There is no definitive answer to this question as it depends on a number of factors, including the wattage of the heater and the size of the open area. However, in general it is not advisable to use a portable heater in an open area, as it could potentially be a fire hazard.Portable heaters are a convenient way to warm up a room, but they should always be used with caution. Many models have an automatic shut-off feature that will turn the heater off if it gets too hot or if it starts to smoke. It is important to read the manufacturer's instructions carefully before using any type of portable heater.In general, it is best to use a portable heater in a small, enclosed space where there is limited air circulation. Portable heaters should never be placed near curtains or other flammable objects. It is also important to keep children and pets away from the heater, and to always unplug the heater when not in use.