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

I once thought using water as a component with a rocket mass heater was a win, win.

then I found out that water a rocket mas heat are a dangerous combinations.  

I don't know ...

https://permies.com/t/224383/rocket-mass-heater-hot-water

Hi L,  yes, it can be done. I'm not sure how well floor heating will work, but in your climate, it has a chance.
Here is one https://permies.com/t/272814/RMH-heats-floor-water
Jeff lawton style https://permies.com/t/155422/Plans-water-heater
https://permies.com/t/190806/built-rocket-water-heater

Hi L,
you say a large thermal mass might be problematic. To me that means I would also dissuade you from using under floor heating, as that uses all the floor as thermal mass and is usually a system with a lot of inertia.
You could supply hot water heaters or the heat exchanger of an HVAC with the hot water.

Benjamin Dinkel wrote:Hi L,
you say a large thermal mass might be problematic. To me that means I would also dissuade you from using under floor heating, as that uses all the floor as thermal mass and is usually a system with a lot of inertia.
You could supply hot water heaters or the heat exchanger of an HVAC with the hot water.

I do believe I have seen underfloor heating systems that have low inertia. Such a system could be suitable for Florida weather, but I could also see running a water/air heat exchanger for forced air heating. Underfloor appeals to me simply because it can create comfort without heating the air as much, meaning my sinuses would be a lot less dry compared to forced air.

Before I started thinking about heating water, I had pondered the idea of adding tubing to my thermal mass, which would allow me to circulate water through the mass to get hot water, and I could also valve in a diversion loop, which would let me circulate the water to a ground loop, so if the weather warmed, I could dump the excess heat into the ground. But I would have to know how fast I could cool the mass to room temperature, in order to know if the idea was practical. That led me to a rethink, and the idea of heating water outside.

Anne Miller wrote:I once thought using water as a component with a rocket mass heater was a win, win.

then I found out that water a rocket mas heat are a dangerous combinations.  

I don't know ...

https://permies.com/t/224383/rocket-mass-heater-hot-water

I had not seen Geoff's video before. Thank you for that link. His design looks exactly like what I was thinking of, and it answered a number of my questions.