It’s important to understand the capacity of your HVAC system.  You may be deciding if equipment can be added to your site without upgrading your HVAC system or maybe you’re simply curious about system performance.  Either way,  it’s important to make sure that your AIRSYS WPUs are appropriately sized.

What happens when my WPUs are undersized?  

Units are undersized when the WPU’s sensible capacity is less than the heat load being dissipated inside the shelter. Here is an example:

  • Site heat load = 10 kW
  • 2.5 Ton WPUs, sensible capacity of 8.7 kW.

This would mean that your site is not protected against ANY HVAC failure. In other words, a failure in either WPU could result in a high temp alarm and an urgent call out to the site. This is bad both for the equipment in the site and bad for the operations organization that will need to perform an emergency dispatch.  In contrast, if this site was “Right-Sized “ then a single WPU failure would only generate an HVAC fail alarm and not a high temp alarm. The HVAC fail alarms can be addressed as a high priority but not an emergency call out.

Recommendation: Plan for one more unit than is necessary for the heat load. This will allow for redundancy during unit failure. 

What happens when my WPUs are oversized?    

Units are oversized when the sensible capacity HVAC system is much greater than site heat load. This is not only inefficient but also makes site environmental control much more challenging

When an HVAC system is engaged in Mechanical Cooling (MC), the latent cooling provided by MC is removing water vapor from the air. MC must run long enough to remove more HO vapor than is added from the outside. With a system that is too large, the temperature setpoint will be reached VERY quickly and therefore MC will only run for a few minutes at a time. Under these conditions, the HVAC system cannot sufficiently dry the humid air, which can result in water vapor condensing inside the site.

Right-Sizing your WPUs 

A good estimate of the equipment load can be made from the output of the DC power plant. Read the voltage and current from the rectifier display and multiply to get an equipment load. The external heat load must also be accounted for when sizing a system. This heat load varies based upon site dimensions, insulation, outdoor temperature, and indoor temperature setpoint. 

The attached calculator will size WPU units based on both the equipment and external loads.

For a detailed hour-by-hour comparison of annual HVAC energy consumption between AIRSYS and other units, please see our Equipment Shelter Utility Calculator.