full life
 
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Old 10-22-16, 04:57 AM
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If the water entering your home really is 100psi high pressure like that used to be more common. While it's not necessarily dangerously high it literally does put a lot of pressure on your plumbing. New houses and those undergoing plumbing renovation are required to have a pressure reducing valve (PRV) installed. This regulates the water coming into the house down to a more reasonable pressure.
 
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Old 10-22-16, 05:11 AM
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Q: Why do you not put an expansion tank on the hot line out of the water heater?
A: The next time someone uses hot water, the water that "expanded" into the expansion tank, which is no longer hot, will come out immediately and make it take longer for real hot water to get to the faucet.
 
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