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Normal pressure and temp?

Santa_Bob Member Posts: 5
edited March 2020 in Gas Heating
Hi, basic question here. I have a Slantfin Sentry S120dp. What is a normal pressure and temp for a system like mine? The feed is at 12 psi, but when the boiler finishes firing, the gauge on the front says the pressure is up to about 25psi, and the temp is at about 250°F. Does this seem normal? I have two smaller boilers (S90s) and they don't seem to get as high, but maybe I just haven't looked at the right time.



  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Does this seem normal?

    i am just a homeowner, not a heating professional.

    But it does not seem normal to me.

    If its pressure when not running is 12 psi and it goes up even a noticeable amount, I suspect a water logged expansion tank. Doubling the pressure is unacceptable.

    If the temperature goes up to 250F, better hope the thermometer is defective, because otherwise you are in for big trouble, perhaps as soon as yesterday. You better get a heating professional in there pronto, like yesterday. If it is really 250F, you are likely boiling the water and the pressure relief valve should be discharging hot water and steam all over the floor. Since you did not remark about that, it too may be defective. You may have a bomb on your hands. I hope I am wrong about this.
  • Paul48
    Paul48 Member Posts: 4,469
    Not Normal

    The maximum hi-limit setting for that hydrostat is 220*. What is the hi-limit set for? If the boiler will fire above the hi-limit setting, it needs immediate attention,
  • unclejohn
    unclejohn Member Posts: 1,830

    Is way to high. Turn it off and call somebody.to check it out. And stay away from the relief valve.
  • Santa_Bob
    Santa_Bob Member Posts: 5
    a bit better.....

    It has a Honeywell L8148E1-1273 Aquastat relay, which I compared to my other 2. They were both set at 200°F, and this one was set to 220+, so I lowered it to 200°F as well. On my initial run after the change, the temp went up to almost 230°F, and the pressure went up to about 19 psi, which is better I guess! The thing is, if this was set incorrectly, then it's been wrong for 7 years, as I never touched it before! I will say I am suspicious of the gauge, as the original one failed, and the replacement was not a Slant-fin part. Also, the pressure relief valve never opens up, and it would at 250°F, right?

    BTW, I replace the expansion tank last spring, and I believe it is still good. Also I replaced the Taco Hy-Vent this weekend, which is why I noticed this issue.

    So I guess the question is, which is inaccurate, the gauge or the aquastat? If it is set at 200°F, and it is going to 230°F, something is wrong, correct?
  • Jamie Hall
    Jamie Hall Member Posts: 21,109
    May I respectfully suggest

    that you shut the system down.  Off.  Dead.  And then set about finding out which bit of it is wrong.  If the gauge is wrong, and you are really running at what the Aquastat is set for, so be it; you just lost a little time.  Get a new gauge.

    On the other hand, if the gauge is right and you really are running up to any temperature at all over 210 or so, you are sitting on a bl___y bomb, my friend.  If the PT relief valve decides, finally, to open and the water temperature is over 212, it is very likely that the whole contents of the unit will try to flash to steam, and you will NOt be happy with the results.

    Turn it off.  Find out what is wrong..  Fix it.  Then, and only then, turn it back on.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Tinman
    Tinman Member Posts: 2,804
    I agree with Paul48

    You need to confirm the accuracy of the limit control and the pressure temperature gauge. Replace as necessary. It's a boiler so the relief valve will open, or should open, at 30 PSI. Water in a boiler doesn't boil at 212 degrees because it operates at a higher pressure. If properly sized, the boiler shouldn't need to operate higher than 180 degrees.
    Steve Minnich
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    the pressure relief valve never opens up, and it would at 250°F, right?

    Not right. The water-steam would have to be at 274F to reach 30 psi. 250F would be about 14 psi.

    "So I guess the question is, which is inaccurate, the gauge or the

    aquastat? If it is set at 200°F, and it is going to 230°F, something is

    wrong, correct?"

    No, the question is have you turned it off yet? Because if you run it without knowing the answer to your question, and if the problem is not the gauge, you could blow the thing up. It is now at least a day later. Have you got the technician in there yet? You may have a ticking bomb in there.
  • delta T
    delta T Member Posts: 883
    Turn it off!

    Yes you could have a defective gauge. You could have a defective aquastat. Maybe both. Maybe the boiler is not reaching 250 degrees. maybe the moon is made of cheese. Until you know don't assume. This is not something that can wait. Call a competent professional to make the necessary repairs, end of story. That being said, here are my two cents:

    1. 200 degrees for the aquastat is too hot. As tinman said, there is no reason (other than an improperly sized/installed system) to run hotter than 180.

    2. You mentioned that you changed the expansion tank. Did you check to make sure it was sized properly based on the btu/h of you boiler and the volume of your system? most residential boilers should run between 12 and 15 psi with very little change in pressure as the temp increases. 12 to 19 even if the temp is getting that high seems to be too much, (though I have not done any math, could be wrong). Never assume that the expansion tank that was in there to begin with was properly sized.

    The bottom line is, there are many things that could be wrong, some of them quite dangerous. I would turn it off and call an experienced heating contractor.

    Jamie's comment about the relief valve is spot on. You will NOT be happy if it opens when it is that hot. I have seen a relief valve blow once in my life and it FILLED a 300 sq foot room with very hot steam (floor to ceiling and too thick too see through) in about two seconds. It can be very dangerous to be standing next too. I was lucky in that I was about 15 feet away and standing next to the disconnect switch which was next to the door through which I very quickly scampered.
  • Paul48
    Paul48 Member Posts: 4,469

    Put on some clean shorts, then got to work?  :-)
  • delta T
    delta T Member Posts: 883
    Yeah pretty much...

    That was an interesting day. I was working on an old a.o. smith burkay coil heater, I think I was changing a gas valve if I remember correctly. turned off the switch, everything stopped, changed the gas valve, checked for gas leaks and started to pick up while it was starting to heat up. I was standing at the door when it went. Upon further investigation the old pump (taco 0012 on the other side of the concrete wall by the storage tanks) had seized up (presumably when I turned off the power and then tried to turn it on again). The flow switch had been jumped out by a lazy maintenance person who saw the flow switch problem (most likely the pump starting to die) as a problem and not a symptom. The combo of no flow and a 225,000 btu flame on a low mass coil boiler like that....well it did not take long. So now I never assume that a system is installed correctly until I know.