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enter image description here This is on the water main for the house, just downstream of the main shutoff valve. Water is going up in the picture, as the arrow shows. Is it a strainer? A check valve? Pressure regulator? It appears to say "Parts No. 135CM2" but I couldn't find anything matching that on google. I cannot read any part of the logo so I can't search on that.

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It's a pressure regulator. The screw at the top turns in (clockwise) to increase the pressure, and turns out (counterclockwise/anticlockwise) to decrease the pressure. The plug at the bottom (adjacent to the screw) can be where a pressure gauge goes for the low pressure side, but it's plugged off. The other plug towards the input is where you can loop back into the high pressure side.

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    Like most pressure regulators, turning the screw in (clockwise) increases pressure. Turning counter clockwise will decrease pressure. – mikes Oct 2 at 9:26
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Definitely a water pressure regulator. I had one of these installed when I bought my house because the water pressure in home was 100psi and I was told by a licensed plumber that plumbing supply is engineered to sustain 80psi max before things like leaks begin to develop over time.

In my area building code dictates that a thermal expansion tank must also be added to the water heater when a regulator is installed. The pressure regulator creates a closed water circuit whereas before it would have been an open water circuit regulated by whatever pressure I had delivered by my water company from their main line.

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    That's right. Before you add a regulator, the water pressure in your pipes can equalize with the whole street network. Now, local pressure surges can go to infinity pressure, and break pipes. Such as water moving through the regulator suddenly stopping when a valve closes, the moving water still has inertia and still moves forward, creating a pressure spike. – Harper - Reinstate Monica Oct 2 at 18:13
  • @Harper to infinity pressure? Wouldn't the pressure relief valve on the hot water tank just dump water out once the local side reached a pressure past the set point? – Fresh Codemonger Oct 2 at 22:57
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    @FreshCodemonger The effect is called water hammer. If a valve is closed suddenly, all the water flowing in the pipe stops. That means it looses it's kinetic energy. Where does that go? The pressure increases so much that the pipe expands, and the deformation of it converts the energy into heat. (There may be other effects). If the pipe is not able to expand because it is made of metal it expands anyway, a little. And the highest pressure is at the valve, very sudden and enough to expand the pipe. – Volker Siegel Oct 3 at 1:44
  • Whether it's effective or not I don't know, but when the plumber we contracted for getting a shower / hot tub upgrade was running the pipes, he added 'T' junction close to all the faucets, with a pipe pointing up about a foot long that was then capped. Apparently it creates an air pocket that is supposed to reduce the effect of water hammer. – dgnuff Oct 3 at 17:12
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judging by the shape it's a pressure regulator, the screw at the tip of the cone section adjusts the pressure.

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