The term “Free Cooling” (FC) refers to utilizing cooler outside air instead of running a compressor to maintain temperature in a space. In the US this is more commonly known as “economizer operation”.Whenever you can utilize outside air for cooling instead of running a compressor you are reducing the electricity consumption to accomplish the task by about 90%. There is another term known as “Partial Free Cooling”. In order to understand partial free cooling you need to understand that pure Mechanical Cooling happens with the outside air damper completely closed. This means that air from the site is being recirculated through the HVAC system and no outside air is being introduced. Quite simply “Partial Free Partial Free Cooling Explained Cooling” is Mechanical cooling with the air damper open. 


Why would you do this? Under certain circumstances there is an efficiency benefit to using partial free cooling. The key condition is when Toutside << Treturn. That is to say the outside air temperature is much cooler than the return air temperature. Sending the cooler outside air through the refrigeration circuit INSTEAD of the warmer return air can result in the supply air temperature be much cooler with the same amount of electricity being consumed. This cooler supply air will cool down the site quicker and therefore allow the energy consuming compressor to satisfy the site temp quicker and run less often. 


Here is an example to illustrate: If Toutside = 70˚F; Treturn = 80˚F and assuming the refrigeration circuit provides a ΔT of 20˚F , then Pure Mechanic Cooling will result in: Tsupply = 80 – 20 = 60˚F. Partial Free Cooling will result in : Tsupply = 70 -20 = 50˚F. 


Limitations of Partial Free Cooling: 

  1. Increased ESP on the supply fan Whenever the HVAC system is bringing in outside air to a sealed environment, that forced air introduction increases the External Static Pressure (ESP) in the site. That increase makes the supply fan work harder which translates to a decrease in CFMs for the delivery of the supply air. If your site has “hot spots” this mode of cooling can make that situation worse.
  2. Reducing Air Filter life that could be used for pure free cooling While you may see some benefit of supplying colder air to the site in reducing the duty cycle of the compressor, that benefit is nowhere near the benefit of the 90% reduction in HVAC power that you get from 100% Free Cooling. If you are in an environment that is subject to a periodic excess of airborne particulates than you are pulling more of those particulates into the primary air filter during partial free cooling. This can shorten the life of the primary air filter which can hasten the clogging of the air filter. 


For these two reasons it is recommended that Partial Free Cooling be disabled on sites with > 12 kW of equipment heat load. Typically these are sites that require an AIRSYS 5 Ton WPU to maintain temperature. For these higher heat load sites, whenever 100% Free Cooling is no longer able to keep the site within the specified temp range then partial free cooling is likely a very small benefit and therefore not worth the potential shortening of the useful life of the primary air filter.


For Instructions on Disabling Free Cooling, refer our helpdesk article here: https://support.tempesthvac.com/support/solutions/articles/6000065700