I'm no scientist but heres my understanding of two ways in which heat has an effect on you HR, firstly your body requires more blood to the surface of the skin for cooling purposes in hot weather. Secondly, blood volume reduces as a result of dehydration, so less blood to circulate the oxygen in your system (So hydration is very important, to minimise this).
Both of these issues drive your HR up for the same pace you might do on a cooler day, simply because your heart is having to pump more blood around your body to feed your running muscles, cool you down and keep you hydrated as you lose fluid due to the fact your running and sweating.
In terms of HR training, I consider my HR to be like the rev counter in my car, I'm not looking for an improvement on each run, but I am looking for an overall trend towards faster times over a period of months. And, thats what I've seen so far, I ran 17 miles on Saturday at 9.11min/mile pace in 28 degrees, whereas two months ago, I ran 8 miles at 10.19min/mile pace and it was a lot cooler. I think thats a good measure of improvement, and an indication to me that my training is working.
I think you just got to accept that you will be running slower if you stick to your HR training for the same pace in the heat. But as you make more medium term comparisons you should see an improvement. Those of you who have only recently started training as its gotten hotter, will probably see big reductions as you run in cooler weather. Your body will still, presumably, make the physiological adaptations you are seeking even though you are running slow. Just do lots more miles if you have the time.
I'm sure someone will correct me if I'm incorrect with my understanding above.
Dex.