I have the same model heatmor that you have and i heat a 2800 sqft house and a 1000 sqft attached garage. I have 100feet of pex supply line and it can be 30 deg out and i never have any snow melting above my lines. I also can comment on if you fill your stove full it will burn it. I started filling my stove about half full and started stacking it in the front. I am getting the same burn times with 1/2 as much wood. I am only shooting for around 8 hours at a time between checking and filling the stove. Here in MN it gets cold in jan with a normal high temp for January around 15-20 deg.
About the damper doors..... I seem to be always chasing mine. I watch the smoke out of the stack and when its coming out at a good clip the door is sticking open. You then have to go and pull the blowers out with the 2 thumb screws and use a scraper and remove all the crude from the flap and the hinge area. Its very very simple and will save you from lots of over burned wood. I usually check mine once a month. Just watch your stack and check it once a month. If you think a flap is open you can tap on the round tube that the blower is mounted to and listen if you hear a flap shut you then know which one is giving you trouble. I also have gotten coals and ash in the back blower housing that can get stuck in the flap and keep it open. These are some of my troubles and i hope it can help u some and others.
I am not sure how many pickup loads that i burn a year but i do have a pick of my wood pile. Let us know more about your setup and we can help you. Dont mind all that smoke from the old smoky i was cleaning up some wet 2x4 scraps from last year that had a summers worth of rain on them.