I could be wrong but I'm pretty sure intensity minutes are calculated on the device and simply reported to Connect and that since the 820 isn't an activity tracker type device it does not calculate this…
Everything e7andy has said on this is correct. I'm here because I was having the exact same issue with my indoor cycling rides not measuring intensity minutes. Or to be more specific my indoor cycling rides were measuring bugger all intensity minutes. I would ride for an hour at threshold and the result on my Garmin connect app was 5 vigorous minutes.
I then rode on my trainer without my watch while it charged. And the result was 0 intensity minutes.
In conclusion and after testing this I have found that when I ride on my trainer the optical heart rate monitor is pathetic compared to the chest strap. I can be at 150 beats per minute and look at my watch and it's registering the same as what the start of the ride was, maybe 60 beats per minute.
The optical heart rate on my watch is always catching up to the chest hrm, and I'm not sure why but when I ride on the trainer it's just not accurate at all, and may never actually reach the actual heart rate in reality.
The only thing I haven't done his connect my heart rate monitor to my watch when I do an indoor ride which I'm not even sure I can do if it is connected to my 1030. But I presume if my watch is getting accurate heart rate data this will fix the issue.
The optical heart rate on my watch is always catching up to the chest hrm, and I'm not sure why but when I ride on the trainer it's just not accurate at all, and may never actually reach the actual heart rate in reality.
The only thing I haven't done his connect my heart rate monitor to my watch when I do an indoor ride which I'm not even sure I can do if it is connected to my 1030. But I presume if my watch is getting accurate heart rate data this will fix the issue.
There are actually two problems - first of all the optical wrist HRM can never achieve the accuracy of a chest strap. And then, when you do not start any activity on the watch, the watch looks up the HR each 15s only, and averages the values.
Hence, for the best results, you need to connect the chest strap to your watch (yes, the strap can be connected to multiple devices concurrently) and you have to start an activity on the watch too, because the watch reads the data from the HRM strap exclusively when in an activity mode. You can discard the activity on the watch at the end, to avoid duplicate activities. All of this was already described in this thread, so feel free to reread the details in the discussion.