Trying to understand house plumbing

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Main water shut off (in house) > PRV with bypass > expansion tank (just replaced) > hot water heater. There’s another main water shutoff at the street.

When I turn the water main off in the house, it shuts off hot water everywhere, and only cold in one bathroom. Cold still works fine throughout the house with full pressure with the house main off. If I turn off the main at the street, ALL water shuts off.

I was going to replace the PRV because my house water pressure is a little higher than 80, but it seems like there’s no point now that the PRV isn’t in line with the rest of the faucets.

The water pressure fluctuates all the way down to 40psi very briefly, then shoots back up to 80-90psi. This occurs every 5 min. I’m on city water so that’s probably why, but I hear a noise from the PRV when this happens. Just sounds like rushing water for 5-10 sec.

I’ve already done a lot of troubleshooting so if you have questions let me know. Just curious if I should replace the PRV or just let it be. Curious if anyone has seen something like this before. Thanks!
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