There is supposed to be a difference between the input and output pipes and this is quite normal.
Where your system differs is the amount of variation as it should only be a few degrees. Now if the radiators are cold at the bottom and the output is much cooler than the input, you have a circulation problem in the system. It could be simply a balancing problem.
There are other possibilities but as you have offered scant information it is rather difficult to be more precise.
The DIY projects section on this website has a topic explaining how to balance radiators.
For proper help you need to give as much information as possible when asking a question. Type of boiler? Description of heating system.? Does it have a hot water cylinder? Is it gravity or fully pumped? Are there zone valves? If so, one or two? How old is it? Is this a new problem? Has anything changed such as new radiators?
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