The trap door at the base of the fireplace that leads to a small metal door outside, what is the use of it:
1 - shove ashes into it
2 - open both during a fire to allow air to enter from outside for combustion, thus not pulling warm inside air from inside the house for combustion.
3 - both
4 - To allow Santa to have an excape route incase Dancer sits on the chimney (again).
5 - something else altogether
Also any tips of increasing heat into the house from a fireplace?
It’s a clean-out door.
What you want is called a Fireplace Heat Exchanger. This is but one example.
The heat exchanger described in one post and pictured in a link in another only works if a blower is attached. Otherwise the heated air emerging from the top of the c-shaped tubes is pulled right back into the fireplace and goes up the flue. In any case (even with the blower) so much heat escapes up the flue that heating with a fireplace is a losing proposition.
Notice that the room the fireplace is in can get really hot when a fire is burning. Don’t be confused by this into assuming the fireplace is helping to heat the house. Note that the air that is sucked up the flue is already heated by the household furnace. This air is replaced by outside air, which must also be heated. The furnace actually runs more when the fireplace is burning.
It’s possible to get a net gain, but it’s slight, and you burn a lot of wood quickly. If you want the maximum heat from a fireplace, you need glass doors to control the flow of air from the house, preferably an external source of combustion air (some prefab fireplaces have this, or you can get it from the ash-dump cleanout door) and ideally some additional heat exchange. With glass doors you’ll probably only get this from a prefab fireplace with such already installed.
In any case a decent wood stove will beat the most efficient fireplace by a huge margin.
Having said this, I have a masonry fireplace with no glass doors and no special heat exchanger. I just don’t kid myself I’m helping my heating bill by using it.
I installed a gas line to FP and bought the “vent-less” gas logs. No heat loss at all from house and the gas logs augment the furnace.
Now if I don’t die from carbon monoxide I will be doing ok.