Heat acclimation not reliable

I also recognized this. I did about two dozens of rides of which most of time the temperature was over 30 C, sometimes over 35 C. Only after one ride, I got 16% heat acclimation, then the value was decreasing again. In former years, with Edge 530, I certainly would have been close to 100%. Non-solar-version 12.16 (and already with older firmware, I had recognized the heat acclimation of 0 when doing long rides in really hot weather).

Not new. It’s never been reliable.  The 1030 was the same. 

Huh, OK. For me, on both 1030 and 1030+ it was perfectly reliable for all rides on days showing temps over 24 C as per Garmin's description of their use of Heat acclimation - even for Indoor rides for which I turned on GPS on hot days in order to get "credit" for exercising in the heat (which seemed fair to me as I have no A/C in my pain cave).

The fact that the 1040 does not report heat acclimation reliably outside - and reports absolutely zero heat acclimation response for indoor rides with GPS turned on and weather widget connected to the internet plus the unit reporting over 24 C for the ride at the activity end summary page is brand new to me only with the 1040 Solar that I now have.

Just the other week, two of us had gone out in +30 and I gained heat acclimatisation with a 1040 and he gained zero with a 1030.  But I’ve found the same using my 1030 also in the past. 

I’ve never fully understood how it is meant to work.  Does the unit need to be over a set temperature at the start of a ride?  Does it need to stay over the set temperature for all the ride?  Is it the average?  Does it need to be over a certain time?  Quite a few factors that I wasn’t sure about.  

Add to those factors ... how hard did you work?  Which may present a new question.  Does it require a power meter and/or heart rate monitor?

I have both as does the other person. Neither us do what you would call “soft” rides.  To be honest it doesn’t bother me. It’s a bit of a non display. What does it really do to help me in what you do?  It’s just a number. My body gives me better feedback than some number on a screen. 

It's supposed to log Heat Acclimation if the temperature, as read by the weather widget that gets temperature data from the internet connection of your phone if the Edge is connected to it, if that temperature is above 24 C. I cannot speak to how long your ride has to be but my 1030 and 1030+ always logged an improvement under those conditions when my rides were high intensity over 30 minutes (like every Zwift race) or moderate intensity over an hour. And now my 1040 logs nothing.

For me why this matters to me under this extended heat wave where I live, is because Heat acclimation changes relate to VO2 Max changes. In the past, once my Heat acclimation has gotten up into the 80% realm I've seen my VO2 Max stop dropping due to heat and resume it's upward trend, and this matches how I feel pretty well. It's fine if not everyone uses their thousand dollar toys for athletic performance training but HEY GARMIN I do, and I need this to work right now please.

On my 1030+, I noticed if I started a ride at or below 72* F (Garmin claims temperatures above 72* F will be eligible for heat acclimation calculcation) and the temperature rose well above that, I would not gain any heat acclimation.  Even if I worked hard for hours.  Nada.  Nilch.  Nothing.

Personally, I don't worry myself over the heat acclimation number.  It's just a fun number and fun to look at.  At the end of the day, it does nothing for me.

I wonder about this as well. In the Bay Area, easy to start from home near the bay where it is cooler, then go over the hills and is 10+ degrees hotter for most of your ride….