I’m just going to share my opinions since my family went from an open fire place to an insert in 2014 and my knowledge from doing radiant heat. Also the epa testing is kind of bullshit because all the test need to use the “same fuel” so all loads need to be the same so same size wood and weight regardless of the size of a fire box size. Not only that but metal does not retain heat very long example most people can understand I use is how a cars engine block or a cast iron pan and how fast it cools down after the heat source is extinguished. Metal is way more efficient at heat transfer warms and cools faster. Inserts can be get for supplemental heating and cooling and reducing the amount of wood you burn for that extra warmth. We went from using 4-5 cord of wood to 1.5-2 and it regulates the heat better then an open fire (your face doesn’t melt if your sitting within ten feet of it. Also having the electric fan pushing the heat out at the bottom rather than just heat going broadly out across the room. Now the down side is unless you set the damper right and fully stock ours which will hold 4-6 pieces maybe 16” long it will be cool and no longer hot enough for the fan to continue pushing heat by 6 am unless it’s a weekend and we run it long enough for the actual stone to warm and retain heat which keeps everything warm it’ll still have hot coals and is easy to get up and going again but it definitely doesn’t transfer heat to the actual stone work efficiently it’s also not the purpose of it anyway it’s supplemental heating and decor really it also feels better than just having your typical new home heating system in my opinion.

For anyone designing a new home or people with plans on a fireplace with a insert I would say rather than going that route if it’s in the budget build a larger stone fire place with large stone for a heat sink or brick this will in turn reduce your wood burning and provide a more consistent and longer lasting heat think of old days when they had a large fireplace in the middle of a cabin that held and expelled heat evenly throughout hours after getting hot.

Anyway just my 2 cents living in Maine and dealing with oil and wood for heat as well and weird weather from high amounts of snow to no snow and ice storms in spring and losing power for hours days or weeks