Hi Ron,
I have not heard of floor heating, but in theory it could work and would be very safe. The heat would radiate up and with the air circulation, eventually the whole kiln (walls, ceiling) would reach a state of equilibrium. Keep in mind the heat source will have to be hotter than your target temp. So if you need to sterilize the load for insects at approximately 140 degrees F, you will need higher water temps. I won't quote any required heat source numbers, as it all depends on volume of kiln, how well insulated, and how air-tight the chamber is.
In my small self-built kiln, I use a PTC heater from Pelonis out of Pennsylvania. It is very safe. I compiled some quotes from their website:
Positive Temperature Coefficient ("PTC") heaters are specialized heaters made from an advanced ceramic material, enabling safe, powerful, and energy efficient heat transfer. PTC heaters have heating elements made from barium titanate-based ceramic stones. The material’s unique properties enable a PTC heater unit to self-regulate; it can run open-loop without needing external diagnostic or feedback controls. As a result, PTC heaters are highly reliable, offering consistent and uniform heat without the risk of overheating. PTC has a unique heat property that allows the PTC heat output to be regulated by the airflow applied. The higher the airflow, the higher the heat output and power (wattage). Therefore, depending on the end design, if the ambient temperature is high, the PTC output power will decrease; if the airflow is high with low ambient temperature, then the PTC output power will increase.
My 1000 watt unit in 2021 was sub $75 (complete w. I recently purchased a 1500 watt unit for around $170 to lower the ramp-up time. My PTC has a constant speed fan and it is controlled by a Penn/ Johnson Controller. Although not needed, I wired in a fan delay relay to have the fan stay on for 30 seconds after the heater is deenergized.