Measuring static pressure

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dean_a1

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Hi -

I have a city water system and am in the process of replacing my expansion tank. Step 1 of course is to measure static pressure in the system (in order to determine the expansion tank pre-charge). However, when I first measure the pressure from one of my hose bibs, it's 60 psi, but after a few minutes it slowly increases to 78 psi.
So the question is - what pressure do I use as the true static pressure when determining the pre-charge in my expansion tank?

Thanks for any input!
 

dean_a1

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Thanks for the responses! So the fact that my pressure slowly increases from 60 to 78 (takes about 3-4 minutes) is an indicator that my reducing valve is defective? That would be bad news - it's in a very difficult spot under the crawl space...
 

Jadnashua

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Unless your water heater was running at the time, trying to replenish a large amount of hot water you recently used...there's nothing else I can think of that causes the water pressure to gradually rise. Was this before or after you started to replace the expansion tank? It's also a sign the tank is defective or improperly filled with air.
 

hj

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quote; It's also a sign the tank is defective or improperly filled with air.

It COULD be, but not likely. The tank helps ABSORB pressure increases but CANNOT prevent them.
 

dean_a1

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I'm fairly sure the existing expansion tank is water-logged, and it's about 16 yrs old - so I am replacing it (haven't done that yet). So I'll pre-charge the new one to 60 psi as Terry recommended. If the pressure still slowly climbs to 78, then appears the pressure regulating valve may need to be replaced?
 

Terry

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I'm fairly sure the existing expansion tank is water-logged, and it's about 16 yrs old - so I am replacing it (haven't done that yet). So I'll pre-charge the new one to 60 psi as Terry recommended. If the pressure still slowly climbs to 78, then appears the pressure regulating valve may need to be replaced?

If you turn down the heat on the water heater, that would be one way to test for a bad PRV.
No heat, no pressure build up.
 
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