The Installation and User Manual for a Rheem condensing/modulating tankless heater suggests its use in a system that heats DHW as well as house heating by means of a fan coil or air handler.  The manual cautions against using the heater for residential heat only.  I had thought it would make a good heat source for a radiant floor.  One job only, no DHW, no heat exchanger, just the tankless heater, a circulator and a single zone floor system of pex in concrete.  I'd appreciate reading some comments, other than the caution in the manual, as to why not to use a tankless heater in this way. 

Well depending on the heat load applied to it, using it for a heat source may exceed a reasonable duty cycle and it will fail sooner than later. Depending on the installation, it may not pass a code officials review to signoff on your completed permit unless it is specificially identified on it's nameplate as being approved for domestic heat(boiler) service. Or at a later date when selling your house, it may not pass a home inspection prior to completing the sale for the same reason...

Thank you, ronmar.  Those are good and helpful comments.  In the manual and on the website it does appear to be recommended for residential space heat as the source of hot water for an air handler for forced air heat (one unit...11,000 to 199000 BTU).  For reasons that are unclear to me, the user manual does not specify its use for radiant, and I wonder what is the basis of that omission.  Are the return water temperatures from an air handler lower than from a water to water heat exchanger?  Are the run times shorter and more frequent with water to air, and if that's a problem, is there some way to work around that issue?

I'd look at the resistance to water flow. A typical tankless water heater has a high pressure drop at the flows you want for radiant (without hot spots).

A heat exchanger could fix the issue (ie, convert low flow/high delta T to high flow/low delta T).

It can do outdoor reset?