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Thread: Water pressure,water hammer and hot water radiators

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    DIY Junior Member Mom7's Avatar
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    Default Water pressure,water hammer and hot water radiators

    I live in an 85 year old house that has a hot water boiler and radiators. I also have terrible water hammer when I use my LG washing machine. I tried using water arrester devices on the back of the washer, but they didn't work. Now I see that they should only be used with water pressure under 60psi. My water pressure is 90psi. If I put in a water pressure reducer for the whole house will I have enough pressure in my radiators to heat a three story house? Thanks for any help.

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    DIY Senior Member jim mills's Avatar
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    Yes. Incoming pressure has nothing to do with pressure generated by the boiler. Could be other factors causing the water hammer. A plumber should be able to help you.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A typical residential boiler only has 12-14psi of water pressure in them...maybe a little higher if you have lots of stories in the house, but nothing like the residential supply pressure. The two systems are typically not connected except through a special valve so that you do not pressurize the boiler anywhere near that high.

    If you add a PRV (code generally requires one when it exceeds 80psi), you also need to add an expansion tank as the PRV creates a closed system. When you heat water both in the boiler and for the house, it expands, and both systems generally need an expansion tank to keep the pressure from rising. TOday, it likely just pushes back out into the supply line, but the PRV will prevent that once installed.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator valveman's Avatar
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    At 90 or even 60 PSI your washing machine is just filling too fast. When the solenoid valve closes, it causes water hammer when the water crashes into the valve from high speed. If you choke back on the faucet that fills the washing machine, the machine won't fill as fast, but there won't be a crash when the solenoid valve opens or closes.

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    DIY Junior Member Mom7's Avatar
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    Quote Originally Posted by valveman View Post
    At 90 or even 60 PSI your washing machine is just filling too fast. When the solenoid valve closes, it causes water hammer when the water crashes into the valve from high speed. If you choke back on the faucet that fills the washing machine, the machine won't fill as fast, but there won't be a crash when the solenoid valve opens or closes.
    I did try turning down the water but it didn't help. I am going to have a pressure reducer installed. Thanks for your help.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Code generally requires the in-house pressure to be NGT 80psi. Note, when you add the PRV, you also need to install an expansion tank...the PRV makes the house into a closed-system. In an open system like you probably have now, when the WH runs, the expansion just pushes back some on the main incoming line, and nothing bad happens. The PRV prevents that from happening (which is why they call it a closed system). WHen you heat water, it expands, and it needs somewhere to go, otherwise, since the pipes aren't balloons, the pressure goes up as the water expands, and it will find the weakest link in the house to leak out - the expansion tank is designed to absorb that, then give it back when you turn on a faucet without leaking.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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