Exact loaction of forced air under fire?

[heus]

I am installing a homemade forced air unit in the back of my firebox. Where should the precise location of the duct be that runs through the back wall of the firebox? I have a 2 1/2 x 12 duct that will enter from the rear of the firebox wall. Should it be halfway between the grates and the floor, or right below the grates? I plan on cutting a hole for it in the morning and wanted to know if there was a specific location for the duct to enter.

[buxtonboiler]

I have mine located about an inch above the bottom of the fire box, under the grates,and centered.

[heus]

Thank you bb

[Bucket Head]

I located mine about two thirds up from the bottom of my ash pit. This way there is room for the coals/ashes that fall through the grate to go below where the air is coming in. This way it takes a while for the ash pit to fill up to a point where the air is deflected or blocked. It has worked well so far. I should add that I have a long and narrow duct and a fairly good sized ash pit. A rig with a small ash pit and a blower with a large duct might not have the room to locate it like mine.

Steve