If a home contains a hot water system as opposed to a forced hot air system, users must account for heat loss from boiler pipes that run through unconditioned spaces such as a crawl space or an unheated basement. This ensures that the boiler will be sized to not only handle for the heat loss in the space, but also through loss that occurs while delivering the hot water to that space.
In this section users can select the pipe diameter, type, insulation, and length. They must also enter the Ambient temperate of that space on a winter design day and the water temperate running through the pipes. Most water temperatures are between 150-180F. Non-condensing boilers tend to be closer to 180F, while condensing boilers are closer to the 150-160F range.
Everyone uses PEX. Why isn't that a choice?