What you’re talking about is factored in to system design - it’s called the 1% design temperature. https://www.greenbuildingadvisor.com/article/design-temperat

What you’re talking about is factored in to system design - it’s called the 1% design temperature. https://www.greenbuildingadvisor.com/article/design-temperature-vs-degree-days https://www.greenbuildingadvisor.com/article/design-temperat... As CP says, you design your system for the conditions 99% of the time, because designing for that last 1% leads to a terrible cost benefit calculation. Same reason you don’t daily drive an armored personnel carrier with a 300-gallon gas tank.