Heat rises. That’s a basic fact of physics, nothing will change that. It’s the reason that the second floor of a house tends to get hotter than the ground floor. It can be a problem in both summer and winter, trying to cool down the house or avoid the upper floor from overheating.
However, an effective HVAC system for a home, particularly a newer home with good insulation, can strike the right balance and keep the second floor from getting too hot and create a good, even distribution of comfort through the house.
You, however, aren’t currently enjoying this situation. At least not if you’re reading this intently. Your house has previously not had a serious problem in winter with a boiling second floor. But now it’s become such a sauna where going outside in the cold seems like a better option. Why is this happening?
Indoor plumbing has a long history, and through it all the materials used for pipes have changed. If you live in an older home, your plumbing system may have outdated pipe material that needs to be replaced. 
