Ive been interested in making one for awhile ,plumbing supply Isle at the local H Depot has the size pipe but the thread size is wrong.Im thinking of just a stainless pan with lid now and finding nuts with the right thread , weld the five plus glow plugs after drilling and welding nuts ,weld lid as needed around the qrt size pan ,whala ,instant small water heater for the rv.Battery powered.

You lost me on this one. Are you talking about using diesel glow plugs for heating elements?

A: Glow plugs are not designed for immersion. They would quickly be covered with corrosion and heating would be almost nil.

B: Glow plugs are designed for a 30 second maximum "on" time. Nowhere near enough time to heat any amount of water.

C: 30 amps or so of draw would quickly kill any battery by the time you got a gallon or so of water hot.

Stick with the propane heaters.

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

Its a bit like youll find on Rvs already ,check into .

one of your points is certainly valid the others I would suggest are questionable, because: A) Glow plugs are designed for immersion in a very much more hostile environment than water! B) cavitation and oxidization occurs at vastly higher temperatures that the water would allow,which in turn would extent the life of the glow plugs, they are sealed internaly to a greater design integrity than any water heating element, and made from extremly high grade nickel alloy, however you are correct that they use a lot of current (even more if immersed in water due to the lower temperature opperation) and would best suit being used durring engine running to minimise charge loss, but never the less they work very well indeed if their voltages are kept correct.