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Wondering why the cold water comes out with full pressure but warm is at 1/4 of the pressure?

What are all the things I can check?

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  • Is the pressure 1/4 in multiple fixtures? Is the flow the same at other locations, kitchen , other bath ? – Ed Beal Mar 11 '20 at 2:07
  • No just that bathroom – Kevin Calf Mar 11 '20 at 2:14
  • Is it a new house, or older? Are the pipes copper, galvanized (steel), PEX (plastic) or something else? More specifically, is there a mix of different kinds of supply lines (especially copper+galvanized)? Do you have easy access to the plumbing, or is mostly closed up in walls? Lastly, when you first turn on that fixture, for just a brief instant, is the pressure good, and then almost immediately it falls off? – Z4-tier Mar 11 '20 at 2:59
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  • I’m curious if you know, if this a new condition or has it been like this as long as you’ve known it? If it has just changed to lower pressure, that is a clue – Old Uncle Ho Mar 11 '20 at 13:15
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If it's only that one sink it very likely is caused by corrosion or blockage between the shutoff and the faucet. The hot water side is especially susceptible to scale buildup. There are a few basic things to check:

  • First make sure the hot shutoff valve is fully on.
  • If the shutoff is old it could have scale buildup in the shutoff valve. Turning it on and off a few times will sometimes free it up.

If the hot is a separate faucet:

  • Remove the aerator from the faucet. Clean the screen of any debris.

  • Turn off the hot shutoff valve and remove the hot water cartridge and check it for scale buildup.

If there is still a flow issue I would replace the shutoff valve and/or the hot water cartridge starting with whichever one looks most suspect. This should solve your problem.

Edit
Another possible cause, but less likely, is galvanic corrosion which happens when two different metals are connected such as galvanized pipe coming out of the wall connecting to copper. This requires a special type of connection (dielectric union) to prevent corrosion.

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