Lots of good info!
Heat, as well as many other outside factors such as humidity, fatigue and even caffeine, can definitely affect heart rate, making it harder to keep your heart rate in a specific HR zone and even change from day to day. This is why at TR we are firm believers that power is the ultimate metric if you are doing interval-based training. 
In short, if you’re out of zone on a hot day, you’re still out of zone. So, it’s important to really listen to your body and adjust your effort accordingly, especially when training in hot conditions. You may need to lower your intensity to stay within your target heart rate zone.