Hydronic kickspace heater on same zone as cast iron radiators?
You can mix a kick-space heater with cast iron radiators in the same zone but it is not recommended. My choice would be a "Beacon Morris". It comes in 3 models (sizes), and It has a built in thermostat that allows the fan to run when the water temperature is above 110 degrees F. The best installation would be with a monoflo tee on both the supply and return lines, however one on the supply will still work. I am guessing that the systems pump runs all the time and is controlled by a reverse acting control that turns the pump on when the water temp. is above about 110 degrees F. You may desire to add a thermostat to control the fan's heater to stop heat over-runs. For best results I have also installed Danfoss radiator valves on each radiator to control the heat in each room.