I have just done my usual 1.5hr road bike ride with the fenix 2 and edge 800, avg heart rate zone 5, yet instead of the usual 1000 calories the fenix says 165 calories and the edge 650 calories, is there something wrong in garmin connect or do you think its hrm / elsewhere related?
Those are what the devices are showing. Garmin Connect doesn't do any recalculation, it simply shows the total numbers in the files that the devices put there. CBORRMAN should look on the devices themselves and at the HR recordings in the files.
I have the same issue on the last 2 workouts - ran 4.19 miles yesterday and burned "70" calories. Shoulda just lifted a glass of wine to my lips for 40 minutes. Heart rate data looks about right. But when I swam last night (no HR monitor) the calories on a 2100 yd swim were 365 or something that looked about right. Any thoughts?
Calorie burn with any device is just an educated guess. Using HR data, they'll likely be slightly more accurate, but the results will be prone to more variance from one activity over the same course and time to the next. Without HR data, you might see more consistency of the data, as the calculations are essentially just doing a set formula that is based on averages similar to how we used to estimate Calorie burn before these niffty devices came along. However, consistency does not mean accuracy.
Many of the Garmin devices in combination with the HR data will use First Beat technology to calculate Calorie burn. There is a pretty good article on it over here http://www.dcrainmaker.com/2010/11/how-calorie-measurement-works-on-garmin.html It's a little old, 2010 but it gives you some food for thought. Also, it seems I saw that Garmin might be using some newer methods in some of their newer devices, but I've not read up on them as I don't use those devices.
At any rate, 70 does seem low for your run, but IMO, the best place to find out why might be in the forum for your device. Sometimes it's just a one time glitch. Other times you may need to do a soft or maybe even a hard reset. And sometimes people have seen changing their activity level in the setup (if your device uses 'activity level') make their Calorie burn look correct.
And if you consistently get unrealistic burns, then if you are in warranty and nothing else works to fix it, call Garmin Support. After they have you jump through some troubleshooting hoops, they'll replace it free if they can't solve the problem.
