A processor, graphics processing unit, and power supply are the main sources generating heat inside a case.
Nowadays, integrated graphics processing units inside the processors are powerful enough for HTPC use.
When choosing a processor, it is advisable to check not only the performance, but also its Thermal Design Power (TDP), which indicates the power consumption of a CPU or graphics card.
The TDP describes the maximum amount of power in the cooling system in a HTPC is required to dissipate.
The main reason why you need to pay special attention in selecting a low power CPU is that processors with over 65W TDP tend to generate so much heat that the cooling fan noise is not any more comfortable for media center use.
The low-power CPU runs at reduced voltages and emits less heat, which requires less cooling.
Intel processors are currently the best choice when it comes to performance and power efficiency.
The recommended Intel options are Intel Core i3 and Intel Core i5.
In case you are wondering, different numbers in Intel Core i3 and i5 often indicate what processor clock speeds they are using, which Intel HD graphics model and whether the voltage has been adjusted or not.
The letters at the end of Intel Core processor models mean whether it is possible to overclock the processor or whether the processor has already being underclocked for better power efficiency.
T-models in the Intel processor line up are great choices for a HTPC as they are undervolted and underclocked meaning that they require less power and thus less cooling.
However, T-models are a bit more expensive and may be harder to find from your preferred store.