Yes the heatmor is filled from the back, I always use an old washer hose (2 ends) If It was plumbed right should be a spigot on return side. You don't have to open the ash door which Is in the back. When you open the main door, 1st one, look down, should see a 1 inch hole in front of the fire door. Pull that flat cover off and you will see the ard rod and the flapper it controls. When you open the fire box door the rod opens the flapper and gives the fire air to lessen the chance of back draft. Also notice the door has a 2 stage latch, when you raise the handle up you can not open the door without bringing it down and around the safety catch. This is so in case of a back draft it won't blow the door into you and or burn your face off, it will catch the door. Always stand to the side with any owb while opening the door. Hope this answers some of your questions and I really hope your system is not totally trashed. They are not cheap and really are a great unit, good luck.
C.B.
C.B.