Boiler pressure changing. Can someone explain why this is happening

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Tcp

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So I have hot water heat. I was doing some pre season checks. System coldand off is at 12psi as set by the automatic fill. I have an isolation valve on my expansion tank with a drain so I can test bladder pressure. So here is where it gets strange for me. The system is cold and not running. I go to isolate the tank and drain after the valve to allow atmospheric pressure on the other side of the bladder. I test and adjust to 12psi air on expansion tank. But then I look on the boiler and the pressure has climbed to about 30psi water. I close the drain and open the isolation valve to the expansion tank and it drops back to 12. Now I just shut the isolation valve and the pressure slowly increases back up to 30. Open the valve and it drops back down.

So then I turn off the valve to water make up and then turn off the valve to the expansion tank and no movement. Then if I open the valve to the water make up the pressure rises again. But if I open the valve to the expansion tank it goes back to 12?
 

Fitter30

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Ex tank needs to be on the return/ suction side of the pump along with the fill line. Take some pics of boiler and piping. Also pressure reducing valve seems to be adding water when it's not supposed to. Drain boiler to 12 lbs with fill valved off. Open fill wait 15 minutes bet pressure raises. Drain boiler down to 12 lbs with fill valved off wait 15 minutes wheres pressure now still at 12 change regulator.
 

Tcp

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So if everything is on pressure stays at 12. Been watching it for a while now. I know my system isn't piped correctly. But has been running for two years. Circulators are on supply side along with fill/prv and tank. Return line has nothing and can't get any one knowledgeable locally.
 

Reach4

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It's a taco 3350-t3 I believe.
New cartridge would seem in order due to your very slow leak, but it's expensive. It would be a lot easier than replacing the device.
https://www.supplyhouse.com/Taco-3350-005RP-Replacement-Cartridge-for-Taco-Series-3350
BUT, https://www.supplyhouse.com/Taco-3350-T3-1-2-Cartridge-Pressure-Reducing-Valve is cheaper. It seems to me that you could take the cartridge from a new unit. I am not a pro.

You can also leave the valve that feeds the water closed until you need water.
 

Tcp

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So I ordered the new part but while I was waiting for it to arrive I decided to try and use the fast fill button a few times in hope of clearing anything that might have hung it up. It seems to have fixed it but if not I'll have extra part. Thanks to everyone who chimed in!
 
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