Heart rate doesn't record accurately on some activities

jup, i had the same happening the first and last time using the OHR of my fenix 5. i selected the cardio workout and started an elliptical training. the first 13 minutes it went up and down between 70 and 90, which i felt was totally wong. then suddenly after 13 minutes it shot up to 150, which was correct (as i can could feel it).

this watch does and shows whatever it likes - a lot of the data is full crap. i highly doubt that i will ever get a garmin watch again.

I've had similar issues as well and it seems most of us have a time related to the issue, almost as if the programming over estimates the normalization.

On Cardio and Treadmill it takes between 5-15min before it spots the correct value and I have fiddled a lot with tightness and location and can never get it perfect.

Annoying part is, my health insurance gives out points and weekly reward for heart rate based exercises and this makes going to the gym difficult unless I wear my Polar heart rate strap.

Do you know how hard I have to run to get above 80% average bpm for 30 min if my first 5-15min says 50-60%.

my health insurance gives out points and weekly reward for heart rate based exercises and this makes going to the gym difficult unless I wear my Polar heart rate strap.

What's so difficult?

If you need an accurate HR tracking don't rely on OHR, just buy a chest heart rate monitor, that will do the work. If I had a choice I would surely buy a watch without OHR and save some money.

Like I said, I have two options, wear a chest strap, which negates the OHR, or use the OHR and run the risk of the first 5-15 min being too low and I have to compensate for it in the second part and that's not always possible.
I like running outside, doing treadmill runs are brain dead and I struggle with it.

It's clear, that HR rate with fenix 5S can't be absolutely correct, but of cause it should be "nearly good", only to have an overview and the calculation of burnt calories could be logically.

My personal opinion is, that the accuracy of HR now is worse than some time ago with other firmware. During hiking, sometimes HR shows very low values in "kein Bereich / no area". Then I take off the watch, clean the sensor (or not) and afterwards it shows nearly correct values.

I note that nearly all of those activities involve gripping and flexing the wrist. You say that you already have the band tight, the other suggestion I would make is to wear the watch higher and further away from your wrist bone. This gives the OHR sensor a deeper blood supply to measure, and is further away from wrist movement.

I had the 735XT and now the 5s on both the OHR is poor at best. I get reasonable results when running, but cycling its pathetic - similar results of 15-25min of my resting HR then POW up to 180bpm. (and when you only have 30min and as another member commented a Medical aid that gives points for fairly vigorous workouts its not a great combo)

Luckily the 5s has bluetooth so I wear my old Polar Strap to get proper results.

Look at the chart.  It's a software bug.  It's either reading elevated heart rate or not.  It's not a position issue.

I've had this problem for years with 3 different Garmin watches.  Great heart rate for runs, and this exact problem for cardio workouts.  It's not a fit issue.  It's software and it's never been fixed.