I'm a homeowner who made the mistake 2 years ago of installing a tankless for my radiant floor heating. In the process of trying to 'make it work' using the tankless heater I already own, I've had the system re-piped into a primary/secondary loop system with a buffer tank.
My question with this setup is, should I be running the tankless at as high a delta as I possibly can? I've always had to run 140 deg. water in the stapled up tubing to get the rooms to come up to temperature with any kind of speed. Lacking the thermal mass of a slab or even the metal plates under the subfloor, I assumed that the higher the temp. differential the faster things would heat up. With my tankless I was thinking to set it at 158, and keep the buffer tank at 140 which would dump out to the floor loops. I'm trying to prevent the short cycling I was getting before. Any thoughts would be much appreciated. Thanks.

The higher the water temp in the buffer tank, the more heat it'll store and the less short-cycling you'll get. However, with higher temp in the buffer comes higher heat loss from the tank. If it's in the envelope being heated, no biggy.
All this from someone with a physics minor and veeeeery little radiant heating knowledge. The big boys will fill you in on the particulars I'm sure.