If I want to do a zone 2 training I want to keep my heart rate at 140 bpm. It would be nice if there would be a possibility that Zwift can adjust the level of resistance (up and down) automatically to keep the heart rate at 140 bpm.
You could also figure out what power output you have at a 140 bpm HR, and set your workout that way.
The thing about HR is that it’s slower adjust than power. If, for example, you’re trying to do a Z2 workout (which might be around 140 bpm), you’d probably have to put out over your Z2 power for a while in order to get to 140 bpm. And that, in turn, would probably start a series of other physiological processes in motion that effectively negate the benefits of the Z2 training.
HR is a delayed response system, as in, it doesn’t track with exactly what’s happening at that very moment.
This has been discussed before and I frankly still have yet to see how it could or would work.
Reaching down for a bottle and having a drink will elevate your HR, and generally speaking, most folks probably don’t want an effort to decrease in response to something like that for some unknown amount of time.
If an HR-chasing ERG mode existed, it would fall into the hole of oscillation often, which isn’t beneficial for anyone, and likely to only be more frustrating than it’s worth; to the point of disabling HR-chasing workouts probably after the first try.
If you want to do Z2, set up a Z2 workout for however long you want. Once your HR stabilizes after warming up, adjust the workout bias to get your HR where you want it to be.
Without knowing how long you want to go for, you may eventually decouple, and have to lower the bias a little bit, or, turn a fan up.
Another thing to take note of is things as simple as poor rest/recovery, a warmer environment, a fan lower than it should be, can all alter your power/HR.
And at the end of the day, there is less benefit to be had when tracking HR per zone versus power; as in, it could over time, lead to de-training. Which is why power based training is the recommended form of training over the past several decades.