Good question to which there is no clear-cut answer.
We do not have much need for radiant heat flooring in the lower deserts of Arizona, so I inquired with colleagues in a variety of geographies and climates to get their experiences.
Some are using electric tankless systems in such applications and are happy with the results. They tended to be in slightly milder climates where usage might not be as intense.
Others in colder climates were adament that electric tankless water heaters are built for intermittent bursts of intense heat and are not designed for long-term, continual use like boilers.
You should talk to a few local radiant heating contractors and get their recommendations. You are going to have to find someone who feels comfortable performing and warrantying the work anyway.
The hydronic system is going to be a closed-loop system, so it will need to be separated from the domestic hot water.
Two other considerations:
- Watch for newer electric tankless water heaters coming out that are specifically designed for hydronic heating systems. They will have the flow meter and other duplicative technology stripped out so that they can be integrated with the hydronic heating system and also manufactured for lower cost.
- Consider a system where you use solar thermal as a source for pre-heating and a tankless system for backup or that additional boost to get the fluid to the temperature you need. The capital costs and the equipment footprint will be larger, but the operating cost savings should be significant. A variation on this idea that uses no tankless water heater is described in this Midwest Renewable Energy Association Fact Sheet. Again, you will need someone experienced to design and install the system.
Good luck, and stay warm this winter!