I will relate my heart rate to heat and cold but try not to give others HR advice. I'm 59 and don't have the same rate as most of the young dudes. My usual maximum is 173-4 bpm. As most, my HR zones are based on the max and is somewhat confirmed using power testing. When the temps and humidity are high I've seen HR's in the high 180's and I could continue racing as if it was Z4/5. I think the very high rate is due to cooling and effort combined. The same effect happens when it is cold, below 40 degrees. I can be doing 10 min hill intervals @ Z4 HR effort, L4 power and not see the HR beyond high Z3. A few winters ago I was slated to do a two hr ride with a 30 min WU then the rest at a solid Z3. I was absolutely killing my self trying to maintain the Z3 HR and at 1:15 into the ride I had to stop. It was 30 degrees, I was a sweaty mess, all blown up and still over an hr from home. That was one of my cycling low points.
Just my $.02, your results may vary.