Automatic set back thermostats will drop the house temperature during the night and bring it back up before you get out of bed in the morning, saving you significant heating dollars. But do they work with radiant heat?
The answer is yes and no -- don't you love that?
If your radiant heat is a very heavy system, like hot water coils inside a concrete slab, it takes so long for the concrete to cool down, and then so long for it to warm back up that it won't stay down for long enough to really save a lot of money, and in some houses it won't be warm in the morning unless you leave it running all night long. The lighter the radiant mass, the more possibility that this could be a useful way to control the hot water.
If you look at the new radiant system in the photo called Plasco, you will notice that the hot water tubes are right under the rug. This system could probably profit from a setback thermostat, but still not as much as a forced hot air system.
Electric radiant systems placed right under the tiles take about 20 minutes to change the temperature, so they react much like forced air furnaces with set-back programs.