Low heat acclimation after 2+ months

Have you checked that the temperatures of your activities in Garmin Connect are listed properly? Not the temperature readings from the watch, but the weather information that shows on top of the map, either on the website or in the mobile app. If Garmin is pulling incorrect data from a local weather station, that could explain the problem. See this thread for more discussion.

I've had my 945 for about 12 days, and already hit 100% Heat Acclimation over the weekend. Granted, I've had a few runs in the mid to upper 90s F, so that surely pushed it along pretty quickly. I do wish Garmin Connect would show the history of the Heat Acclimation value over time, and not just the latest value.

Definetly a low heat acclimatization is not something to "worry about", except if you are preparing for a very hot race or so. It does not take *any* personal variables into account, the only thing it depends on is the temperature, humidity and duration of your activities.

Obviously it will get things very wrong if the temperature (and humidity) values it grabs from some weather station near you are wrong, but other than that you can expect the number to climb 5-20 points if you run while it is hot, to remain stationary on the first day of not running in the hot and to start dropping on the second day of not running in the hot: So it is relatively fast moving, even just a week or two of cooler runs will drop you down to almost back to zero.

You can see acclimation history on Garmin Connect web here:

https://connect.garmin.com/modern/report/-1/all/last_seven_days

If the link does not work this is under "Reports / All activities / Training status".

If you are regularly physically active outdoors in sufficiently hot weather (consistently above, say, 75F), you shouldn't get stuck at a certain Acclim level. There are certainly climates and environments on the planet to which you could never truly fully acclimate, places like Death Valley in the Summer for example. Instead, what your watch is telling you is more like the degree to which you have been sufficiently active/exposed over time to acclimate as much can be reasonably expected. 

I may have muddied the waters, but I hope that makes sense.

Here in Finland it's pretty rare to have enough 'hot days' in a row to complete the acclimatization process. 

I run in the Southeast USA.  Not only is it HOT here (no matter the time of day - temps are above 70F) but there is constant humidity as well.  I know the garmin algorithm doesn't take into account the humidity, but I am surprised that my Heat Acclimation has never gotten above 35% since I got the 945 in mid-May.  While I do run in the morning most (when temps are usually mid 70s) I do a few weekly runs in the late afternoon as well, when temps are 90+.

It makes me wonder, if I can't get enough stimulus to get acclimated here...WHERE can I go to get acclimated?  I guess Death Valley as HermanB mentioned above.

I managed to hit 100% in Albuquerque on my MARQ before it started declining over the last 2 weeks when I both ran less and ran earlier in the day. The way this works seems to have some limits. I wish Garmin would do some post processing on the numbers so that you had a better chance of getting the weather right even if you hadn't been able to get the stars to align for the sync process at the beginning of the run. There were days that warmed to over 90 degrees while i was out running and i saw acclimation drop. That doesn't seem to make much sense to me, but I think it highlights some of the limits of the current implementation. 

you need to run regularly, 2-3 runs per week will not cut it.

I do 6-8 runs every week on average.  On days when I double, I'll run early AM (usually mid 70s) and then again in the late afternoon (temps 90+).

I can't have heat acclimation because the main temperature displayed by the watch is 10°C under the real temperature (and this is the temperature shown by Garmin Connect). If I move 20 km from my usual location I get the right temperature: I ask Garmin, should I change town?

I went on holiday and there everything was fine, Garmin Connect and the watch had the right temperature and I had 6% acclimation in two days. Now I'm back 0% acclimation and I'm running with 30°C every day.

I get all kinds of wonky acclimation numbers. I biked to work, 20 mins and it was 57deg F, and it gave me 9% acclimation. I ran 12 miles in 105 degree heat and got 3% acclimation. There is just no consistency at all and there are inconsistencies between the way my Edge 530 and FR 945 implement them as well.