Why can't you just close the window on the draft blower?
RC you can close it but draft still pulls up from the bottom of ash pan and through the holes there is a gap above the roller grate in the front and back.
The plates are made with an air gap in them the one in front allows some of the draft to go into ash pan and some to come through holes for the over the fire draft.
For the holes in the back plate I took some fiberglass insulation and my poker and plugged the gap by opening ash pan and stuffing it from from the bottom to block draft in the back if you have been burning for a while it may have plugged itself off with ash.
For the front I took a 1/4 inch plate and cut it to fit over the front plate of furnace that has the holes in it and then drilled and taped it so the bolts line up with existing holes and used 2 bolts to hang plate by sticking bolts through the existing holes. I did this thinking I could switch back and forth from coal to wood if need should arise. I started furnace up in october and have had it going on coal from that time on I did try to burn wood but like the coal so much better.
I had same problem with my furnace til I made these 2 modifications now I have no problem with getting fire the length of grate sometimes the back will burn up before the front.
Draft will take to path of least resistance so it will flow up through the gaps in the front and back rather than through the coal bed.
I had to add a draft inducer to my chimney pipe due large flue on and outside wall which caused poor draft.
I hope you have CO monitors installed.
Stove size coal is approx. the size of a baseball and some as large as a softball.
Hope this helps you out.