Boiler Pressure

Discussion in 'Boiler Forum' started by Engine1, Jan 13, 2012.

  1. Engine1

    Engine1 New Member

    Messages:
    13
    Location:
    New England
    Hot water boiler pressure is at 18 cold, 25 or so when hot. The new regulating valve is adjusted to the lowest possible setting. Expansion tank has 15PSI. The two zone baseboard loop is on a first floor.

    I don't know if this important, but the feed is into the return line and the pumps are on the supply side.

    Any reason for concern here? The manual says the pressure should be at 12.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    If the expansion tank is properly sized and the pressure is adjusted correctly, AND if there's no additional water being inadvertently being introduced to the system (autofill valve leaking, a leak in the heat exchanger with the WH, etc.), AND, you've gotten all of the air out of the pipes, the pressure should be stable, hot or cold. The only way to correctly measure the pre-charge in the expansion tank is when there is no water pressure applied, otherwise, it will read the same as the water pressure.

    Assuming you have an autofill valve, it may no longer be shutting off properly. Once the system is filled, you could shut off the autofill entirely as the system should be closed and never lose any water. If it does, the autofill valve will mask the leak problem and keep the pressure in the safe to operate range (unless it is leaking, then the pressure will rise).

    12psi is probably the minimum pressure otherwise the safety pressure switch would prevent the boiler from firing. A common residential boiler relief valve is often 30psi, so the usual desire is to operate somewhere above the minimum and less than max...a figure of 14-16psi is fairly typical. You don't want it to be bumping off the minimum because it might fail to fire up. Too high, and it would be dumping water. Adding fresh water to a typical boiler is not a good thing. It works best when it is in there for the duration as it reaches an equilibrium with the metal parts.
    Last edited: Jan 13, 2012
  3. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Jim is right.

    I would add that the typical residential boiler pressure is 12psi and that the pressure in a diaphragm tank should be equal to the design static system pressure all at
    room temperature. The proper pressure is determined by the measured height of the highest radiator. A slight increase in pressure is common
    and acceptable (5psi or so), but if you are approaching the PRV max of 30 you have a problem with the size or condition the expansion tank.

    As for safety pressure switches; they are generally found on all high efficiency condensing boiler and never found on low efficiency non-condensing cast iron
    boilers.
  4. Engine1

    Engine1 New Member

    Messages:
    13
    Location:
    New England
    Was wondering about that pressure safety switch. There isn't one (that I know of) on on this cast iron 85% system. Thanks for clarifying.

    I removed the diaphragm tank and it had a pressure of 13PSI, connected it earlier read 15PSI or I'm just off a bit or inadvertently let out 2 - either way it appears OK. The baseboard hot water loop supplies a typical first floor, so all indications are that it shouldn't need much more than 12 PSI when cold. The pressure valve adjusting screw is unscrewed as far as it can be which sets it for the lowest possible pressure. The nut holding it is snug - not tight. The device is a Watts B1156F and the label reads that it is set for 14-17PSI. Is that simply the wrong valve?

    Right now it's 2.2F outside, it's running with a setpoint of 195 (likely in boost mode) and at 25PSI.

    Will anything bad happen if it's running at a higher than called for pressure?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    The spring in the autofill valve may no longer be able to adjust to where you want it and a replacement may be good.

    You don't want your boiler dumping water from the safety valve. If it never exceeds 25psi, it should be okay. Just like the radiator cap on your car, the minimum pressure is to prevent the water from flashing into steam, which can be dangerous. There'll be some max your boiler is designed to handle, often indicated by the safety valve, but not necessarily...you'd have to read the spec sheet as to the max safe pressure. The higher you need to pump, the more pressure you need in the boiler to keep it all liquid. Steam in a hot water boiler is the bad thing, and maintaining at least the minimum pressure is required. Too much, and it may leak. How high that is, depends on the equipment.
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,137
    Location:
    Maine
    In any case the auto-feed valve should not be left open. fill the system to 15 lbs and shut the auto fill off. Once the system is pressurized there should be no need to add water ever unless there is a leak somewhere. Go here www.heatinghelp.com for a detailed discussion of autofill valves and whether or not to leave them on. Also, because new code regulations prohibit dumping the drain port from any back flow or check device with an atmospheric vent on the floor we no longer install auto feed/bfp valves on any hydronic systems. We fill the system with either a hose or a pump, set it and leave it. Depending on system head pressure and the high limit setting, on a cold day the system will fluctuate 2 to 3 pounds normally and, depending on where your circulator is mounted it may show more variation than that. Installed on the return, which is pretty normal for packaged boilers and having the expansion tank on the feed side, the system will experience larger pressure changes than if the circulator was mounted on the feed with the expansion tank before it. In any case though, if the pressure relief ain't weeping and the pressure is below 25 lbs then forget about it.
  7. Engine1

    Engine1 New Member

    Messages:
    13
    Location:
    New England
    Can you post or PM a link the details on this. I don't see it online.

    Thanks.
  8. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,137
    Location:
    Maine
    IPC and IRC code.

    All BFP's that have atmospheric vents must be discharged by indirect waste to a vented trap or sump.

    This was adopted IIRC in 09 but it may have been 10.
  9. Engine1

    Engine1 New Member

    Messages:
    13
    Location:
    New England
    Would that apply to a Watts 9D-M3 in a residence, and if so can you link to or post the section that requires that?

    thx
  10. Engine1

    Engine1 New Member

    Messages:
    13
    Location:
    New England
    Two questions:

    1) Is there an advantage to having a system operate at, say, 12PSI instead of a higher pressure of, say for comparison 24PSI?

    2) Can someone refer me to the rule where it's written that pressure relief valves must be upright and not horizontal?
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    1. You need enough pressure so that with the burner and pump on, your boiling point stays high enough to not allow the water to flash to steam. Unless you have a really tall building, generally, around one atmosphere is the sweet spot. Most boilers are designed with a safety pressure relief valve designed around 30psi. So, at about one atmosphere, you have 100% or more safety zone before it releases (you don't want that to happen). At 24psi, you've dropped that safety margin to 25%.
    2. You'd have to go by the manufactuer's specifications. If they say it must be mounted upright, then there's some engineering reason for it. Best not to tempt fate. If you have a runaway boiler - one that is in the process of superheating the water, if that safety valve doesn't open, you have the equivalent of a bomb in the basement. A steam explosion can level a house.
  12. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    The circulators may be pulling water from a fill valve and adding to the static pressure. We often use automatic fill valves and often leave them on for the first season. This is particularly true for radiant floor and old cast iron radiated systems as air will come out of solution long after we are gone.

    Directing overflows from backflow preventors and more especially, pressure relief valves is a mistake, regardless of the code. If a safety valve opens, the "proper" drain, will effectively hide the event. This is both silly and potentially dangerous. Yet another example of folks who don't have enough to do, sitting around a table trying to answer the ever-important question; who is the smartest guy in the room?

    I think it more than interesting that plumbers (most likely boiler installers) fret irrationally about full-time fill valves at 12psi, while the potable water system is perfectly fine at 80psi. It is true that we don't want fresh water in a hydronic system but good service satisfies this serious requirement.

    On most residential fin-tube systems a #15 diaphragm tank is plenty. Confirm the tank pressure at 12psi and the static-cold (room temperature) system pressure (confirmed with a reliable gauge) matches the tank pressure and you are all set.

    Heatinghelp is a good site, for DIY and a very controlled sales pitch, but don't expect to get candid answers or express an honest opinion without a good self-righteous scolding (both public and private).
  13. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,137
    Location:
    Maine
    Heatinghelp is an excellent site. A lot of "professionals" drop out because they often give wrong information and there are quite a few experts on there that will jump on them for it.
  14. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    It is not where the circulator should be mounted, but it's relationship to other components. All system should be filled between expansion expansion tank and system piping.

    For those that are not plumbers, an indirect waste pipe - be it gap or break - will run all evidence of a failure down the drain, thus effectively defeating one of its main purposes (to alert maintenance personnel of a problem). In the case of BFP, the leak is ususally of no consequence. In the case of a PRV, the event should be obvious, immediate and the evidence should be apparent and long lasting. Unless you are watching the drain 24/7, you will miss any event.

    I think you are mixing up commercial steam boilers with residential hydronic boilers, but if you would care to sight time and location of these rare events please proceed. The idea that a fill valve would cause more trouble than it cures is irrational, but some "professionals" would rather charge a service call to top off a hydronic system, after the air has come out of solution, than charge a one time fee to install a proper fill valve; be it automatic or manual.

    A lot of "professionals" (one who does something for a living) tend to run in a herd. It is all about singing in the choir.

    The man who tells the truth should have one foot in the stirrup.
  15. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,137
    Location:
    Maine
    The reason for the drain is because when BFP's let go they can and often do cause quite a bit of property damage.

    Please read Dan Holohan's book "Pumping Away" and get back to me with your first comment.

    Again, there is absolutely no good reason to install a fill valve on a boiler at all. If you have air problems then you need to fix the air problem and not rely on the autofill to cure the problem which it not only won't but it will also continually add new air to the system causing rust and sludge.
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