If you had a heater that was continuously producing heat and a machine that could absorb that heat and convert it into usable energy, it might seem like you could have unlimited energy. However, the concept of unlimited energy violates the laws of thermodynamics.
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed; it can only be converted from one form to another. In this case, while you may be able to convert heat energy into another form of energy, such as electrical energy, the total amount of energy in the system would remain constant.
The second law of thermodynamics states that in any energy conversion process, some energy is inevitably lost as waste heat. This means that even if you have a machine that efficiently converts heat into usable energy, there will still be some energy lost in the process, typically as waste heat. This limits the efficiency of energy conversion.
Therefore, while you may be able to extract usable energy from the heat produced by the heater, you would not have unlimited energy. There would always be losses, and the total amount of energy in the system would remain constant or decrease over time due to inefficiencies.