US State Boiler Code

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Dan Rowe

Well-Known Member
Feb 12, 2010
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This thread is my attempt to find rules for US State model or small hobby boiler code or construction rules. These are not easy to find. Please check your state and see if the link is correct or if an exemption has been found. Please post any new information so this first post can be updated.

As far as I know the only country to have a Model Boiler Code is Australia. I actually hope that some one can prove that statement wrong.

In the United States where I live the regulation of most types of boilers is at the State level. This means that there are 51 codes one for each State and one for the US Capitol. I keep a list of State boiler codes that I have found on the web. This stuff keeps moving around and changing so links go dead very frequently.

A few hints on reading one of the State codes and finding the parts needed for hobby use. Search for “exemptions” “hobby” “model” “miniature” “state special” “historic”.

I have not been able in all cases to find exemptions for the model hobby type boilers I do not know if this means the full code applies to model hobby use or not.

Exemptions Section 25-12-7

Sec. 18.60.210 Exemptions

R20-5-429 Variance

010.01-019 A9 State Special

751. Boilers and Fired Pressure Vessels Not Subject to These Orders

9-4-104. Exemptions

Sec. 29-231. (Formerly Sec. 19-426). Exceptions. The provisions of this chapter shall not apply to: <snip> (6) antique or model boilers used in public, nonprofit engineering or scientific museums and operated for educational, historical or exhibition purposes having a shell diameter of less than twelve inches and a grate surface area of less than one square foot;

5.3 State Special

300-6-1-.11 George State special boilers
300-6-1.12 Non-conforming or non-standard boilers



Boiler and pressure rules repealed March 29,2010

Sec. 5 (430ILCS 75/5) Exemptions
(7) Steam boilers of a miniature model locomotive, boat, tractor, or stationary engine constructed and maintained as a hobby and not for commercial use, that have an inside diameter not exceeding 12 inches and a grate area not exceeding 1 1/2 square feet, provided they are constantly attended while in operation and are equipped with a water level indicator, pressure gauge, and a safety valve of adequate capacity.

The definition of "Model Boiler" is now online for Indiana:

680 IAC 2-1-2 Title; scope; applicability; definition

(f) The term "regulated boiler or pressure vessel" does not include any of the following:

(12) Commercial toy boilers and miniature model boilers constructed as a hobby that do not exceed a size specified by the

680 IAC 2-4-30 "Model boiler" defined
Authority: IC 22-13-2-8
Affected: IC 22-12-4; IC 22-15-6
Sec. 30. "Model boiler" means any boiler that does not exceed any of the following limits:
(1) Two (2) cubic feet total gross volume (exclusive of casing and insulation).
(2) One and one-half (1 1/2) square feet of grate area.
(3) One hundred (100) psig maximum allowable working pressure.

Enter &#8220;89&#8221; click submit

44-915. Act inapplicable to certain boilers and pressure vessels


Under Mechanical Safety / boiler


Maryland 33 Model Steam Boilers*
Explanation and worksheet
Excel spreadsheet





Missouri Revised Statutes


48-726 (8) Boilers of a minature model locomotive or boat or tractor&#8230;&#8230;


New Hampshire
Section 157-A:6 Exemptions.

New Jersey
12:90-4.1 Scope of subchapter
12:90-4.10 Inspection of boilers

New Mexico

New York

North Carolina

North Dakota



Hobby Miniature Steam Boilers 918-225-0390

ASME Code adopted
3a.166. Miniature boilers and kitchen equipment

Rhode Island

South Carolina ACT and Regulations as of June 13 2006.pdf

South Dakota




Section 3

40.1-51.8. Exemptions

Washington DC

Washington (State)
70.79.070 Miniature hobby boilers
70.79.060 Construction, installation must conform to rules
70.79.320 Operating without inspection certificate prohibited&#8212;Penalty.

41.18 Exemptions from periodic inspections
41.43 Wisconsin special vessels.

No Boiler law
Last edited by a moderator:
Hi Dan,
I was checking to make sure that what I was posting for Ontario, Canada was current and came across this form ASME;
Boiler and Pressure Vessel Code - 2007 Edition - Codes & StandardsThe 2007 Edition of ASME - International Boiler and Pressure Vessel Code is ... and territories of the United States and all the provinces of Canada. ...
Do you know if this has any relevance to our size boilers?
Hi Gerald,
I read through the ASME code a few years ago and found a page with the definition of a miniature boiler but not many specific rules pertaining to them.

Several States have addopted the ASME code in whole or part to be the state law, one of these is Pennsylvania.. I just added Washington State (thanks PaulG) and Pennsylvania. If you search the PA code for "miniature" you will find the current ASME definition of the size restrictions for the classification.

Quoted from PA boiler regs:
Miniature boiler--A boiler which is not more than 16 inches inside diameter of the shell, 5 cubic feet gross volume, excluding casing and insulation; 100 psig maximum allowable working pressure; and, 20 square feet of heating surface.

Cheers Dan

Fortunately most of these state codes exempt small boilers used for hobby purposes. They typically exempt things like: under 12" ID, under 150 PSI, under 1.5 sq ft grate, under 5 cu ft capacity, not for commercial use. Almost no hobby boilers get over those specs.
Indiana -

Essentially adopts ASME. You can buy a complete copy of the AMSE Boiler & Pressure Vessel code online for just under US $14,000. .... (so, you are expected to abide by a set of rules that will cost you a fortune to obtain?) If anyone is aware of a cheap, legal way to acquire a copy of the ASME, I'd love to read it.

Indiana has several exemptions - the one I like is farms, provided the boiler is used 'for agriculture purposes'. So, just hook that model steamer up to a battery charger and use those batteries in your flashlight around the family farm and I think you're good. "Model Size Boilers" are also exempt, provided they meet specs established by the boiler board - never been able to find definitely documentation for those; but rumor is they defer to AMSE.

Some model builders I talk to don't like to take the code too seriously - but watch out for home owners insurance - if you try to file a claim for damages caused by a boiler out of code it will probably be declined.

Anybody know of a good deal on a 10+ acre farm in central Indiana???

It's important when reading this ASME stuff that one be very careful of nomenclature. For decades, to the ASME (and anyone who adopted their code into law), a "miniature" boiler did not mean a model or hobby boiler. Miniature boilers were a seperate category of commercial boilers such as laundry and hot water boilers, steam heat boilers, espresso machines, popcorn wagons, etc, so when reading be careful not to assume that by miniature boiler you are reading about a model or hobby boiler.

Also some of the code descriptions of exemptions have nasty little Gotcha's . . for instance one of them gives a list of exclusionary specifications, which would exclude someone with say a 4" diameter model locomotive boiler, and then at the end it says in so many words "provided an ASME approved safety valve is fitted." Even a small ASME compliant steam safety valve is very large compared to a 4" boiler and in most cases would be unworkable, if for no other reason than it could empty the boiler when it opened. In another code essentially the same exclusions are offered but then at the end says in so many words "a fusible plug shall be fitted in the firebox" and this can be a deal breaker. First there is general opinion that fusible plugs in a small copper boilers are a waste of safety effort and a royal PITA, but more problematically if not already fitted at initial construction they are extremely difficult if not impossible to successfully install later.
To quote Rick in the notice to this section “Don't Guess, Just Ask
I wish the answer was that simple with US State Boiler Law.

As Harry mentioned the definitions of the legal terms are important, and they are not used the same way in all cases in different State codes. Even the term hobby can be misleading as it is also used for full size traction engines in some states.

The states where I have found a clear exemption or in the case of Maryland a model boiler code I left a note before the link to help locate the section we are interested in for hobby model use.

In most of the State codes I have read there is no mention of small hobby boilers. I like to find the ones with clear workable exemptions like Texas where I live, or even a model code as Maryland. This is a small hobby and it has a good safety record so it has gone unnoticed by most State Regulators.

The consequence for not keeping a good safety record is clearly stated in the Oregon State Boiler Code under Hobby Miniature Steam Boilers:
(2) This exemption continues as long as:
(a) There is no explosion;

Oregon and Alaska are two of the states that have one of the gotcha clauses mentioned by Harry and have the requirement of an ASME approved safety valve. I have never seen an ASME {V} stamp safety valve smaller then ½” NPT. These will work fine with large scale models but I agree with Harry they are not useful for small scale boilers. ASME stamp info:

The whole ASME code is not needed sections 1 & 2 would be enough but even that is over $1000. This is why I bought all 3 sections of the AMBSC and it is a real bargain for model boiler design information. I could not find the silver solder color code needed to know which alloys to use so I googled the given reference and for a few more dollars I downloaded the section of the Aus welding code I needed. If I had known about this group at the time I am sure that someone here would have been able to supply the color code information.

I was browsing the NJ Boiler code

12:90-4.1 Scope of subchapter

(a) This subchapter shall apply to the design, construction, inspection, installation, repair and alteration of steam or hot water boilers, including, without limitation, model steam boilers, except as provided in (b) below.

(b) This subchapter shall not apply to:

0. Steam boilers having adequate relief devices set to discharge at a pressure not greater than 15 psig when such boilers serve dwellings of less than six family units or other dwellings with accommodations for less than 25 persons;
0. Hot water boilers having relief devices set to discharge at a pressure not greater than 160 psig and hot water boilers limited to temperatures not exceeding 250 degrees Fahrenheit when such boilers serve dwellings of less than six family units or other dwellings with accommodations for less than 25 persons;
0. Any steam or hot water boiler having less than 10 square feet of surface;
0. Any steam or hot water boiler having a heat input of less than 10 kilowatts or less than 40,000 BTU per hour;
0. Any steam or hot water boiler under the jurisdiction and control of the United States Government when actively regulated by a Federal agency; and
0. Any steam or hot water boiler used solely for the propulsion of a motor vehicle regulated by the Motor Vehicle Act, Title 39 of the Revised Statutes.
It appears the key here is less than 10 square feet of boiler heating surface and/or less than 10KW /40,000 BTU per hour heat input. to be exempt from the code.
so you need to burn a bit under under 2 lbs of propane per hour max (BTU per Pound 21,591) a half gallon of ethanol (1 gallon of ethanol = 84,400 Btu) or 3-5 lbs of coal (1 pound of coal = 8,100 to 13,000 Btu)
This is where language of codes gets dicey and becomes subject to "interpretation", which means, its depends upon the official you happened to be talking to on that day. There are no "and/ors" in the section so my interpretation would be that all you have to do is meet One of the exclusionary criteria to be excluded. The wording I see says (for example) if you have less than 10ft/sq of heating area your boiler is excluded and it then doesn't matter how much heat you put to it. However I know from years of dealing with codes that what a literate and rational person might think a code says and what a codes official says it says can be two completely diffierent things.
GWRdriver said:
However I know from years of dealing with codes that what a literate and rational person might think a code says and what a codes official says it says can be two completely diffierent things.

Never have truer words been written.

Last weeks example. New surveyor arrives at our steam tug for its' annual survey. On inspecting the ships Stability Statement he noted with great concern that some 3 tons of concrete in the bilge under the boiler was not annotated in the statement.

"How long has that been there?" he asked. He was advised by my replacement that it had been there since the ship was built . "Well a verbal advise is not good enough and it will have to be removed." This after a whole 3 secs of consideration and an obvious lack of any understanding of ship stability.

The Director of the Museum rang me asking, what did I know etc. etc. I asked what could I do to help. He asked would I give them a Statutory Declaration saying that I was present when the Inclining Experiment was carried out and that the concrete was under the boiler at that time. This was done and another bureaucrat had covered his backside after 14 years of it not being a problem.

Best Regards
The exact statement for the New Jersey boiler code exemption is “3. Any steam or hot water boiler having less than 10 square feet of surface;” to say that means heating surface or grate surface or any other surface is an assumption made by the reader.

A bit of further reading will take you to:

12:90-4.10 Inspection of boilers

a) All steam or hot water boilers or similar equipment potentially capable of generating steam as described in (b) below shall be inspected and be subjected to a hydrostatic test, if necessary, at least once each year at 12-month intervals. This inspection shall be a complete internal and external inspection as construction conditions will permit. All hot water heating boilers shall be inspected internally at 24-month intervals and shall be inspected externally every 12 months.
(b) Steam or hot water boilers subject to the inspection of (a) above shall include those listed in (b)1, (b)2 and (b)3 below, except as provided in (c) below:

1. Steam or hot water boilers having 10 or more square feet of heating surface;
2. Steam or hot water boilers having a heat input of 10 kilowatts or more; or
3. Steam or hot water boilers having a heat input of 40,000 BTU per hour or more.

I think that the inspection section clears the air a bit to get to the intended meaning of the code.

I was a marine engineer and US ships are inspected every year. This includes the boilers. I mostly worked on low speed diesel ships but they have a small donkey boiler for steam heat. I will agree that most of the Coast Guard inspectors I met on a professional level were not qualified engineers and really had little knowledge of what they were inspecting. One of the ships I was on had a monotube donkey boiler. This would lead to problems as the standard inspection sheet says to check the low water shut down and alarm. I have seen ships engineers try in vain to explain this to the inspector…. the sheet says low water shut down and our logic sounds like BS to him. The USCG approved device for emergency shut down for this boiler is a limit switch mounted on the external boiler insulation cover which is some what flimsy. When the tube coil expands due to lack of water and over heating the limit switch was set to shut down the boiler and sound an alarm. The usual drill after logic failed was to distract the inspector and press the limit switch to simulate a low water shut down and alarm.

On one inspection I had an actual Coast Guard engineer. I explained the system to him and stated that I did not have any confidence in the device but I was willing to give it a go. He simply asked me if I had set the limit switch to factory specifications and I answered with all honesty “Yes sir.” His response was “Good enough for me.”

Tin thanks for reading through that one I have updated the list in the first post.
Cheers Dan
The first message on this thread is now a complete picture of the US State Boiler code at this time. The link posted by Quake was a big help as it was mostly correct with a few broken links.

Wyoming and Idaho do not have boiler laws. The law in Idaho was repealed March 29, 2010 due to insufficient funding for the program.

As I said before for the most part the States have adopted the ASME code language which works well for industrial power boilers. Several states have a State Special where a design that does not conform to ASME code can be built.

The rub is the ASME code is so dang expensive that it does not really work with a hobby budget. There are several books that explain the code and that may be a cheaper way to go to work out a design, but several States require a PE to sign off on the boiler design for a State Special.

It is complex and messy but that is the way it is.

Cheers Dan

I'm not a great reader of mobo jombo, but what I'm gathering from the info down below is that its ok to build and play around with my 3.5" 100psi loco boiler without getting inspected nor going to jail ???

What are the requirements for pressure vessels in Wisconsin?
Answer: Code requirements for Pressure Vessels
Comm 41.16 Initial inspections.
(1) BOILER AND PRESSURE VESSEL INSPECTIONS. (a) Except as provided in par. (b), boilers and pressure vessels shall be inspected by a certified inspector before they are placed in operation.
Note: See s. Comm 41.41 for installation registration requirements.
(b) The inspections specified in par. (a) are not required for boilers and pressure vessels exempted from periodic inspections in s. Comm 41.18.
(c) Where the boilers or pressure vessels specified in par. (a) are installed in a city of the first class and inspections are made by the city, the city shall keep a record of the inspections and shall submit a copy to the department.
(d) Where the inspections specified in par. (a) are performed by a certified inspector other than a department inspector, the certified inspector shall file an inspection report with the department and shall affix the Wisconsin registration number as required in s. Comm 41.36. The inspection report shall be filed with the department within 30 calendar days after completion of the boiler or pressure vessel installation. If the report is not filed within the 30&#8722;day period, the department shall perform the inspection.
Comm 41.18 Exemptions from periodic inspections.
(1) EXEMPTED EQUIPMENT. Except as provided in sub. (2), periodic inspections are not required for:
(a) Boilers or pressure vessels which receive regular inspections by United States government inspectors;
(b) Heating boilers located in private residences or in apartment buildings having less than 3 living units;
(c) Expansion tanks for hot water heating boilers;
(d) Boilers used exclusively for agricultural purposes;
(e) Pressure vessels having an inside diameter not exceeding 6 inches with no limit on pressure;
(f) Pressure vessels having a volume of less than 5 cubic feet and an operating pressure of less than 250 psig;
(g) Pressure vessels with a volume of less than 1&#8722;1/2 cubic feet with no limit on pressure;
(h) Pressure vessels having an internal or external operating pressure of not more than 15 psig with no limitations on size;
(i) Hot water supply boilers and water heaters, and hot water storage tanks in which the temperature does not exceed 210 F;
(j) Vessels used for the storage or processing of cold water, including those with air cushions;
(k) Pressure vessels which are used in accordance with the regulations of the United States department of transportation;
(L) Air receivers having a volume of less than 12 cubic feet and an operating pressure of less than 250 psig; and
(m) Pressure vessels used in processing and storing of fermented beverages at temperatures not exceeding 140 F.
(n) Any pressure vessel used as an integral part of an electrical circuit breaker.
(2) EXCEPTIONS. In individual cases, the boilers and pressure vessels exempted in sub. (1) shall be subject to inspection by or on order of the department upon the complaint of any person or upon the initiative of the department when there is reasonable cause to suspect that the construction, installation, maintenance or operation of the vessel is not in keeping with the general purpose and intent of this chapter.
Comm 41.28 Safety Rules.
(1) MAXIMUM ALLOWABLE WORKING PRESSURE. No boiler or pressure vessel may be operated at a pressure in excess of the maximum allowable working pressure stated on its current permit to operate.
(2) ALTERATION TO SAFETY DEVICES. No unauthorized person may remove or tamper with any connected safety device.
(3) INSTALLATION LOCATION. Boilers and pressure vessels shall be so installed that there will be sufficient room between the vessel and any ceiling, wall, partition or floor to facilitate the connection and operation of valves, pipes and other appurtenances, and shall be installed in a manner that will not block any inspection opening.
Comm 41.32 Pressure gages for air receivers.
(1) GAGE LOCATION. Air receivers shall be equipped with an indicating pressure gage so located as to be readily visible.
(2) GAGE DIAL. The dial of the pressure gage shall be graduated to approximately double the pressure at which the safety valve is set, but may not be less than one and one-half times that pressure.
Comm 41.37 Maintenance. (1) CORROSION PREVENTION. All boilers and pressure vessels shall be installed and maintained in such a manner as to prevent excessive corrosion and deterioration.
Comm 41.41 Installation registration.
(1) BOILER OR PRESSURE VESSEL INSTALLATION REGISTRATION. (a) Except as provided in par. (b), the installation of any boiler or pressure vessel shall be registered with the department by the installer before the operation of the boiler or pressure vessel. Registration shall be in writing on form SBD&#8722;6314.
Note: Copies of form SBD&#8722;6314 are available at no charge from the Safety and Buildings Division, P.O. Box 2509, Madison, WI 53701&#8722;2509, telephone 608/266&#8722;1818.
(b) Registration with the department is not required for:
1. Boilers and pressure vessels exempted from periodic inspections in s. Comm 41.18; and
2. Installations in cities of the first class if an installation registration form has been filed with the appropriate city official.
Comm 41.42 ASME code vessels. (1) ASME CODE COMPLIANCE. Except as provided in ss. Comm 41.43, 41.44 and 41.45, boilers and pressure vessels shall be constructed and installed in accordance with the ASME code. Boilers and pressure vessels designed to other national or international standards may be approved if the design has been accepted by a nationally recognized independent third party.
Note: The department will recognize the applicable case interpretations of the ASME boiler and pressure vessel code as being acceptable.
Note: The ASME code specifies that persons installing boiler external piping by welding are required to possess the appropriate ASME credentials.
(2) REGISTERING WITH NATIONAL BOARD. (a) Except as provided in par. (b), boilers and pressure vessels constructed and installed in accordance with the ASME code shall have the manufacturer’s data report registered with the National Board and shall bear a National Board number. Copies of the registration shall be provided to the department when requested
From ASME Section VII Division 1
UG&#8722;125 GENERAL
(a) All pressure vessels within the Scope of this Division, irrespective of size or pressure, shall be provided with pressure relief devices in accordance with the requirements of UG&#8722;125 through UG&#8722;137. It is the responsibility of the user to ensure that the required pressure relief devices are properly installed prior to initial operation.
(a) When a single pressure relief device is used, the set pressure marked on the device shall not exceed the maximum allowable working pressure of the vessel. When the required capacity is provided in more than one pressure relief device, only one pressure relief device need be set at or below the maximum allowable working pressure, and the additional pressure relief devices may be set to open at higher pressures but in no case at a pressure higher than 105% of the maximum allowable working pressure.
The way I read it you are exempt by Comm 41.18 (e), (f), and (g).
One of those exemptions would have been enough but hey three is great.

I have spoken to one of the State Boiler Inspectors from WI at a a steam show in Edgerton WI and he was a very reasonable fellow and had a keen intrest in historic steam.

Dan would I still be subject to an Initial inspection?

As I read it you are not subject to initial inspection, but they can require an inspection if someone complains that there is an unsafe operation.

Although I live in Texas I have emailed the boiler inspector in WI and got an answer in a few hours which was a big surprise to me as it was a Sunday, and it was a clear understandable answer. The same question to the State of TX took a few months and it was clear that the person had no idea what he was talking about.


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