I have no idea the only experience I have had is just the tankless water heaters which are all over the place in Europe. I am not sure how solar works for a boiler either I have a boiler but gas fired.

"Joe Macker: What happens if the incoming water to the tankless is hotter than the tankless is set to? Does it just let the water pass through?"

Yes. Just make sure it's not hotter than the safety limits of the heater to avoid triggering a thermal fuse or other such protective device.

I have no direct knowledge, but when reading up on using tankless heater using preheated water from a geothermal system, I read comments that some tankless heaters won't sense the need for heat if the incoming water temperature is too high.

I don't quite understand how your new boiler and hydronic baseboard space heater is expected to work.

Most heating system boilers are designed to be run as a closed loop system, so they heat and circulate water through a sealed hydronic system that has some type of heat radiating element (radiator, baseboard heater, pipes in slab, etc.). You could use a heat exchanger and use the boiler water to indirectly increase the temperature of water going into the tankless heater, but with a closed hydronic heating system you can't feed the boiler water directly into the tankless water heater.

Also, many baseboard heaters are designed to run at temps up to about 180 degrees, which is quite a bit higher than you want your hot water in the house.

Bruce

@Bruce in Northern Virginia the closed loop of the boiler will have a point where it bifurcates into a pipe that goes to the cold-water inlet of the tankless water heater.

What happens if baseboard heater is run at temps of around 120F. I don't see any downside other than taking more time to heat up the space.