The 18” fireplace inserts will run continuously until it is turned off or the unit’s safety features are invoked.
For the 23” or larger models, the heater is programable, so that if you select 80°, the unit will turn off and on as needed to maintain that temperature setting. If you prefer the unit to run nonstop, press the arrow button on the remote until the digital display on the fireplace reads "on." This will keep the heater running until a new selection is made. The arrow button also lets you cycle through the different temperatures plus the "on" selection, and you can set it for anything from 64° to 82°.