Your PC could be running much hotter than it should. That can lead to lower performance, lessen the life span, & at the extreme, cause permanent damage. Running hot may be something you have no prior control over, e.g. motherboards with one AMD chipset very commonly used components that weren't up to the task, so they could literally burn up [the smoke was quite disconcerting].
Unless you run an app like HWMonitor, Core Temp, maybe Afterburner, RealTemp etc. you may never know. That's not to say that they're a complete solution. They read the sensors that are already there on your motherboard & in the CPU, which aren't that accurate anyway until you get to pre-melt temps, and sensors [&/or motherboard circuitry] can go bad. The only way to spot trouble like the meltdown I experienced is with something like a non-contact thermometer, which I promptly went out & bought after said meltdown.
It used to be in vogue for motherboards to monitor temps to 1st warn you things are getting too hot, & then shut down the PC if things got any hotter. Recently they seem to rely on the CPU's own throttling -- as temps approach critical the CPU reduces its clock speed, without notice to you. How hot is critical? Neither Intel nor AMD like to say, so Google on the CPU in your PC for recommendations. You can use CPU-Z to tell if your CPU is throttling itself -- check or watch the clock speed under load.
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The perfect electrical conductor hasn't been invented or found yet -- anything that conducts electricity has some resistance, & resistance means heat. Heat is harmful, besides increasing resistance. There are 4 areas where PCs get hot -- the CPU & GPU of course, but also the chipset [where you'll most always see a heatsink], & the section that regulates electrical power [which usually only has a heatsink on more expensive motherboards]. Directing sufficient airflow through the case for adequate cooling is half art, half science. Often big, very visible fans are a marketing gimmick.
Marketing also makes it more confusing that it has to be... enthusiasts & hard core gamers [yes, often the same thing] push the limits, meaning more heat, and they're also more sensitive to the undesirable effects of all that heat, e.g. a CPU will produce more errors the closer it gets to its limit. Long story short there's an awful lot of marketing hype focused on cooling, so you'll find fan boys and conflicting claims and overcomplicated tech advice.
What you need to know is that there needs to be good airflow across the motherboard [the main, large circuit board] -- ideally that air should flow over your mechanical hard drives too. It doesn't hurt to think of the ads you see every once in a while with a car in a wind tunnel, e.g. a fan blowing from the side may just create more chaos, contributing little or nothing to, or even hurting the smooth flow of cooler air from outside the case. Dirt & dust & plain ol' crud accumulate where air enters the case, on the fan blades, and on the surface of any & all heat sinks. The 1st two hurt airflow, while the last hurts the heat sink's ability to transfer its heat to that passing air.
Much ado is made about heat sinks. Ideally a heat sink makes 100% contact with the chip(s) it cools, so the smoother & flatter the heat sink's contact surface the better. A lot of importance is attached to the thermal compound that goes between the heat sink & chip(s) to bridge even microscopic gaps. What you need to know is that sometimes that compound deteriorates with age. Aftermarket CPU heat sinks can be huge -- besides worrying about whether they'll physically fit in the case, you also need to worry about too much weight stressing the motherboard.
Water cooling is an alternative with a list of caveats. It can be a lot more effective [lawn mowers are air cooled -- cars use liquid], but you have moving parts that break, the potential for leakage [which can spell disaster], & radiators that can be a lot harder to clean than a heat sink's fins. There are 2 types of water cooling -- closed loop & not. You can have a system with individually replaceable components that you fill, & later monitor the level of coolant, or the popular closed systems that once installed you [in theory] never worry about. Note that while you can clearly see if a fan, say for a heat sink is working or not, you can't see if a water cooling pump is turning, or check coolant levels with a closed system.
Whether you use water cooling or not helps determine the case you should have. More open cases work well with water cooling, but aren't as good about that directed stream of airflow that can be critical for a conventional CPU heat sink. When I 1st went to water I had a closed style case -- when I put the same guts into a new, much more open case the temps dropped on average 10 degrees C. When the cooler went bad I stuck the old heat sink back on while waiting for a warranty replacement -- temps rose an average of 20 degrees C, a bit higher than the same heat sink in the old, closed case.
You may not need to buy a new case for better cooling -- most any case can be modified fairly easily. I've drilled holes or cut out the external drive bay covers on the front of cases to provide air intakes. I've mounted fans in external drive bays, or behind the decorative plastic front of the case. I've mounted all sorts of fans internally, sometimes to braces I added, or using wire ties, or even glueing them on heatsinks with silicone sealer. For braces to mount fans to I've epoxied on coat hanger wire, or cut empty cans with tin snips, even on occasion breaking down & buying u-shaped aluminum stock at the hardware store. In the old Pentium 3 days I made air ducts from plastic shampoo bottles. When money's tight go for more small fans rather than fewer bigger ones -- I bought 80mm fans from Newegg 10 at a time for ~$1 each.
