Depending where you are and your insulation floor heat can work well to 30-40 F. . lower then that and you need radiators with fans to help out.
I have 2 inches of insulation with in floor as well as radiators with fans. From a below 0 cold start, using radiators and floor, I can get the bus to 65į in less than an hour. Once warmed up I can turn off the blowers and just the floor will maintain down to 10į or so then I need to run some of the blowers.
Even at below 0į my boiler cycles so I know I haven't reached it's limit.