Despite what Garmin says on their sales literature about their wrist-based HR monitoring, it's dangerously inaccurate. I have a heart condition and I have been concerned that the HR readings on my Venu were low ever since I got it. From day 1 the readings were lower than the FitBit it replaced. After going back and forth with Garmin about it - they even sent me a replacement Venu and the problem persisted - I decided to get the HRM-DUO and see what it said.
Pictures speak louder than words. This is a timed workout on a Bowflex HVT. It's exactly the same workout each time. Same duration, same resistance, same rest periods. Looking at the caloric output, I was remarkably consistent, too. I am using the same time-marker in both examples to show the HUGE difference between the Venu and the HRM-DUO readings. 30:03 into my workout
First up - Venu wrist based HR readings. @ 30:03 in, the Venu was reading 153 bpm and I felt with every inch of my body that the reading was not right.
"Ok, but wrist HR readings LAG behind your actual HR, so you're not being fair!" Noted. Let's go forward in the workout 1:40 to give the Venu time to catch up. At 31:41 into the workout, my Venu said my HR is 161 BPM. 
Post workout, here's the zone breakdown. The Venu said I was mostly in zone 3. Acceptable for a dude my age.
Now the exact same workout with the HRM-DUO getting the HR data. At 30.03 into the workout, my HR was a whopping 180 BPM. A difference of between 20-30 BPM against the Venu, Also note the distinct work/rest cycles reflected in the HRM-DUO data. This is a far more accurate representation of the interval-workout I do on the HVT.
Here's the HRM-DUO HR zone breakdown:
Again, this is the exact same workout. Same resistance, same work/rest cycle, and pretty much the same caloric burn. Wildly different heart-rate data being provided between the Venu and the HRM-DUO.
Yes, it's common knowledge that a strap is the best way to get HR data, but there are far too many people on here saying that the Venu (and Fenix) HR sensors are fine and accurate enough. They're not. Far, far from it actually. It is my opinion that the heart-rate data provided by the Venu should be seen as nothing more than a cute curiosity and something to show you that your heart is beating, and that's where the Venu's HR capabilities end.
Anyone that wants accurate data needs to either go to a different wrist based device or get a strap.
