HR Broadcast

Yes. Manually select it on when you want it. Turn it off afterwards.

well, that's what I do ... it works, but can't say it's the best user experience one can have

The broadcast of HR don't work, the HR show is not accurate not even close. 

What I do is activate broadcast during activity, and then, when I zwift I start any activity on the watch, that way the measure oh HR is accurate, in the end I discard it on the watch

Ganfas said:

The broadcast of HR don't work, the HR show is not accurate not even close. 

The OP was talking about broadcasting during an activity. 

Well worth submitting a feature request to Garmin and asking them for the ability to broadcast HR during an activity that can be set per activity. As you've found out, currently it's all or nothing. 

If you are regularly active then you will likely be charging your watch frequently. In which case leaving it on probably is not going to be really noticeable in terms of battery consumption. Yep, that's a very sweeping statement but mine stays active over winter when I go indoors on the wind trainer and I'm not aware of a significant battery hit. But I do charge once or twice a week depending on how active I've been.

Can now broadcast HzR on Bluetooth while recording indoor cycle. How? Start indoor cycle activity, then hold up to get menu, scroll to wrist hesrt rate, select, scroll down to broadcast select press start again and Bluetooth HR start. Using it now in Zwift

It easier if you put HR Control in the Control wheel of the light button. much less button usage.