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System charge and psi

jgaudet Member Posts: 87
I just redone my system I have a wood boiler and oil boiler that are connected to each other. My new fill valve is pre set to 15 psi and my expansion tank is preset to 12 psi. I checked the expansion tank while it's isolated from the system and it does have 12 psi. On both my wood and oil gauges they both read about 14-15 psi but when I plug in a water pressure gauge I made that goes from 0-30 (wich should be more accurate) on either of my boilers drains it is only showing just over 10 psi. Any ideas what this could be or how I can go about fixing it so everyything has the proper psi settings and I'm not using up expansion space! 


    EBEBRATT-Ed Member Posts: 15,517
    With the system full of water the gauges measure static pressure so a gauge at the bottom of the system might read 12 psi, two feet above that you would read 13 psi up two feet more you would read 14 psi etc.

    So it depends where your gauges are and some gauges are not very accurate
  • Zman
    Zman Member Posts: 7,569
    How tall is the system?
    It would be a good idea to establish your design pressure then calibrate everything to match.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • jgaudet
    jgaudet Member Posts: 87
    My wood boiler is in the garage and my oil boiler is in the basement about 60 feet away and the highest baseboard above my oil boiler is about 7 feet 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,830
    edited October 2021
    Your static pressure can be very low in this case. You only need about 6 PSI. The woodstove may be a problem. It sounds like it is one of the higher points in the system. Since the woodstove may become much hotter than the oil boiler you may want to operate at 12 or 15 PSI in order to keep the boiler from flashing into steam from time to time. If the basement pressure is 12 PSI then the boiler in the garage will be 2 or 3 PSI lower. So let's say 9 PSI, then your water may boil at about 235°. Add to that the pressure created by the circulator pump then the boiling point will be higher, maybe 245°. If the circulator is pumping away from the woodstove the pressure in the boiler will drop when the circulator is operating. This will lower the boiling point of the woodstove to maybe 225°. So if I had your system, I might select 12 PSI in the oil boiler in the basement and see how things operate. If there are no problems, then you are good to go.

    If you experience boiling noise in the Wood Boiler, then increase the pressure of the system by adding air pressure to the new expansion tank (measured while disconnected or isolated from the system pressure) and increasing the water pressure adjustment to match on the auto feed valve. Do no go over 18 PSI cold static pressure. this will leave you a margin for expansion much lower than the 30 PSI relief valve pressure.

    You also have Grundfos multi speed circulators (I have seen from previous posts) I think your Woodstove Circ is pumping into the return. That is good. Try increasing the speed to create more pressure in the wood boiler if noise is an issue.

    Post some Pics of your work here for everyone to admire.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • mjstraw
    mjstraw Member Posts: 41
    With the system full of water the gauges measure static pressure so a gauge at the bottom of the system might read 12 psi, two feet above that you would read 13 psi up two feet more you would read 14 psi etc.
    Wouldn't it be the other way around?
    Indiana PA

  • jgaudet
    jgaudet Member Posts: 87
    Ok thanks everyone. I have added some more air in my expansion tank to 15 and see how I get along with that! Here is a few pictures of my system rebuild and some of the before pictures! I know i should have went primary secondary but I'm not that smart with this stuff yet so I just changed it up a bit. I have a circulating pump pumping into my wood boiler that circulates the water between the 2 boilers and then I have another circulator on the supply side of my oil boiler with a spirovent on top of my 14 gallon expansion tank pumping away into my zones. Ps I just have to clean up the wiring a bit more.