All my kids have asked me not to heat the tub too much during the summer, when my son had it he could not get the heater to work so it was unheated with him, but they only used it a couple of times last summer.
I have found that I like my tub hotter than anyone else likes it, I will go in it when it is at 40c and enjoy it, my wife and son that is still at home find that far too hot and prefer it to be about 36c
Normal summer temps here run to the mid 30s and I have seen it as high as 38c