Keeping your HTPC cool is very important. As with any computer, if your HTPC overheats it may cause some serious and permanent damage.  Here are some tips on air flow inside the case, and case placement that can help you keep your HTPC cool and running well for hopefully a long time.

Case or Chassis Location and Fan Placement

The type of case you use will widely depend on the space you have to use, your room/décor, space, so on and so forth. Despite that many different factors can come into play you will want to follow some core rules when it comes to placing your case and fans inside the case.

Generally speaking you will want to place your CPU in a well-ventilated area that has lots of space to not only pull in cool air but to push hot air out effectively. You will want to note how much space your case gives your CPU, different CPU heatsinks have different heights. You wil also want to make sure it is quiet so you do not hear it running while watching a movie, it should typically be under 30 dba.

Typically any shelving unit with an open back should be just fine. Closed back units do not offer effective ventilation. You do not want to store your HTPC on the floor either.

Some chassis have multiple fan mounting locations. This allows you more control over how air will flow through your HTPC. You will want an airflow that pulls in plenty of fresh/cool air and exhausts warm air. You will also want the airflow to pass over critical components (Chipset, CPU, Voltage Regulators, Graphics cards, etc.). Typically you will have fan(s) located along the front and/or top pulling cool air in while fan(s) along the back push hot air out.

Installed Cards

You should next consider the cards you are going to install and the impact they will have on airflow. Some cards, like graphics for example, can be full size and very long. These can seriously impact airflow. Other cards are low profile and less restrictive on the airflow. You need to consider these factors when choosing where you are going to mount system fans.

CPU

The more powerful the CPU you choose the hotter it will get. Since the CPU is one of the most important items on a HTPC (or any computer for that matter) you will want to take special care when cooling this component. Unless you are over clocking (which really is not necessary with a HTPC) the cooling system that comes stock with your CPU should be just fine. 

Cabling
Cabling is one of the most often overlooked topics during a system build that has a profound impact on the thermal properties of your HTPC. It is also one of the more challenging issues to resolve. The more cables you have in the way the more airflow restriction you will have. This can typically be resolved by using zip ties or cable sleeves to help group the cables together.