Can E+ model an oil heater ?

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I believe you are describing what is commonly called an oil-filled heater or oil-filled radiator which use the oil within the heater as a thermal storage medium. The oil within the heater is warmed by an electric heating element and the surface of the heater stays warm and never gets too hot to touch. Because of the moderating affect of the oil, the electric heating element does not cycle on and off as often. If this is the kind of radiator that you are describing, I would suggest that you model it using the ZoneHVAC:Baseboard:RadiantConvective:Electric object in EnergyPlus. Ultimately, the energy added by the electric heating element gets added to the zone. The extra lag due to heating the oil cannot be explicity modeled but could be approximated by using a higher radiant fraction to the floor of the zone. This would cause a delay in the speed that the heat added by the electric element in the heater enters the space.