Some of the newer high efficiency water heaters are set up to allow for a hydronic heat loop, so you get both domestic hot water and hot water for heating. However, I have no experience with how well they work or how complicated they are to maintain. This company seems to favor the Polaris brand, but I think AO Smith has a similar type of water heater. https://www.radiantec.com/energy-efficiency/how-hot-water-heaters-for-radiant-heat-save-money/

Most traditional hydronic systems e.g., radiators) operate as a closed system where you fill them and it just circulates the same water every time it runs. The water soon loses all oxygen, and from then on the risk of rust in the pipes is very low. I'm not sure whether the system above (Polaris) is just a separate heat loop in the water heater, or if you are actually circulating fresh water for heating.

Bruce

This is probably not helpful, and merely anecdotal...My high school math teacher built a similar system for a greenhouse that was hooked up to 55 gallon barrels (black) that sat on the south side of the room. I don’t know the specifics, but he must have had a pump to circulate the water. It worked very well, and we lived in a very cold climate (40 degrees below zero).

Thanks Bruce and Susie for the help, because it is separate from the rest of the house and concrete I may try the black barrel method. It just happens to face south as well.