! Rocket water heater

I want to build a rocket water heater.

Background.

I live in Florida where the weather tends to be warmer. Winter here is an on and off thing. We normally have lots of pleasant weather in the 60 or 70s (Fahrenheit) with the occasional cold snap that can go anywhere down to freezing. Usually the cold only stays a few days at a time. The longest cold snap I have ever experienced here was 3 weeks. It was memorable because it killed off a lot of the non-native parrots. They are social nesters and can huddle together for warmth, but after a couple weeks without being able to forage, they starve. Anyways the fluctuations mean that a large warm thermal mass in my home could be a liability if the temps suddenly go back to 70 or more.

My proposed solution would be to built an external water heater, and use piping to transfer the heat into the house, with the bonus of hot water for the taps as well. The boom-squish problem dictates an unpressurized water tank, but my thought was a tank with a built in heat exchanger, that lets me transfer the heat to my municipal feed. The water in the unpressurized tank can't go above 212F, and since no heat exchanger is perfect, the water coming out of the exchanger would, by definition, be less than 212F. This could then be mixed down to get the temp needed in the house, say 140F for the taps, and lower for radiant floor, or possibly more for space heating.

Has anyone done something like this?

Incomplete diagram attached