The 'fat burning' zone doesn't exist

Stop worrying about what ‘heart rate zone’ you are in because those standards are based off of poor metrics. No two hearts are the same. Each heart is the product of genetics, age, and fitness level. In fact, you could be doing more harm than good basing your training intensity off of a heart rate monitor.
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And while we're bursting your bubble, we should also tell you that the 'fat burning zone' does not exist. We are big fans of low intensity exercise for recovery and beginners, but thermodynamics tells us you’re burning fat at all times and in the long run, things like high intensity weight training and sprinting will burn more fat than if you spent your ENTIRE workout in the ‘fat-burning zone’.

What about the 'red zone' (a HR over 90%)? Well, that's not always good either. If you’re always in the ‘red zone’, you’ll be too tired to get in the high quality reps you need to build strength, keep proper technique, and you’ll be too tired to come back and do it all again tomorrow.
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Focus on getting strong quality reps and what you’re eating, and less about what ‘zone’ you’re in for a better long-term outcome.