Most likely radiant ceiling heat then, but we still can't be sure.It is too old to be a radiant in-floor system, I think, at least from systems I have seen. The modern infloor systems are 15 amp and only 120 volt. A question is why 2 20 amp circuits? One circuit should be plenty for one room, and you only found one thermostat. Something isn't adding up. If you saw 2 120 volt 20 amp breakers, they should be joined with a tie-bar. If that is the case and the breakers are installed incorrectly on the same bus, that would explain the slow reaction time of the heater. It would then be only getting 120 volts. I didn't make myself clear, there were (2) 20amp 240V circuit breakers for the system. The room was good sized, which was built over a large 2 car garage. Stopped back after my morning inspection to check the ceiling with my tic tracer, and found that there is radiant heat in the ceiling. I was going by the thin layer of ceiling texture that convinced me there wasn't any wiring in the ceiling. But using Mike's suggestion I was able to verify that there appears to be panels as the tic tracer glowed when going all across the ceiling (and then went out when tripping the breaker) instead of going off/on as you pasted by a wire.