Question: I keep seeing the recommendation in your answers to previous questions that unvented logs shouldn't be used more than 3-5 hours per day. Can you please explain the reasons for this? What problems would running them 8-10 hours per day cause? Does the 3-5 hours refer to the high setting or any setting? Thanks for your help. I've learned a lot from what I've read so far. Answer: Many of the unvented appliances sold today are oversized. This can cause problems with excess moisture, carbon monoxide and stale air. These units are not made for continuous heating. In most cases, the 3-5 hours relates to a low or medium setting...depending on the size of the room or open are that the appliance is used in. If the area is very open to 1000 sq feet of home or more, then a higher setting might be OK...but still limit the number of hours.