That's an awesome stove your dad has. I am sure it will burn less wood being that it is the top EPA efficient stove. The H1 isn't perfect, and is older technology. So, far though I can't complain. I load it with (4) 14-16" Beech Logs and it burns for around 12 + Hours. After the fire goes out the stoves stones are still hot for hours and radiating heat. I didn't haven't to pay very much for it, and it seems to be doing the job. From what I have read though, that stove your pop has is really something special.
My brother uses this one and I know it would heat the heck out of his house in central PA. It has moved to NW Illinois and is heating a much bigger farmhouse successfully, so I know it cranks.
Now, when you say 14" logs, are you talking about unsplit diameter? You aren't throwing in whole logs are you? How do you get those burning without splitting them? I never had any luck unless we are talking a small log or large branch.