I totally bought in zone 2 training since the end of the summer season. While it’s counter intuitive in the beginning, it’s doing miracles if you stick to it. The biggest challenge for me was to do it outside. I live in a very hilly region. But I nailed it in the end. One thing I started to think is actually hinted at in one of the blogpost of Alex : what’s the point of doing a ramp test to get your FTP? The ramp test seems to me more a function of your anaerobic capacity. So I’m puzzled there. I did something different which works great at the moment. Probably not very scientific, but it works. After doing zone 2 for some time, my heartrate at the same power levels dropped significantly. I know, heartrate varies, not very reliable,… However, I noticed over a couple of days of zone 2 session a drop of at least 10%. So without doing another FTP test, I added 10% to my previous FTP. And bang on, my heartrate went up to the ‘target’ part of my estimated zone 2 again. And while cycling outside, I notice the new FTP is rather accurate. Any thoughts ?
So happy to read this!
FTP itself is aerobic but the ramp test goes up into the anaerobic zone. The ramp test does a good job at estimating your FTP, and an FTP within the ballpark is important to correctly calculate load. But for the rest, it is rather useless in my opinion. Unless you like ‘bragging’ numbers 
For Z2 training, HR is the best way to go. If you do it purely by power, you will neglect the fact that all days are different. Sometimes your body is better or worse at accepting the load you put it through.
What you see with the dropping HR is that you are becoming more efficient. Efficiency factor (Watts/BPM) is a metric widely used in training analysis.
Now, let’s be realistic, the gains you are seeing now, will not continue. It will gradually become slower and slower, because now you are at a point where your fitness is rapidly evolving. But the better you get, the slower/smaller the gains become. You see, I’m not saying that you shouldn’t use Power, you should use it in a different way depending on the type of work you’re doing.
At that point, it is a good idea to add in some sparse higher intensity work. The aerobic work you’re doing now, is the best way for beginners to rapidly gain fitness. Look at it as ‘raising the floor’. But once the floor comes close to the ceiling, you will need to raise the ceiling. Just to make more room for the floor to raise even higher.
It is so difficult to get the message through to beginners, that the way to go is ‘easy’. Stories like yours, may convince a couple more people that it does work a lot better then the ‘No Pain, No Gain’ philosophy.
You gain basic fitness fast and have close to no risk of getting injured or overtrained. But you need to have a bit of patience for your body to react. This work makes up the base of a pyramid, a base that will be so strong that you can build high peaks on it later on.
So continue to have fun and ‘Smell the roses’!
Agree with all of the points made. 2 additional remarks:
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HR is indeed variable, can be different from day to day. With Trainerday, the +/- is what it takes to tune it. It’s HR before Power. I had a conversation on it with the GCN guys and they actually asked the question on one of the videos of Dr. Inego San Milan. HR is closer to representing what it’s all about : what energy system you are using. And that’s what you need to train, not the power as such. I’m currently looking into non invasive lactate measuring devices. Unfortunately one of them was too early on the market and the company from Texas is no longer around. There’s another one I’m looking into, but it’s quite expensive. Don’t understand why it’s not used more often. But that’s another story in itself.
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When doing Zone2 work outside, you need to look at a combination of power and heartrate. For the simple reason that heartrate is dragging. Especially on climbs, look at your power of which you know it’s your zone2 ceiling. Don’t go over it (which means : switch back to real low gearing and have the willpower to drive so slow uphill). The heartrate will catchup and you stay in the zone.
What I did since end of september is zone2 all week indoors. Saturday going out for a ride without looking at numbers. Like somebody on a diet with one day of “no diet”. On Sunday I do a zone2 ride outside. Since this week I started to do one HIIT session indoors in the middle of the week. Taking the advice of somebody else on this forum : first a good portion of Zone2, the HIIT at the end, and not the other way round.
It’s all big fun to work with, and the results are stunning 
You are on your way to make good progress for years to come
You already understand when to prioritize power and when HR is more important.
As I said, HR gives a picture of the internal load and is what you should follow for endurance rides. Never compare from one ride to another, average over multiple weeks and compare periods to see progress.
Power is external load and gives a better picture on performance improvements, especially in the higher intensity zones, where HR becomes unusable due to the lag.
As a low HR focused guy for last 12+ years. I love to hear peoples success. I have not been consistent enough to get fast but my aerobic engine significantly improved.
Is the heart rate that good at estimating whether am I working in zone 2? Currently I feel like to be below or around VT1 could be a better “metric” perhaps?
My subjective example experience from today’s workout on a trainer indoor was that I was at VT1 (nose inhale, longer mouth exhale) but heart rate was creeping above the zone 2 rate and I was sweating. Then a cold wind gust came through the room (felt really nice) and the heart rate jumped down by 3-5bpm. This repeated every time another gust came in.
At that point I realized that I’m seeing higher heart rates at similar wattages than 2-3 weeks ago due to being overheated (it was freezing outside weeks ago, impacting room temperature). However, I’m also seeing that the wattage wher I’m able to be still at VT1 is getting higher as well.
Working at zone 2 power, VT1 but higher heart rate diminishes aerobic adaptations? Anyone with any experience and / or thoughts about this?
First things first : it seems like you don’t have a proper ventilator for an indoor training. Get that first. There’s a great GCN video where one of the presenters did exactly the same workout with or without ventilator. The difference was 10 bpm if I recall. Which is normal, you are overheating. Even if the room temperature is low (like close to freezing), you still need a ventilator. Reason : when you are outside, you are moving and the air is cooling you. Not so indoors. So if the room was let’s say 10° and now it’s 20° ©, don’t look any further, has nothing to do with your aerobic adaptation.
Secondly : heartrate is an indicator, but it’s not perfect. It’s “the best available”, if you take lactate testing out of the possibilities. Zone 2 is really about what energy system you are using. Which is only be accurately defined by lactate measurements (and to be honest, even then some specialists have different opinions on what zone 2 exactly is). Heartrate is the next best thing, but it’s not perfect. It can be impacted by variables like : overheating (like you experienced), but also getting a cold or a flue.
So in summary : get the cooling fixed and keep in mind it’s an indicator, not a hard metric KPI.