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# expansion tank/pressure/pumping away

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Member Posts: 18
Circ is not running, temp in boiler is 140* F, pressure gauge reads 25 psi on cast iron boiler, circ is on return line, tank is on discharge line. If tank is set to 12 psi, How can the gauge read 25 psi, assuming the gauge is accurate?

• Member Posts: 6,505
edited December 2018
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Doesn't sound like you're pumping away, but into (thru the boiler). If the gauge is correct, you have 25psi in the boiler/system.
Hopefully you checked the tank pressure isolated from the system (dropped all pressure in boiler or valved off and/or removed).

There was an error rendering this rich post.

• Member Posts: 18
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STEVE.. i haven't yet tested the expansion tank. If the tank is 12 psi can the boiler read 25 or is that impossible? I know that I am not pumping away. The circulator is not even running, the system is heating by gravity at this point. Perhaps with 25 psi there is enough flow created to satisfy demand. It's a one story home with a 60,000 boiler (probably only needs 25-30,000). Thanks for response
• Member Posts: 18
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what do you think about this? 25 psi is enough to force flow through system piping and still live in a 20 degree differential which signals circulator to not need to run.
• Member Posts: 23,650
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25 psi with the pump not running is a static pressure. It isn't forcing anything to go anywhere; if you are getting any circulation it's by gravity.

When the pump turns on, that tank pressure won't change. The pressure at the inlet to the pump, though, since it's all the 'way round the barn, will drop by whatever pressure differential the pump can create.

Now all that said -- if the pressure gauge is accurate, the tank is also holding 25 psi -- which means it has either lost it's air charge (but you would see that because the pressure would rise when the water heated up) or the system is simply overpressured by whatever feed system you have. If you were pumping away I would suggest trying opening a drain somewhere a bit and draining just enough water to drop the pressure to 12 psi. Since you are pumping towards the pressure tank, though, you should drop only to 15 psi.

And see if it holds. If it does, and you went to 15, the pressure tank charge should be adjusted to 15. If it doesn't hold, there is a problem with the feed device if any.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,956
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Reading the air pressure in the tank while there is 25psi on the water side will teach you nothing; it can only be read accurately with the water pressure at zero. If the tank was precharged to 12 psi like it should have been and still remains there, the water side should reflect that and also read 12 psi static. It's entirely possible to mismatch pressures in a system, the air bladder won't stop the auto-fill or valve operator guy from overfilling, BUT what this does is overcompresses the bladder and restricts it from being able to cushion the expansion- which then leads to premature failure of the bladder. If you have a purge valve between the system and tank, I would close it and relieve all pressure from the water side of the bladder. Then check air pressure with a low pressure gauge and reset to 12 psi if necessary. Then reopen the purge valve and drain enough water from the rest of the system to hit 12 psi. Pressure has little to do with thermosiphon/gravity circulation, so at this point you'd want to turn on the circ and watch the pressure gauge to determine what happens next. What make and model boiler is this?
• Member Posts: 9,546
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Simply put if you are sure the tank is set to 12 psi properly. By properly isolated from the system.

Then either the system is overfilled, or the gauge isn’t accurate. That system pressure needs to be at ambient when checked that means boiler also.

The bladder can handle 25psi. However it lowers the acceptance volume, and could cause the boiler pressure relief valve to pop at 30 psi when system is up to temp.

Try a psi gauge made up to fit the boiler drain, and check accuracy of boiler gauge.