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Thread: Water pressure loss when using two faucets

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    DIY Junior Member jskd82's Avatar
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    Default Water pressure loss when using two faucets

    I just bought a home and notice that the pressure is ok when I have one faucet running but when i flush the toilet or turn on another water source the water pressure drops. I am connected to a county water line and the water main is running thru my property. The house was built in 1975 and it does have a pressure regulator....it may have two if I am not mistaken. What could be some possible causes? Also the shower pressure isn't really all that great when it is running by itself. I need to change the shower fixtures and faucets as one of the shower knobs is stripped and water is dripping because it can't fully close. Thanks for any input.

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    In the Trades Gary Swart's Avatar
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    The first thing you should do is get a pressure gauge. You may be confusing low flow with low pressure. At this point, you are guessing about how much pressure you have. Check both sides of the PRV. It could be the PRV is shot or in need of adjustment. Since you have a PRV, there should be an expansion tank between the PRV and water heater. The pressure in the expansion tank should be the same as the PRV is set to. Next, what kind of pipes are in your home? If they are galvanized steel, they very well could be corroded nearly shut and needing replacement. Are there valves in the lines that could be partly closed? These are some of the things you need to do/check.

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    DIY Junior Member jskd82's Avatar
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    Thanks for the quick reply. I'm not sure what an expansion tank is and I don't believe that there is one installed in my house. I am not sure what kind of pipes they are but I would guess that they are galvanized.... I will try to take a look next time i'm at the house. It is a slab foundation house and all the pipes are in the foundation so I would have to run all new piping and have it go up under the roof. Would I be able to do the new piping change myself or is it really hard to do? I can do soldering but not sure what other major things need to be done to replace the piping...... I will see if I can find a gauge and hook it up to the washing machine hose, is this a correct way to test pressure? also is there any way to tell if the piping is corroded other than cutting the piping? Thanks. Btw I live in Hawaii so maybe we don't use expansion tanks here?

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    Commercial Plumber markts30's Avatar
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    If your piping is galv. then your problem is most likely restricted flow due to corrosion on theinside of the pipes...
    The galv piping corrodes on the inside and causes a severe restriction in ID and flow resulting in exactly what you are describing...
    Usually your only cure is a re-pipe

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    DIY Junior Member jskd82's Avatar
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    Ok I went and checked the piping and it turns out that it is actually copper piping. I bought a pressure gauge and tested the pressure on one of the washing machine spigots and the gauge read 52 psi. I didn't think of turning on another faucet and see what the pressure dropped to until now lol. Anyways what could be causing this problem since we have ruled out galvanized piping? I should also add that the water heater is over 20 years old....could it be something to do with the water heater? Thanks.

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    DIY Junior Member jskd82's Avatar
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    anyone got any theories?

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    Forum Admin, Expert Plumber Terry's Avatar
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    I think Gary had it right when he mentioned the PRV valve going bad.
    I've been replacing a lot of them lately, for exactly the same symptoms that you have described.

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    DIY Junior Member jskd82's Avatar
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    ahhhh ok thank you very much for the reply. I will try changing the valve when i get a chance and give an update as to what happened.

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    In the Trades Gary Swart's Avatar
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    An expansion tank is a cylindrical tank that is installed in the main supply line after the PRV. It has an internal bladder and is air charged to the same pressure the PRV is set to. The reason for this is that a PRV has a check valve inside and creates what is termed a closed system. The reason this tank is necessary in a closed system is because when the water heater heats water the water expands. In an open system, this expansion is absorbed by the water main and causes no problem. However, in a closed system, the expansion has no place to go and causes a rapid and extreme rise in pressure in the water heater. The expansion tank gives this water a place to go. Usually the increased pressure is enough to trip the TP valve on the heater.

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    Licensed Grump GrumpyPlumber's Avatar
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    53 psi isn't terrible, the norm is 60, max 80.
    I'm with Gary & Terry, my guess is on the PRV not working right.
    "The biggest regrets we have in life are the chances we never took."

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    DIY Junior Member jskd82's Avatar
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    Thanks guys I changed the PRV today and now the pressure is fine when running multiple faucets.

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    Plumber Cass's Avatar
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    You must install an expansion tank if you don't have one or you will be replacing your water heater sooner than you want to.

    An expansion tank is a must when you have a pressure reducing valve regardless of weather you had one before or not.

  13. #13
    DIY Junior Member jskd82's Avatar
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    Not sure if it matters but I have a tankless LP heater....and as stated earlier no one here in Hawaii that I know of uses any sort of expansion tank

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