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30 lb T&P is Relieving at 33, yet boiler temp is only 175

ETET Posts: 11Member
Looks like 30 lb. T&P is performing as it should.

Seems odd for this residential boiler to be running much above 25psi at these temps.

Aquastat is pretty much ruled out as we just have too much pressure at this temp.

Tips appreciated.

Possibilities include the flue which appears clear, internal unit cleaning which has been done (not certain how well), and I guess there is always the chance the circ pump is delivering lower than spec'd gpm. It circulates seemingly well enough, but just have too much pressure for the temp.


  • 4Johnpipe4Johnpipe Posts: 479Member
    What is the cold start pressure? Is they expansion tank set at the correct pressure 12-18 PSI for most residential applications is normal. Is this a new problem or was some work recently done to the system?
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  • ETET Posts: 11Member
    The B&G water feeder is regulated to 15 psi. The street pressure is about 45

    The expansion tank is 65 years old and high on the wall.

    Never been touched. I guess it's time.

    I guess you drain it, pre charge with 12 psi of air, and then open the water supply?

    Never done one without a bladder.
  • JUGHNEJUGHNE Posts: 5,652Member
    Tank is water logged. If your lucky you can isolate the tank from the system and drain tank empty. Atmospheric air pressure into the tank is all you get for "pre-charge ". Put tank back into the system and your fill valve will pressurize the head of air.
    Any air removed from the system should go into this tank. If any air eliminators are added to the piping than the tank will waterlog quicker. There is probably an "air scoop" near the boiler that directs any air into the tank.
    If you bleed air out of the system you will eventually bleed air out of the tank.
  • kcoppkcopp Posts: 3,324Member
    edited June 2015
    Boilers have Pressure Relief valves only...T&P valves are for tank type water heaters. So the PRV could care less as to what the boiler temp is. You either have a tired PRv or your expansion tank is water logged. There should be a drain on the tank.
  • george_42george_42 Posts: 60Member
    if there is a domestic coil in the boiler, a pinhole in it will also cause the relief valve to go off
  • MikeL_2MikeL_2 Posts: 178Member
    An indirect water heater is another place where boiler and domestic water can mix..............
  • ETET Posts: 11Member
    This system has no domestic hot water.

    We'll drain the expansion tank, and refill the system and see what happens.
  • Tony MassiTony Massi Posts: 86Member
    If there is no indirect or coil then it can be the feed valve is over filling the boiler or your not draining the expansion tank all the way till its completely empty. If you don't put back that cushion of air into the tank you'll have the same problem. If you have the same thing after you drain the tank the feed valve is over filling, it could have some dirt in it.
  • ETET Posts: 11Member
    I think we can eliminate the feed water question if we shut off the feed valve after the system is full, and at that point we should just see 12 psi on the system, then we'll open the valve to the expansion tank and test run.

    As poster Jughne says, it appears that the old expansion tanks used in the 1950's just held ambient air, and that the water filled boiler system ultimately pushes against what was just ambient air pressure in the tank. That could explain why this unit has no air fittings!

  • JUGHNEJUGHNE Posts: 5,652Member
    With the feed valve off and you open to the air filled expansion tank you will only pressurize up to the height of water above the tank. Opening the feed valve set at 12 or 15 will pressurize the air tank to that amount. Then you could shut off the feeder. Fire the boiler and see what happens with the PRV.
  • ETET Posts: 11Member
    Good luck and bad luck.

    We drained the old expansion tank, reconnected the tank. Then opened the valve to the expansion tank. Somehow we started with 0 psi on the system gauge.
    We fired the boiler and somehow this time even with the feeder valve open, the pressure stayed at zero for 10 minutes. I know that sounds like the feeder valve regulator, but I somehow doubt it.

    Not trusting that was going to be the reliable result, we tried it again the next day. This time we started with 12 psi on the boiler, but did not drain the expansion tank again. On the second trial, the pressure went to about 33 although the temp never went over 190. So we still have the problem. It's hard to imagine the expansion tank could water log that fast.

    Perhaps there is another step to the process other than draining the expansion tank then opening the expansion tank valve and firing up the boiler?
  • JUGHNEJUGHNE Posts: 5,652Member
    Could you post a picture of the tank showing the drain valve on it and a picture of the boiler connection to the tank. (step back for the second one to include the boiler & tank together)
  • JUGHNEJUGHNE Posts: 5,652Member
    edited June 2015
    ET sent me these pictures. Has anyone ever seen an air tank like this? Or the rather unusual copper tube (3/8 or 1/2 OD)connection to the system?

    Hopefully the pictures attach

    Could this be one of the first bladder membrane expansion tanks?
    ET says it is 65 years old. I thought I saw a picture of the first Extrol tank made and it resembled this?

  • kcoppkcopp Posts: 3,324Member
    That is a Diaphragm tank. Actually looks like a European Zilmet tank you find in a wall hung boiler. I would be swapping it out for a Amtrol type tank. You will need to do some re-piping.
  • JUGHNEJUGHNE Posts: 5,652Member
    ET, when you dated the system at 65 years, we assumed that you have a plain steel compression air tank for the thermal expansion of the water. Typically hanging from the ceiling in the boiler room.
    But from the pictures it is some type of pre-charged diaphragm tank. If it was in good shape and you went to drain it you would have no water come out of the tank. So the air side of the diaphragm/bladder has leaked and is waterlogged. The best guess is to replace it with a pre-charged Amtrol type expansion tank.
    Then you must also have some type of auto air elimination to remove all from the system as no air will go into the tank as previously mentioned

    What type of radiators do you have...old style upright cast iron or base board copper tube. How many stories tall is the house?
  • ETET Posts: 11Member
    Thanks for posting the picks Jughne.
    This unit is in a one story home and it's baseboard copper tube.

    There already is an air eliminator in line at the rear of the boiler, just as the hot water distribution lines leave the unit.

    Are you saying that we may need a second air eliminator if we install an Amtrol style of tank? If so are you saying that needs to be inline in the supply line to the Amtrol or must it be inline with the primary circulation of the hot water system??

    The current expansion tank is simply teed into a branch of the drain line and though piped into the boiler system, it is not in the direct path of circulation, but more at a dead end. The easiest place to install an Amtrol unit would be into that same 1/2" line. Does that sound like reasonable approach?

    Other Amtrols we've installed have usually been right after the water feeder and again not in the main flow of hot water circulation.
  • JUGHNEJUGHNE Posts: 5,652Member
    We now know that you already have an Amtrol type of tank on the system, it is just waterlogged and most likely not repairable.
    If you have that air eliminator there that should be all you need.

    If you connect a new expansion tank where the old one connected, the system should work like it did before. (just bear in mind that the tank will contain some water while operating and will be top heavy if just screwed into the existing fitting.)
    That is not the ideal location but will probably suffice unless you want to do major piping changes at the boiler.

    I would check the air pressure of the tank with it still in the box, not connected to anything. Should be 12 PSI. Then the water feeder should be set to bring the system up to 12 PSI.

    Is this your own house?
    Dan has a good book for this system called "Pumping Away".
    You can buy it above at the company Store.

    If you wanted to correct/improve the piping of this boiler then that book would be a good reference. You have all the components there except for the new tank.

    Please let us know how things turn out!
  • ETET Posts: 11Member
    Solved with a new tank as suggested!

    We'll never know if the old one had a bladder or if it was fully emptying, or if it was pulling a vacuum, etc. I had hoped we could get to the bottom of whether using ambient air was even feasible, but it became too much trouble to find out!

    Thanks especially to Jughne!
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