As per post . I was of the understanding particularly with the modern MAN riddled with sensor they don’t. Just rely on 220 v AC water heaters ? Wether on AC shore power or generators. Partly because because of extra sensors these days Eg with MAN “ coolant expansion tank pressure “ it’s got a range typically 250-900 mb s and a time to reach with in said range . So if one engine had to heat up a large clap cold water tank from its pipes , that might take too long and trigger a MDDS ( or modern eq ) alarm ? Say a hot tank for a 4/5 cabin 90 ftr as an example. Or are calorifiers just small compact boat thingys , assuming larger boats with bigger motors naturally have genys running which provide AC for electric elements when not at the dock .