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expertgasman
In PC I talk our technical industry language.
When talking to a lay OP then I write in simple language which he will understand.
I could have said "geyser" but that term is only used now by very old people as it has fallen out of use.
When I was at school we used to talk about "old geysers" but that was for old men! No longer used in text speak.
Alternatively you could have said multi point Or water heater