Should I disable HR in fenix 5 after connected HR on chest or no?

Like in Title

In manual there is no info about that,

I had a chest HR from Fenix 3 and I wonder if I want to use it should I disable HR in watch or not?

The manual says that when the OHR is set to Auto, it will automatically turn itself off when a chest strap HRM is connected. See the manual here: http://www8.garmin.com/manuals/webhelp/fenix5/EN-US/GUID-358696A6-F98D-4772-8507-AF73499DB129.html

HTH

Yes but sometimes generate issue external and internal HR sum and show you high pick
https://goo.gl/photos/sX62GK6yVDN9Sezu6

So I need to manually disable it even if I had auto right now

So I need to manually disable it even if I had auto right now

Mine is on Auto and works great, I never change anything.

Yes but sometimes generate issue external and internal HR sum and show you high pick
https://goo.gl/photos/sX62GK6yVDN9Sezu6

I don't think that's the device adding your HR together - that looks more like an issue with your chest strap picking up noise.
But I might be wrong. One way to check is to peek under your watch and see if the optical lights are still flashing while you've got your chest strap connected.

If your strap is connected and the optical lights are flashing and it shows a high HR like in your picture then you should flag it as a bug.

I agree - that looks more like a chest strap issue. Poor or intermittent contact between your skin and the chest strap can lead to high readings as the strap makes and breaks electrical contact as well as picking up the electrical signals of your heart. This is more common at the beginning of a workout (as is the case in the photo) when your skin is dry - once you are sweating, the strap is better able to maintain contact. In dry atmospheric conditions, static electricity in the techical fabric of your running shirt can also cause stray electrical signals.

Tighten the chest strap a bit, and make sure you moisten the contact pads before you start exercising.

This is happen always for me in f3hr and f5 , when I put hrm after training on the table, so we can say fenix don't​ switch automatically but sum up both hr

Yeah I'm pretty sceptical that it's summing up. On my F3 (non HR model) I would occasionally have weird HR spikes recorded after training when I had taken off my chest strap. This would happen if I'd just finished a really sweaty session and was close enough to my gym bag that the watch would stay connected.

So I would say it's more likely that the chest strap is still broadcasting but it's sending nonsense results rather than a summing up issue.

In fact I've experienced a totally different behaviour - if the chest strap drops during an activity I would've expected the OHR to kick in, but instead I just get zero heart-rate data (or a flat 70 value).

if the chest strap drops during an activity I would've expected the OHR to kick in, but instead I just get zero heart-rate data (or a flat 70 value).

I did some testing the other day and found that as soon as the chest HRM is detected, the OHRM switched off. If the Chest HRM is then disconnected, about a minute later the OHRM switches back on. Not sure if this will be the case during an activity though.