What is this?

Water pressure reducing valve. (prv)

Make sure that area stays accessible in case repairs are needed.

clawlan said:

Yea, it is really stiff when turning it. I might err on the side of caution and just shut off at the street instead.

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I agree, should be replaced with a brass ball valve at some point.

However, you say it's a brand new home so it's probably just stiff and needs exercising. Try spraying some wd40 and work it off and on a couple times a year.

Looks like it got painted and that's what might be making it stiff. Just a suggestion.

clawlan said:

House is about 15 years old but the prev owners were very poor/lazy DIYer's and jacked up a lot of stuff. I'll hit it with some PB Blaster and see if that loosens it up at all. I can turn it, but it is really stiff.

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Yea, see if that helps any and clean the paint and drywall mud off the handle and stem.

If not, check the packing nut that's right under the handle and make sure it's snug but not too tight or over tightened. Sometimes if it's over tightened you won't be able to turn the handle very well either. Once you get it freed up, open it all the way then back it off (close it) just a little so it doesn't seize up like that again.

Or, as suggested, replace it with a full port brass ball valve. You would have to turn the water off at the street to replace it. If you go this route, check with your local municipality first to see who is responsible for this. Then it's just a matter of coordinating with them, you and your plumber when you want to do it.

Up to you. You will eventually have to deal with it sooner or later as those gate/globe valves never seem to work when needed. Just a suggestion.