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Low Pressure Heating Boiler Regulations

This is probably not a popular subject to pursue, but does anyone have any idea where to find regulations (not manufacturers requirements in their IO Manuals), that directly apply to Maximum Allowed Working Pressure (MAWP) of 15 psig or less operating pressure in less than six family buildings?

Every place I look exempts them from the regs....it feels like they drop into a black hole.

An example, there are occasional discussions on HH about using low pressure gauges because the operating pressures are nearly undetectable on the the 30psig gauges. But what regulation actually requires them? I won't go into the basis for the gauge range, because that is not the question. The question is what regulation requires it.


  • pecmsg
    pecmsg Member Posts: 4,837
    The 0 - 30# gauge is required by code because that's the Maximum operating pressure. Nothing wrong with adding a second gauge, say 0 - 3#'s 0 - 5#'s as long as the 0 - 30 is still in use.

    Gauges are most accurate when operating in the middle of the scale.
  • Jamie Hall
    Jamie Hall Member Posts: 23,271
    The fundamental regulation -- and I can't quote the paragraph, it's in NFPA somewhere -- is that one must have at least one pressure gauge which reads to twice the pressure rating of the pressure relief valve on a heated pressure vessel -- such as a boiler. For some reason which I don't understand there doesn't seem to be such a regulation for water heaters... but I don't write these things.

    There's nothing that says you can't have more gauges or whatever ranges suits your fancy. But you have to have that one.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JohnNY
    JohnNY Member Posts: 3,229
    edited March 2022
    Small point but the A in MAWP stands for Allowable. Not Allowed. My take on that is it's an engineering/mechanical guideline from the manufacturer as opposed to a Code. But then sometimes I make things up in my head that make life easier for me.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003
    I used the 0-30 psig gauge because it is a common point of discussions on HH. What I'm not finding is something of wider coverage. It almost looks like the exempted boilers don't have any regulations, which seems a bit ridiculous to be true.

    As a result of some DMs with another HHer, I asked three radiator vent manufacturers about their MAWP. Their numbers were 1) working capacity up to 15 Pounds, 2) operate between 4-5psi. We are rated to 10psi, and 3) tested at 5 psi and maximum operating pressure should not exceed 2.5 psi. It seems to me that if something went wrong and the pressure rose to the 15psig relief valve setpoint (same as MAWP) two of the radiator vent brands could be compromised and leak or worse.

    I have seen nothing even close to some kind of regulation for the exempt category.

    Now, trying to be a bit rational, since most, if not all, of the boiler models that are exempt are also used in situations covered by the regulations, one could "assume", the exempt category is "protected". This is not a very satisfying conclusion.

    Let's keep trying.
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003
    @JohnNY Here's the Oxford dictionary definition for "allowable"......

    "allowed, especially within a set of regulations; permissible."

    I didn't try an American dictionary.

    Nice try.
  • JohnNY
    JohnNY Member Posts: 3,229
    @JohnNY Here's the Oxford dictionary definition for "allowable"......   Nice try.
    I guess if you’re willing to let the facts cloud your judgement…
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • Jamie Hall
    Jamie Hall Member Posts: 23,271
    With regard to maximum working pressures and maximum withstand pressures for vents (and traps, by the way) -- in a sense you are quite right, and there is a bit of a disconnect. To further look at this, first -- maximum withstand pressure is usually 15 psi for devices intended to be connected to smaller steam systems, such as what we are looking at. In turn, those systems are required to have a pressure relief valve, sized to fit the combustion equipment, which will open and limit pressure to the same 15 psi. That is in the codes. I haven't run across any with a maximum withstand pressure less than that -- though there may be some.

    But -- it's important to examine just what "maximum withstand pressure" really means. It means that they will not fail catastrophically at that pressure, such as by bursting. It does not mean -- most emphatically -- that they will function at that pressure. It doesn't mean, in fact, that they will ever function again having been subjected to that pressure, at least more than once. Maximum working pressure is usually much lower -- as you note, often 3 to 5 psi or in that range. That is the pressure below which they can be expected to function as intended, and not fail, or at least have a reasonable life. To see how this works, consider the two functions of a trap (and, often, a vent): one is to release air, but not steam, and the other is to release water, if present. The first function requires that the trap or vent be able to open if subject to temperatures below steam (more typically, around 210F), regardless of the pressure. This means that whatever temperature sensitive element is in there is able to open the valve working against the pressure of what is inside. Over the working pressure, this won't happen and the vent or trap won't open when wanted. The second is similar -- there must be an element in there (usually a float) which will open a valve when water is present, but not (or at least be inactive) when it isn't. Again, it may not be possible for that element to open the valve against the internal pressure.

    Not sure whether that addresses your concern?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003

    @Jamie Hall Thanks, as usual. I've been looking at it as a structural integrity thing.

    I don't know if you ever read on HH about my experience with a baseboard hot water expansion tank that failed in a house I owned while my oldest son was in college. Fortunately, even though someone was in the house at the time, he was not hurt. Anyhow, that's part of my risk averseness, etc.

    I don't wish that on anyone's bucket list.

    What struck me is there are MAWP numbers for the Pressuretrols (the instruction sheet uses the term maximum operating pressure, but the sheet (8-75 vintage) may be older than the more modern terminology). Obviously the 30psig gauge can handle the MAWP. But nothing is said about vents, and the radiator vents are in occupied spaces. So I thought I would search around to try to find anything in actual regulations.

    What I find interesting in a way is that there is an exemption for buildings less than 6 families. The only guess I can make is the size of the boiler for that size building vs. smaller and how much "damage" it can do. What else could be the reason, other than lobbying and politics?

  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003
    @pecmsg Can you point to a regulation where it states 30 Psig is maximum operating pressure?

    My understanding is that and MAWP are essentially the same.

    The relief valve has a 15 psig setpoint and a small amount of accumulation pressure to pass full flow, but that is no where near 30psig.

    Once upon a time I read someone say the gauge is 30 psig so you can see how high the pressure actually is, but then you have to be there or the gauge has to have a secong pointer that shows the highest pressure the gauge has seen. I haven't seen that on any of the usual gauges and nothing in the regs that I have read says anything either.

    Your point about half range is valid, maybe that's why. I don't know.
  • MikeGordon
    MikeGordon Member Posts: 10
    ASME Boiler and Pressure Vessel Code, Section IV (Rules for Construction of Heating Boilers) mandates a MAWP of 15 PSI for steam boilers built under the code. In addition to the MAWP, Section IV also requires the boilers must be manufacturer equipped with a steam pressure gauge and that it must read at least 2 times the MAWP, which is 30 PSI. Water boilers built under section IV have their own MAWP (160 PSI) and maximum water temperature requirements (250F), among others.

    The reason for the 2 times MAWP gauge is safety. If the boiler should ever exceed the MAWP, the 30 PSI gauge will still shows a reading. A lower pressure gauge may spin completely around showing perhaps zero PSI. If someone sees that, and opens up the boiler/piping, serious injury or death will result.

    High steam pressure boilers are available. If the MAWP is greater than 15 PSI, the boiler would have to be built to the requirements of ASME Section I (Power Boilers), which is a very different standard.

    All of the state and local mechanical codes require boilers be built to ASME code, and installation requirements flow down from there. Each state has their own requirements for operation and inspection. Very generally, residential installations are except for annual/regular inspections while boilers installed in commercial applications may likely require them. Depends on your location if a 6 unit building is considered commercial, i would guess it would be.

    Hope this helps.
    EBEBRATT-Ed Member Posts: 15,511
    @MikeGordon is correct

    ASME Code is the back bone of regulations for any pressure vessel fired or unfired.

    As far as I know every state accepts this. However there may be states that have more stingent rules.

    I have never heard of the 15psi or less in a 6 family as @SteamingatMohawk mentioned but it is possible I don't claim to be an ASME expert

    But they have different codes for "heating boilers" and "power boilers" and just about anything else

    Problem is to buy there codes cost big $$$$$ and I don't think you can even view them on line for free.

  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003
    I agree.

    The MAWP vs gauge range is stated as 30psig specifically, not 2 x MAWP. But that is a non-issue as the MAWP is defined as 15psig. The result is the same.

    The six family requirement is in the New York law. I probably should not have mentioned it in this discussion. But it is still interesting to know. Scratch my head for the reason why??

    I found a free copy of the 2013 Section IV at :


    Section HG does specifically state in HG101.1 Service Restrictions: " The rules of this section are restricted to the following services: a) steam boilers for operation at 15 psig (100kPa)..."

    So, I did eventually find what I was looking for to some extent. It looks a lot like the other sections, but I have not compared them to determine any differences. It talks about the boiler and the equipment like the relief valve, steam gauge, pressure gauge, etc. It essentially doesn't appear to go beyond the boiler and its immediate attachments, like radiator vents.

    I assume there are more recent versions of the boiler code, but don't intend to spend the $$$ to get access.

    Thanks for the feedback.
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846

    If the boiler should ever exceed the MAWP, the 30 PSI gauge will still shows a reading.

    Unfortunately, that reading, more often than not, will be 0.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
    EBEBRATT-Ed Member Posts: 15,511

    As far as I know ASME covers the boiler or pressure vessel only in a residential setting. It also covers the safety equipment attached to the boiler or pressure vessel (pressure & temp gauges, safety or relief valves , vacuum breakers, high limits and operating controls and low water controls.

    You have to read carefully to determine what boiler piping (if any) is covered under ASME.

    On high pressure steam over 15 psi the boiler the first piece of pipe coming out of the boiler and the first stop valve have to be hydrotested as part of the boiler.

    Found this out the hard way. We installed a high pressure boiler and had to remove the first stop valve because the boiler would not fit through the door with it on. When the inspector showed up to inspect I wasn't there and an employee told him "yeah, we reinstalled that valve"

    $3000K for the hydrotest

    B31.1 Power Piping

    B31.3 Process piping

    B31.9 Building piping
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,003
    Yes, that's the impression I get in looking at it. I just find it somewhat surprising that, so far, the system it is attached to has no regulations, other than what the boiler manufacturer or agency having jurisdiction says. Boiler installation manuals that I have read over the years say nothing about steam vents.

    In a way I wasn't surprised when I got the info from three manufacturers that I mentioned earlier in this discussion.

    I found the international mechanical code and plumbing code on line. They do not say anything about steam vents.

  • ethicalpaul
    ethicalpaul Member Posts: 5,701
    If you were there what would you have said @EBEBRATT-Ed ? :wink:
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
    EBEBRATT-Ed Member Posts: 15,511

    It was a flanged 2" globe valve, 4 bolts and a gasket. WE should have put it back on after we slid the boiler in place.

    It's been so long I don't remember the exact details or why the inspector was there before we even started piping....can't remember. HP Steam boiler come shipped with the factory pressure test as part of their documentation. So you show that to the local state inspector and your good to go with the boiler....then he looks at the rest

    Another reason for ASME. It crosses state lines and (maybe) country lines. If not you couldn't get Buderus etc in the USA.