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Boiler Pressure Spike while under heat

Chris72Chris72 Member Posts: 4
Hi All,

My Oil heating system has been spilling water from the PRV for the last few days. After some observation, it only happens while the system is heating. The pressure remains constant (10-15psi) while at rest but when heating, as the water temperature increases toward 170 degrees the pressure slowly rises to and past the 30psi threshhold causing water to leave through the PRV.

While doing some research on lunch at work, I assumed it was a bad expansion tank so I ran out and grabbed one. However, I inspected my tank at home and it seems fine, not waterlogged as I expeted it would be. It's only hot on the very top and there is air in it. I haven't checked for the exact air pressure in it though. I have started to doubt it is the tank.

My system is less than 10 years old and last year I replaced the pump and PRV.

Thank you for your help.


  • JeffMJeffM Member Posts: 159

    Those symptoms sure sound like a bad expansion tank, so keep looking at that. Check the pressure at the air valve on the tank, I would bet there is none. I'd try taking the tank off and checking it out thoroughly.
  • Chris72Chris72 Member Posts: 4

    Thanks for the reply Jeff

    I will certainly look at the tank first. In fact I'll probably change it out regardless. The tank does only get hot on the top and is not watterlogged. Would the tank fail in another way? Could it just be too little air pressure in the tank?
  • billtwocasebilltwocase Member Posts: 2,385
    Might still be bad

    I've had some that the diafram would not compress. Had air, but wouldn't release from the inner sides of the tank. I would first take the pressure off the boiler, to zero, then test the air pressure. Should be 12-15 with zero on the boiler side. Also, any change to piping? Is the tank isolated between a flow check and zone valves?  Pics will help
  • Chris72Chris72 Member Posts: 4

    Thanks Bill

    I'll definitely try that. There's been no change to the piping and the tank is off on the side, kind of hard to describe, a picture would be useful so, I plan to work on it tomorrow so I'll see where it goes and take a few pics.
  • billtwocasebilltwocase Member Posts: 2,385

    It may be unlikely, but does the boiler have a tankless coil in it, or have an indirect? May be a pin hole, or even a bad feed valve
  • Bob HarperBob Harper Member Posts: 732
    prv release

    Undersized or failed exp tank. If not on an isolation valve, install one so the pressure can be tested with it isolated from the system without dropping the system. If you get water out of the Schrader valve, the diaphragm is blown and the tank needs to be replaced. The charge should equal the operating pressure of the boiler.

    Failed feed valve. Isolate the valve to test this. If bad, clean strainer first.

    Bad PRV. Test annually and replace every 3 yrs or as needed.

    Yes, leak from hot water coil but this would be constant as would bad feed valve. PRV release on firing generally from undersized, waterlogged or otherwise inadequate exp. tank.
  • earl burnermannearl burnermann Member Posts: 126
    Lost it's charge

    You could take the tank off and check the pressure with a tire gauge. It may have lost its air. Would recommend replacing it in this case. I've pumped a few back up but they just loose their air again in a few days.
    If the women don't find you handsome, they should at least find you handy!
  • billtwocasebilltwocase Member Posts: 2,385
    I wouldn't

    add an isolation valve. Although nice when replacing an extrol, by just closing it with pressure on the system, it is only trapped in the tank, and you won't get an accurate PSI reading. Shut the valve off, and remove the tank. You will see what I mean
  • WeezboWeezbo Member Posts: 6,232
    edited December 2013
    a blow out gun co2 cartridge ,

    with an adapter to a shrader fill fitting can change the tank pressure up , while in place, about 3 lb's .

    if a tank has the diaphram "moored " even pressurizing it to 28 psi and releasing it may not bring results , usually , it makes a distinctive noise . that only means that it was moored . on those usually you are also working some guberage loose, so how much may depend on what type of water you are dealing with or fluid ..if the system fluid is clean ,i think , it is safe to let the air out one more time and refill it and let it roll.

    what i did one time when one was stuck is i cut the entire tank apart to see what was it that produced the mooring and i could see a light black almost soot type mixture coating the sides ..

    thats prolly when i had way too much time on my hands and wanted to understand every thing there was that caused certain reoccurring problems with products ..

    to think that back then a 30 was a fraction of what it is these days ...

    oh this may help too . the travel , prv strainers if at a distance to the supply house do cause problems. while there are plastic parts made for the bottom of strainers they are not so easy come by ...if the screen is plugged that may only indicate the fast fill is the culprit.. so , by watching, while you clean the screen with the fast fill valved off look for and listen .... as air eliminators can be installed in the wrong places ...and become addmitance valves...older Grundfoss circs also have the rather dubious screw fitting to eliminate air and lube a wet rotor they have an o ring that is also not easily found on the shelves of many wholesalers so be careful ,... if any circs are growling ...and you loosen them to get the air out of the system of the circ....

    coils can indeed have particulates that dependent upon size and water pressure work as an occasional two way ,valve and sometimes cork, as pressure on the potable re wedges it into place...

    hope it helps someone if not you.

    *~//: )
  • Chris72Chris72 Member Posts: 4
    Mission Accomplished

    Hi All,

    So I swapped out the expansion tank, which was certainly more difficult than it should have been (limited space and on way too tight with what appeared to be an sealant rather than t-tape). but brains and brawn prevailed. It turns out that the tank, although it did have air in it was still nearly filled with water. With the new tanks installed, the system worked perfectly.

    Thank you all for the advice and have a Merry Christmas!
  • billtwocasebilltwocase Member Posts: 2,385
    Merry Christmas Chris

    good luck to you
  • Ron Jr.Ron Jr. Member Posts: 527
    Had that happen too

    with new expansion tanks . The diaphragm gets stuck to the sides and it doesnt pull off when you fill the boiler . We always listen for the small pop when filling the system . 
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