QUESTION: What inspections are required for an electric water heater on a towing vessel?
RESPONSE: To answer this question, first you must identify some characteristics of the electric water heater.
46 CFR 136.110 defines a pressure vessel as “… a closed tank or cylinder containing gas, vapor, or liquid, or a combination thereof, under pressure greater than atmospheric pressure.”
An electric water heater is a closed tank that contains liquid and is under pressure greater than atmospheric pressure.
The following italicized text contains excerpts from the regulatory citations and not the language in its entirety.
§ 143.300 further defines requirements for pressure vessels as those that are greater than or equal to 37.4 gallons and over 15 pounds per square inch. Under this set of requirements, there are some that are applicable to an electric water heater:
(a) be equipped with a spring loaded relief valve that prevents pressure from exceeding the MAWP by more than 10 percent.
(b) be externally examined annually and relief valve tested in accordance with § 143.245 (i.e., twice every 5 years with no more than 3 years between testing)
(c) be marked with MAWP
(d) if installed after July 20, 2018, must meet ABS rules (see § 143.540)
Since the water heater will operate at the pressure of the potable water system, the gauge requirement of § 143.300(a) is more commonly applied by having a pressure gauge on the output side of the potable water pump or thereafter in the piping system.
While not applicable to Subchapter M vessels, one can look at Subchapter T (46 CFR 182.320) and Subchapter K (§ 119.320) for similar regulatory guidance. Those regulations put specifications that apply to a water heater that is UL listed, under 120 gallons, and heat input is less than 200,000 BTU per hour.
Additionally, the water heater manufacturer likely provides documentation to guide the owner on maintenance. A quick web search of some major water heater manufacturers identified that at least annually the temperature pressure relief valve should be tested, and one was found to call for the relief valve to be reinspected once every three years and replaced if necessary. The vessel owner/operator should consult the manufacturer’s documentation for the make/model installed on the vessel.