how much psi for my boiler?

Discussion in 'Boiler Forum' started by oshwa, Dec 31, 2013.

  1. oshwa

    oshwa New Member

    Messages:
    4
    Location:
    Minnesota
    What should my water psi be for my boiler when not operating and then when it is operating? On the side of the boiler it says MAWP 50psi. The reason I ask is that I had a leaky water valve that I tightened up and it stopped leaking but this caused my psi to drop drastically. I added water so that it is 25psi when it is operating at peak. It will shutdown to cool off when the temp reaches 180 degrees and that is when it is at 25psi. The psi will then slowly drop along with the temp. I was wondering if I should increase psi due to MAWP being 50psi. It is a two story house.
  2. oshwa

    oshwa New Member

    Messages:
    4
    Location:
    Minnesota
    Check that not 180 degrees it is reaching only 170 F degrees. On this side it says max water temp 250 F degrees. Should it be reaching closer to that as well? I guess I am confused on how the maxes should relate to what you actually operate it at? Also if I had more psi for the water will that in turn increase the water temp?
  3. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,127
    Location:
    Maine
    Water pressure should b about 15 lbs

    Max temperature no more than 205
  4. oshwa

    oshwa New Member

    Messages:
    4
    Location:
    Minnesota
    so do I need to decrease my psi to be 15? or should it be 15 psi when not operating? Currently it is at 25psi when at peak operation and it is having a hard time keeping up but it is very cold here in mn right now. Also I do hear water moving in the radiators and I did not hear that before the leak occurred so I was guessing I needed more pressure if anything not less. I will listen if explained a little...... Also I did bleed the system of air. The radiators do not have a bleeders but the system as a tank that I bled and that helped.
  5. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,127
    Location:
    Maine
    15lbs all the time

    You need to get all the air out of the system.
  6. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    No boiler guys answered yet?
    The standard setting is 12 lbs cold. 1 PSI will raise water in a column 2.31 feet.
    The pressure should be set to what is required in total height of system. For example, if a boiler is in the basement, and its a 2 story house with baseboard radiation on the second floor, it could be 8' + 9' + 1' = 18' (boiler to upper baseboard).
    1 PSI / 2.31 = 0.4329
    18' x 0.4329 = 7.7922 minimum PSI required cold temp
    With pumped applications because of circulators will see a pressure differential across the impeller, its advised to adjust for this. Industry leaders agree on 3 PSI for residential size circulators.

    Thus 7.8 PSI + 3 PSI = 11 PSI required (12 PSI is a industry chosen number perhaps because of what if its radiators that are taller on the second floor)

    So 12 PSI is a starting point but not necessarily correct for all applications, for example a 3 story house likely requires a higher setting.

    The second part of your concern/question-
    Water expands as heated and on a closed system whether its pushing against a rubber bladder with a air charge behind it or a direct expansion tank charged with air it will increase in pressure as it is heated and expands.
    There is no magic number on what it should be as its different depending on the cold start pressure, the height of heating system, the type and size of expansion tank and the operating water temperature.

    To give you an average operating pressure, a two-story house, operated at 180 degrees, with baseboard, with a properly sized bladder type expansion tank will go from 12 PSI cold to 19-24 PSI when hot. This is well under the residential boiler relief valve usually set to blow off at 30 PSI.

    If the gauge is accurate, the expansion tank is sized correctly and you are still hearing noise with proper air elimination installed in the correct location, try increasing the cold pressure an additional 2-3 PSI.

    Increased water pressure does not increase heat output. However if the pressure is low and circulation is not taking place properly there could be reduced heat output delivered.
    The operating temp of the boiler should be matched to the structure heat loss/circulation flow rate/heat emitter design and output. These are all related and affect each other.
    Last edited: Dec 31, 2013
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,892
    Location:
    New England
    The normal residential boiler has a safety valve set at 30psi...so, unless you have a need for higher pressure to ensure the water doesn't boil and flash to steam (like the pressure cap on your car), and you want a margin for error, in most circumstances, in the range of 12-15psi is fine. When water is heated, it expands (as does air), and in a closed system like a hydronic heating system, expansion means pressure increase. To overcome that, you need a properly sized and setup expansion tank. IF that is there, the pressure in the system will NOT change much, and may not be visible on your pressure gauge.

    If the pressure changes significantly, you have a problem. Running at an unnecessarily high pressure will put you closer to the safety pressure release, and dumping water in that manner is not a great idea. That valve is not designed for regular use...it should only ever be opened to test and if an actual safety fault issue occurs.
  8. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,127
    Location:
    Maine
    Engineers.....making the simple complicated LOL

    Hey, MAWP is maximum allowed water pressure. All residential boilers must meet that minimum standard

    Pressure relief valves pop at 30psi, also per code

    Most water feed valves come set at 12 to 15 lbs and should be left alone unless you are pushing water up 5 stories or so.
  9. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    Yes but what I wrote above is only hydronics 101 stuff. Many "experts" have been stumped at install problems not knowing the simple things.
    BTW 5 stories would require about 23 PSi cold pressure to get heat
    Of course then you would have 50# relief valve on that boiler also.
    Which would be a typical commercial boiler application.
    Commercial water boilers are ordered with 30, 50, 75, 100 and 125 psi relief valves as an option
    Last edited: Dec 31, 2013
  10. oshwa

    oshwa New Member

    Messages:
    4
    Location:
    Minnesota
    Ok so I think I have a better understanding but yet still confused. 1st off I have a 6ft basement 9ft main floor and it is baseboard heat so 1ft for upstairs. So 16ft. So 7psi + 3 psi= 10psi. This should be my cold temp psi then? I have an expansion tank but no bleeders on the radiators. Now to go back a little when my leak occurred I needed to bleed the expansion take as it had air in it. I have tried to bleed it a few times here and there as the water as been circulating in case air is circulating or something. Anyways no more air has bleed out when I have done this (at least that I can hear). Alright back to now. I turned it off and let it get cold. When the temp read 70 F degrees (which is the lowest it goes) the psi was roughly 8psi. Please remember when running it has been reaching 170-175 F degrees and the psi has been 22-25psi. Once it reaches that it turns off for awhile and then goes back on and goes through that cycle. So my question: is it operating correctly? Should I increase the psi to 10psi when it is cold (even though it is reaching 25psi when operating/hot)? Just leave it be? Thanks for all the info!
  11. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    Set it to 12psi cold, and charge the expansion tank properly. (Search youtube or similar for primers on how to do that.) If you have high-volume radiators & plumbing on the heating system you may need to bump up on the expansion tank sizing if it's hitting north of 25psi when the system is hot.

    At 8psi cold it'll be under negative pressure at the top of the system- it's easier/better to purge air by tapping-in or venting somewhere near the highest elevation of the system, which requires the pressure to be positive there.
  12. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,127
    Location:
    Maine

    Cold 8lbs, hot25lbs. Expansion tank is waterlogged. If it's an older style, non bladder tank you need to shut the valve to the tank off and totally drain the tank. Then open the valve and adjust system pressure to 12 to 15 lbs.
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