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After nineteen years as a HI it occured to me that there's something about water heaters which confuses me. I've seen MorFlo, Rheem, and Ruud water heaters with a capacity of 119.9 gallons and AO Smith heaters with a capacity of 119 gallons. I've never seen a heater with a capacity of 120 gallons or greater. Can anyone explain this to me? The only thing I can think of is that, possibly, a water heater 120 gallons or greater is classified as a different appliance with entirely different construction and safety requirements.


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It will take more research than I have time to do but the key seems to lie in the application of different ANSI standards to gas burning appliances. The standards vary with sizing and affect venting, combustion air requirements, etc. An inquiry directly to a manufacturer should yield fruitful results.

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I googled a bit. One site had a 119 and a 120 gallon unit. The 120 met ASME specs, the 119 did not. Anything larger than that...and I found up to 5000 gallon units all met ASME specs. Can't find the actual requirements but I suspect that 120 or over has to meet tougher (boiler?) standards.

I did find the following...


This section covers requirements for construction and continued service of pressure vessels for the transportation of dangerous goods via highway, rail, air or water at pressures from full vacuum to 3,000 psig and volumes greater than 120 gallons. "Construction" is an all-inclusive term comprising materials, design, fabrication, examination, inspection, testing, certification, and over-pressure protection. "Continued service" is an all-inclusive term referring to inspection, testing, repair, alteration, and recertification of a transport tank that has been inservice. This section also contains modal appendices containing requirements for vessels used in specific transport modes and service applications. Rules pertaining to the use of the T Code symbol stamp are also included.

Seems like it costs money to get any more info.

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I'm sure you are right about the hydro(static) test. Each tank would likely have to be individually tested and stamped, a fairly lengthy process, adding significantly to the production costs. I'm also wondering if there might be periodic inspections/re-testing required along the lines of high pressure air storage tanks. That of course would be totally impractical in a residential water heater.

Hopefully, Norm will come back with the full story.

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