Immersion heaters were a new convenience when I were a lad! The water was heated by a back boiler behind the coal fire. It circulated to the hot water tank which was in the airing cupboard.

What did we do for hot water when the fire wasn't lit? That's what kettles were for:-) Incidentaly, unless you are using a combi-boiler you will still have a hot water tank, even if the wtaer is heated by gas. It's all these non-plumbing typpes confusing the issue by calling the hot water tank a boiler. One think the hot water tank should never do is boil. You are in trouble if it does!

Cheers

DtG