Boiler question.

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Vinny

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hello everyone. I’m a plumber for many years but mostly work in new construction projects. My hot water boiler safety relief valve was leaking. I noticed it stopped leaking and the pressure dropped to 5 PSI. I opened the pressure reducing valve and it brought the pressure way past 30 and was climbing to almost 70 psi. I shut off the prv and it was still feeding water into it. So I shut off the my water feed, and knocked the pressure down to about 25 psi. Should I change the PRV as well as the relief valve? I’m not too familiar with the pressure reducing valves and how much they need to be opened.
 

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Dana

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Unless the boiler is in the basement of a 5 story building it should be set up to run at 25psi. Measure the vertical distance between the pressure gauge on the boiler and the highest point of the top floor radiator in feet, then set up the system to run at 3psi + (0.455 x vertical distance).

The pressure relief valve may have picked up some grit when it opened up. Sometimes grit can be cleared by opening it manually for a very short blow-down, after which it stops dripping.

The system's expansion tank needs the air pre-charged to the system pressure before the system gets fully filled & pressurized. If that is not done it may not have sufficient expansion volume to accomodate the expansion of water when the system heats up. There are multiple threads on this forum covering the topic of how that's done.

The "auto-fill" pressure reducing valves are a solution-problem. They too often seep, overpressurizing the system. While it's convenient to have when filling/pressurizing the system, it's generally better to close the isolation valve once the system is properly filled, avoiding the problems created when the thing inevitably fails.
 

Fitter30

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Water height 2.31' per pound 10 lbs= 23.1' with boiler and pump off, expansion tank piped on suction side of the pump. From the base of the boiler to the height of the highest heat emitter take that measurement divide by 2.31 add another 3-5 lbs. Thats a full system with everthing blead. Bladder tank set air pressure same as water pressure, plain steel tank if its full of water when boiler heats the water pressure in system will rise rapidly. System will have to be drained if there isn't a shutoff in tank line to drain tank and drain valve in tank.
 
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