Kezo, with respect you are wrong. A radiator giving out 5000 BTU at Delta T 60 will give out less than 4000 BTU at Delta T 50 and over 6000 BTU at Delta T 70. Ignoring balancing and flow rate for now, the size of the radiator determines how much heat it can provide to the room. If the radiators are sized to heat rooms at 80 degrees flow temp they will not heat it adequately at 60 degrees flow temperature.