water pressure up and down

orgnovaJanuary 4, 2012

Water pressure is fluctuating up and down. Could the pressure regulator valve be going bad? Are they rebuildable?Could it be the water company lines? Thanks in advance.

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Could be failed PRV. They can be rebuilt but I prefer to replace them.

Could be thermal expansion if you don't have a expansion tank which is code required if you have a PRV. If you have a thermal expansion tank the bladder may have failed.

Read this thread... http://ths.gardenweb.com/forums/load/plumbing/msg121638422077.html?9 or you can always search the forum.

    Bookmark   January 4, 2012 at 3:15PM
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When you say 'up and down', from what to what?

    Bookmark   January 4, 2012 at 3:23PM
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Went to check the pressure in the thermal expansion tank and when I did, black water came out. I'm guessing the tank is bad. Checked the water pressure where it comes into the house and it was reading 80#s, which is kind of high. Would a bad thermal expansion tank cause high pressures? Thanks.

    Bookmark   January 5, 2012 at 7:35AM
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Just noticed I don't have a backflow valve before the thermal expansion tank. I'm guessing I should have one installed too.

    Bookmark   January 5, 2012 at 8:37AM
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If you got water out of the schrader (air) valve on the expansion tank then it has failed.

That is the test for a bad tank but the pressure should be set (checked) with the tank EMPTY of water.

What do you mean by "I don't have a backflow valve before the thermal expansion tank"? It would be smart to have a ball valve before the expansion tank so it's easier to replace it.

    Bookmark   January 5, 2012 at 9:51AM
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Code requires a full bore valve on the cold water line feeding the tank and when an expansion tank is installed it is required to be on the tank side of the shutoff valve.

    Bookmark   January 5, 2012 at 11:18AM
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I just finished refurbishing my Watts PRV: a rebuild kit was $50 and a new valve was in the $250 range. However, taking it apart to rebuild was a many-hour pain due to internal corrosion of a specialty nut inside. I would have replaced the valve with a new one but that wasn't easy either: the idiot plumber installed it so close to other lines that it couldn't be removed without first cutting out those other lines and re-doing them. I've since decided to redo this plumbing, but wanted to get the PRV functioning in the meantime, hence the rebuild kit approach.

    Bookmark   January 5, 2012 at 11:13PM
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Have you ever wondered why we don't install a pressure gage on a potable water system that is supplied from a municipal main?

The answer is that the normal pressure variatons from day to day are so great that a typical homeowner would drive himself, his plumber and the water supplier nuts complaining about his/her pressure.

Those of you who have a home well system know only too well that your pump is contolled by a pressure switch in your pressure tank and when the pressure falls to 40psi the pump starts and continues to fill your pressure tank until it reaches a cutoff point at 60psi, yet even with that 20psi differential in pressure there is very little noticable difference in how the fixtures perform.

For a municipal water distribution system the line pressure on the municipal mains is not generated by pumps, but rather it is generated by "Vertical Static Head" pressure from water storage towers located throughout your community, as a consequence the actual static head pressure at your main water valve is determined by the vertical height differential between the elevation or your main water shutoff valve and the actual top surface level of the water in the storage tank.

The municipal storage tanks are then fitted with a float control that works in the same manner as the fill valve in your toilet tank. When the water level in the tank drops to a certain level pumps are started which lift water up to the storage tank. Now generally the storage tanks have enough capacity that in a worst case scenario they can supply the theoretical load for the area they serve for two or three days, which accounts for why you still have full water pressure even in instances when some dissaster has wiped out the electrical power in your community.

Even though the tank may be able to supply water for two or three days, in most regions the pumps supplying the tanks are set up to top the tanks off at night. In this manner your municipal supplier can save money by operating the pumps during off peak hours, thus getting a reduced electrical rate while still maintaining a close average pressure on the system.

It must also be understood that the water tower system is designed to provide basic vertical static head pressure to the majority of the community it serves, BUT, if you have the mansion on the hill overlooking the town, the vertical differential from your house to the top of the water in the tank is less, ergo, your line pressure is less and you may in fact have to install a boost pump to maintain code minimum when the water level in the tank is low.

On the other hand, if you live at the bottom of the hill your vertical differential is much greater than the majority of the community thus your pressure is higher and you would need a PVR to prevent over pressure when the tank is full.

I have prepared a simple illustration that may help you see how this effect works.

    Bookmark   January 6, 2012 at 1:46AM
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That was a timely and informative post. After I rebuilt my PRV last week, I put a gauge on the system to monitor the pressure. I had the PRV set for a pressure reading of just under 70 PSI, but in the morning the floating, second indicator needle on the gauge showed that the pressure had had a peak of 8-10 PSI overnight above where I thought I had the PRV set. At first I was thinking that there was still something wrong with the PRV. Then I realized that the PRV doesn't manage to a single set point, it simply provides a certain amount of reduction at a particular setting, and if the city provided water pressure increase, so will the gauge reading. You're right: until I figured this out I was driving myself nuts....

    Bookmark   January 6, 2012 at 2:46AM
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One more thought on variations in recorded pressures. I had been thinking that all of my variation from the pressure I was wanting my PRV to maintain was from city supply pressure fluctuations. I have since realized that the functioning of the thermal expansion tank can also raise the pressure by several PSI. The expansion tank provides a safe place for expanding water to go so the water doesn't raise the pressures really high in a closed system. However, the expanding water compressing the air space in the tank still does raise the overall pressure (which falls back off as the water cools).

    Bookmark   January 7, 2012 at 5:23PM
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