fireplace
I never used this trap door. Presumably, it's to dispose of ash on a regular basis.
Is there any particular reason that using this is a bad idea? There's a door/ access panel in the basement to clean out the accumulated ash, which makes me wonder what the benefit is of using this. I can just fill a bucket and carry ash away every fer fires.
This is going to sound bogus but I just got this tip the other day from a friend and it worked great on my woodstove. Take a wet paper towel and dip it into some cool ashes from inside the fireplace, just enough to get a thin coating of ashes on the wet towel. Then scrub the glass with the wet towel. Within a few seconds you'll see even the toughest creosote start to disappear. I usually finish with a dry paper towel just to get it really sparkling. I don't know how or why this works, but I suppose it's probably some combination of abrasion and a dissolving action. It really is amazing.
Just FYI, if your glass is totally covered in creosote, it probably means your fireplace isn't burning hot enough, and/or you're burning too much sappy wood like pine, and/or you're burning wood that's not properly seasoned. The creosote on your glass is also being deposited on the inside of your chimney, which is bad. Pine can be burned but needs to be well seasoned and ideally should be mixed with seasoned hardwood.
Similar to @BMitch's answer, I used a magnetic cover to cover the vent below my fireplace. We found this was the primary cool air leak for our fireplace and I bet it is yours too. It works great, and is very simple to remove before starting a fire. Since it is black like the fireplace, it is not even noticed. I bought one from this site selling the exact size I needed and in black.
Here is my fireplace with it on and removed.
Best Answer
The benefit is that (with the door below closed as per normal practice) you don't need to wait for anything to cool (it's going into a nice masonry chamber, nothing to burn) and you don't need to carry ash through the house. You dump the ash (hot or cold) down the hole all winter, and when spring comes you shovel it out from the basement and go spread it on your garden (unless you are in an alkali soil area. In acidic soil areas wood ash is quite beneficial to gardens.)
@jstola's nightmare scenario is frankly a bit far-fetched. Those cast iron doors don't just fall open by themselves, and any smoldering combustion byproducts would be filtering up by the trapdoor, into the chimney, and out of the house.