A crucial part to installing a radiator is correct sizing, balancing the system, and allocating the rads in the right place. A bathroom with a giant rad in it – it’s going to be too hot. If you’re in a living room and you put a small rad in it, it’s never going to be warm enough. What’s most important is for the radiator to be matched to the size of the room and what the demand is on that space.
Sometimes customers happy with their radiator performance track record, upgrade their hydronic systems by replacing their existing radiators with new or refurbished cast irons. In cases like these, it’s best to calculate the BTU output from those radiators and find the ones that best matches what was there. In the cases of new construction projects, there will be a heat loss design layout for the project that will specify how much energy each room needs; the radiator is selected based on what the heat loss is for that space.