When the coals build up, do you just wait until it burns all the wood, then scoop out a majority of the coals and ash, leaving enough coals to get the wood burning again?
I have the same problem with my QuadraFire Insert. I forget the model, but it's rated high enough to keep a 2,500 sq. ft. house warm. Since I have 1,700 sq. ft. it does a relatively good job, but since it isn't centrally located the West side of the house gets cool - 20°F cooler than the East side of the house where the insert is. I can keep the East side of the house 90°F warmer than outside temps if I push it, but the coals build up pretty quickly.
I can leave the insert unattended for 12 hours and while the house has cooled down considerably, there is still enough coals to get a new fire going without doing anything other than stirring them up, putting wood on them and opening the air intake 100%.