According to the blueprint, fat burning occurs in the 55-75% range.
For a rough estimate of your max HR take 226 (220 for men) and subtract your age. It's not a perfect science, as I'm 27 and can get my HR up to 200 without puking or passing out.
For example, if you're a 40 year old woman :
Max HR = 186
75% = 140
55% = 102
I knew a girl who was working out with a trainer I was using who would get her HR up to 180 during minimal work, she wouldn't be breathing hard or anything like that. If this is your case, I would probably check with a doctor to make sure everything is working right.