Review Index:

Passive vs. Active CPU cooling with Thermalright Coolers

Manufacturer: Thermalright


Today's review is more of a comparison than a true review — we are comparing three of Thermalright's top-of-the-line CPU heatsinks in passive, pseudo-passive, and active cooling modes.   Our three contenders include the Thermalright HR-01, the XP-120, and the Ultra-120.  What we really want to find out is how the HR-01, which is designed for passive cooling, compares to the XP-120 and Ultra-120, which rely on active cooling with a fan.  The HR-01 will be tested with, and without, a Thermalright 120mm Fan Duct.  All testing was performed on an Intel P4 Extreme Edition dual core 955 CPU heater inside a closed mid-tower case.  To read a detailed review of the individual heatsinks, please click on one of the links below.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />


Thermalright HR-01-775

Thermalright XP-120

Thermalright Ultra-120


All three Thermalright coolers are constructed with solid copper bases, multiple heatpipes, and large cooling fin arrays.  The HR-01 and Ultra-120 are tower-style coolers while the XP-120 has a lower profile. 



HR-01                                Ultra-120



The HR-01 was specifically designed for passive and pseudo-passive cooling and incorporates wider spacing between the cooling fins to minimize air resistance.  The Ultra-120 and XP-120 heatsinks both use more closely spaced fins for greater surface area and are designed to be used with a 120mm fan of your choice.



As you can see in this close-up picture, the HR-01 fins (on the left) are spaced further apart than the Ultra-120 fins (on the right) to help maximize passive air-cooling.  Since the two heatsinks are approximately the same size, this means the Ultra-120 has a lot more fins (52 compared to the HR-01's 31), which also gives the Ultra-120 a lot more surface area to dissipate heat from.  And, the tighter spacing requires a fan be used to effectively move air thru the heatsink.


  • Passive cooling — does not use a fan or other means of forced-air cooling.  Relies on natural convection cooling.

  • Pseudo-passive cooling — does not use a fan mounted directly onto the heatsink but does use forced air cooling from a nearby fan.  May incorporate a fan duct or shroud.

  • Active cooling — uses a fan directly mounted onto the heatsink for forced-air cooling.

Not too long ago, low-powered CPUs relied on a finned, aluminum heatsink without any cooling fan, to keep them cool.  Even today many modern components rely on passive cooling (RAM chips, voltage regulators, <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Southbridge controllers, and the majority of surface mount components).  Passive cooling provides two big advantages; no moving parts and silent operation.  However, as components have advanced and heat generation has increased, the heatsink surface area required to dissipate more heat became too large and designers started incorporating small fans onto heatsinks in order to keep their size small. 


In very basic terms, a heatsink relies on two key properties for dissipating heat: (1) surface area, and (2) mass flow rate of the fluid medium (air or water).  In other words, for a heatsink to dissipate more heat, the surface area must be increased, or the flow rate of the cooling medium must be increased, or both (assuming other variables are held constant).


A little bit of airflow goes a long way when it comes to cooling a heatsink.  For passive cooling to work, natural convection creates the required airflow.  The air in contact with the heatsink fins becomes warmer as heat is transferred out of the heatsink and into the air.  As the air becomes warmer, it also becomes less dense and is displaced by cooler, denser air.  This is why warm air rises.  In an ideal situation, cool air will flow in from the bottom and sides while the warm air exits out the top and away from the heatsink.  The airflow generated by passive, convection cooling is typically minimal, which is why passive coolers must have a large surface area to compensate.


On the other hand, a relatively small, compact heatsink with minimal surface area can be made to dissipate the same amount of heat as a large passive cooler, by using a fan to greatly increase the airflow over the heatsink.


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