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Hydronic high pressure ?

yaca Member Posts: 9
Hi All, Believe my boiler is operating at a higher than usual pressure. I noticed a few days ago the bell &Gosset safety relief valve was discharging water and it’s designed to discharge at 30 psi . I banged on expansion tank and it was filled with water. The water fill is a Caleffi 553 with gauge and the gauge was reading 30 psi even though it’s was set at 15. Most of these parts were 7-8 years in use so I decided to replace the back flow preventer, caleffi 553 with gauge and expansion tank.

After a few hours of running the boiler( Dunkirk hydronic natural gas 210000 btu input ) with 3 zones ( one for each floor and a priority for indirect water tank )the safety relief valve discharge water again. The boiler gauge reads 190 degs at 28 psi, the caleffi water fill gauge was reading 25 psi. The expansion tank is precharged at 12 psi and the water fill set at 15 psi. 
I’m confused because I’m using a webstone valve for expansion tank mounted upright, handles of operation are in the correct position but if I bang a screwdriver to top part of expansion tank it seems empty , same sound as bottom of tank. 

I didn’t see a plug on port of expansion tank but shouldn’t the top part of tank sound differently from bottom part of tank , or am I confused on that, it’s a Calefactio HGT-30. The boiler at least 15 years old and it’s normal operating rarely ever discharges and water in the past from relief valve. 

The boiler model is a Dunkirk PWD-7B

Any suggestions greatly appreciated 

thank you


  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    Sounds to me like the tank is toast You cannot tell how full an expansion tank is by pounding on it, particularly the bladder type. There should be a Schrader valve on the end of the tank away from the water connection; you could try measuring the pressure there (tire gauge) but... I suspect it's done.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pedmec
    pedmec Member Posts: 968
    What type of heat emitters are you using? radiators baseboard, etc.. How did you determine what size expansion tank to buy.
  • yaca
    yaca Member Posts: 9
    The tank is brand new and suspect something with the tank as well , it shouldn’t sound empty but I’m starting to think maybe the port is blocked , 

    This is a personal heating system so the size I guess was  determined when boiler was installed , many years ago.

    baseboard heating and at the end of the zone for each floor a cast iron radiator in hallway before it returns to boiler

  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    The tank is brand new? How was the initial air charge on the tank set, before you connected it to the system?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • neilc
    neilc Member Posts: 2,700
    can you post a picture or 2, wide angle, tank, boiler, circ, and connecting piping, all in one shot,

    you said tank orientation, is the air valve up or down?
    wondering if the tank auto valve matters directionally also,
    is it a simple check and it's fallen shut?

    is the boiler doing the domestic hot also?
    is that leaking thru?
    known to beat dead horses
  • exqheat
    exqheat Member Posts: 185
    May want to adjust pressure reducer to lower pressure. How tall is the building. should be between 10-20.
    John Cockerill Exquisite Heat www.exqheat.com Precisions boiler control from indoor reset.
  • yaca
    yaca Member Posts: 9
    Air valve is down, but the new expansion tank was stamped with a pre charge of 12 psi , had to get a second air gauge to verify but the tank was actually pre charge at 25 psi. Considering  I couldn’t find my air pump I figured it’s pre charge at 12 and if the water feed is set at 15psi I should be ok until I get a new air pump to balance pressure. 

    I tried to post pics but come up blank when pasting. After reading pumping away by Dan holohan and some other books I relocated circulator pumps onto to supply side of zones and water feed at the expansion tank the system runs great , This was done seven years ago , floors reached a higher temperature on those really cold days, guessing no air in system. Previous install had circulators on the return side of zones and always had air in system.

    For some strange reason I was thinking a functioning expansion tank would have water on one side of the bladder and when you tap on the tank it wouldn’t sound hollow . With the air valve facing the floor tapping on tank from bottom to top should sound different? 

    Thank you for suggestions, greatly appreciated 
  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    It's that high pressure in the expansion tank that's doing you in. You need to isolate the tank from the system (I hope there's a valve...) and drain all the water out (watch out, it will be heavy!). Then set the precharge with a compressor to your desired cold pressure, then reconnect it. Not hard if there's a valve to block it off. A colossal bore if there isn't one...

    You would think that tapping on a tank would suggest water level, but particularly with bladder tanks the bladder makes it very hard to tell just where the water is -- or isn't. So it's not reliable.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 7,373
    If the air charge is set to 25 psi, then the tank won’t accept any water until the system pressure exceeds that. There’s your problem.

    The air charge pressure should be the as what the fill valve is set at: 15 psi.

    You have to isolate the tank as Jamie described so that there’s no water pressure on it and then reduce the air pressure to 15 psi. You don’t need a pump to let air out - unless you let too much out.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed Member Posts: 15,510
    what @Ironman said. remove the tank and set the air pressure to 12-15 psi. if the tank came charged to 25 the tank wont accept any expanded water until the system pressure rises to 25 psi
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,817
    The air pressure in the expansion tank is actually the water pressure in the system compressing
    the air in the expansion tank. You need to check the air pressure in the expansion tank when it is not connected to the water pressure in the system. That should be able to be tested if you have an expansion tank isolation valve. If you don't have that valve, then you must lower the water pressure in the boiler to zero. Then you can check the tank pre-charge pressure. .
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics