Heart Rate Monitor Help :-)

The "fat burn" zone is meaningless. As Dr. Tim Noakes points out in his book The Lore of Running, even if you burn a higher percentage of calories from fat when exercising at low intensity, you will burn more fat calories total at a higher intensity.

Moreover, as long as you are consistently maintaining a calorie deficit, you will lose weight. That lost weight will come from one of two places: (1) fat tissue and (2) lean body mass - mostly muscle.

It's a good idea to do some strength exercises while losing weight to ensure that most of your loss comes from #1, not #2. As long as you do that, it doesn't matter whether the calories you burn while exercising come from fat or carbohydrates, because you'll adapt to use more fat during everyday activity. Almost all of my cardio is at moderate to high intensity, about 7-10 hours a week now that the summer is here, and I've lost quite a lot of fat.

It's also worth remembering that most HRMs are best at estimating energy expenditure for steady-state cardio, such as brisk walking, running, cycling, etc. at a near-constant effort. The more you vary your effort, the less accurate their calorie estimates become.

I spend about zero percent of my time in the fat burn zone. I like to go right to max, apparently.