Question: I haven't been able to find anything on your web site that gives the results of heat loss comparison tests. What I'm looking for is a comparison of the amount of heat loss vs gain when using: 1) an open fireplace 2) a fireplace with an insert that has doors (glass or otherwise) 3) a fireplace with an insert that's using outside air for combustion It seems I've read that the efficiency of the "outside air" system is significant but I can't find anything that states just how much more efficient it is. Answer: This stuff is discussed in certain articles and Q and A's, but here's a quickie. Open fireplace - about 0-15% efficient, sometimes even negative Newer Fireplace Insert - EPA approved - 60-75% Efficient Outside Air - No increase in efficiency. Outside air helps a lot with open fireplaces, since these use a large quantity of air. It may also help a small amount (less than 5%) with stoves, however sometimes the cold air introduction from outside can lower the combustion efficiency.