HR monitor differences

What kind of treadmill sensor is it? In my experience, the handheld type that you have to grip are never accurate for me. Even my expensive Precor treadmill HRM is terrible.

I think they are Precor treadmills, but I tried 2 different ones and my heart rate was the same on both and then the same on my watch when it's not tracking my run
Only when I start my run does it seem to be off.

It's too hard to tell if the watch is correct, or the treadmill is correct. If you have access to a chest strap heart rate monitor, that would give you an answer. Do you mind linking an activity or two here? Or attach a picture of your heart rate graph?

I usually trust my watch over a treadmill, to be honest, but it could be the opposite in your case. That being said, sometimes my 235 takes a few minutes into a run before it levels out, so that might be what you're seeing.

I don't know how to link activities but I can tell you that what I see is in the activity my HR may say 170 but back out totally and scroll down and it will say 155 and the treadmill will be close to the 155 also. So something in the activity is not working right. I'm not sure if it's wrong when I run outside or not.

I don't know how to link activities but I can tell you that what I see is in the activity my HR may say 170 but back out totally and scroll down and it will say 155 and the treadmill will be close to the 155 also. So something in the activity is not working right. I'm not sure if it's wrong when I run outside or not.

What's your cadence? Perhaps it's a cadence lock issue (maybe wear the phone tighter?)

I'm having a hard time following the last post about 170 vs 155. Your watch says 170 during an activity, but your phone says 155? Without being able to see the activity, I'm not really sure.

What I'm trying to say is that my watch will show a higher HR during the activity, like in the run data fields but if I stop it right away and back out to the main clock and arrow down my HR is much lower and that HR will match the treadmills. So it's like it's measuring high during the active sessions. I can start an activity and it's high already. A couple of times it didn't even show a HR for a few minutes. And my watch has been either tight or just a tad loose. I tried both ways.

Ok, I'm following you now. It seems like you have a couple of things going on.

1. When it doesn't show your heart rate for the first few minutes, it's because it doesn't have a lock on your heart rate. This is an issue that most of us have been facing and it can be incredibly annoying. We usually have to wait a few minutes for it to have a heart rate lock before starting the run. There are other workarounds that people have discussed in this forum, but none seem to work for me. I just wait.
2. In regards to your watch heart rate being way higher than the treadmill, Flipstone might be right in that it might be a cadence lock issue. Look at your heart rate and cadence charts in Garmin Connect to see if your heart rate matches your cadence.

Do you know what firmware your watch is currently running?

I know the HR is the most recent version and I think it's like 5.2 or something that just updated. My cadence is usually in the low 170's. I'm not sure I understand how that and HR would be the same though.