This is from Cleveland Clinic:

"At 50% to 60% of your maximum heart rate, you’re using 85% fat to burn calories.
At 60% to 70% of your maximum heart rate, that drops to 65% fat being used.
And at 70% to 80%, you’re only using 45% fat to burn calories."
How Heart Rate Monitors Benefit Your Health.

I have tried slowing down to hit the lowest intensity range. I can do it on the recumbent bike/elliptical. I cannot seem to do on the rower: even if I go slowly, my HR rises.