Recently I found out my heart rate during ERG workout is lower than race or free ride even the average power output is the same, and sometimes race power even lower then ERG workout but HR is much higher, for example, recent experience on riding Z1/Z2 on Zwift free ride my average power is 15x watt, and my HR will be 128 ~ 13x, but if I just do purely Z2 WO for 2 hours, my HR mostly stay at lower 120 when my power output is same 15x watt, does anyone have similar experience? and how do you improve that to bring down your HR during race or free ride?
What is your cadence in comparison? If you ride with a lower cadence in erg, that could explain the lower HR.
I thought about that, but both are 89, 90 around, not much different.
I would think that would be normal for a more structured w’o to induce a lower HR as oppose to a race. A race will be random, perhaps your adrenaline/ nerves will be different and maybe your less structured over your fluid/food intake during.
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For racing, that is understanable, but for a social z1/z2 pace on Zwift is something I don’t really understand. One theory I have is that because ERG mode locks at certain power, so the muscle works on a more steady pattern, less variation, and in free ride, I am constantly changing power output to try to match the pace, if drop, a bit higher to move up the position, if ahead, relax and drop back. This variation pushs up the HR and never had a chance to recover to a lower HR. Interesting thing is I tried a short workout in resistant mode which is more closer to free ride but my HR is just like ERG mode, steady and lower than free ride pace.
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Yip that’s the theory, the constant random change of pace/power etc drives up your HR as opposed to a more constant state ERG workout but I am surprised you are finding it in a Z1/2 social ride too.
On a short free ride there’s perhaps less of the random ness as to power fluctuations 
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I reopen this thread because I have as of late experienced this and felt like sharing.
I lately have very low heart rate at relatively high power outputs (measured with the same power meter) during endurance/tempo ERG mode workouts.
I never see such aerobic efficiency outdoors.
This is even more pronounced when I use my naturally low cadence instead of pushing myself to have a higher cadence.
I wonder if indoor workouts are the best option to develop real world cycling fitness because muscle groups and fibers involved I think somewhat differ leading to different effort for the body at the same power output.