While researching ways to heat a small bathroom (28sq. ft. total), I came across a post that someone put some narrow piping above a bathroom subfloor and cemented it in under the tiling. They were using a tankless recirculation loop as a form of hydronic radiant floor heating.
I'm actually installing a tankless, though I planned to do so without a recirculation loop. I also planned to put electric radiant heating in the non-shower parts of the bathroom (~15W/sq. ft.). The post got me thinking about whether a recirculation pump really helps at all, whether it can be used for radiant heating like that, and how quickly it will heat the floor, especially compared to the electric heating. I'm thinking I might go with radiant in the whole bathroom, only under the shower where there's no electric, or nowhere.
For reference, I'm putting the radiant heating in because I don't expect the mini-split to adequately handle the bathroom. The shower is probably 10ft. of plumbing from the tankless, and the farthest faucet is maybe 25ft.