How to Estimate a Custom Air Handler Change Out

Changing out a custom air handler may require a lot of attention to detail, but with the right checklist handy it should be no big deal!

Here is a short step-by-step process to help you with your next custom air handler change out. 

Step #1: 

Gather all the physical dimensions 

Here is a list of the most important physical information you will need to gather while on site:

  • Overall height, width and length
  • Order of each section type in direction of airflow (for example -> mixing section, filter section, DX coil, fan section, final filter)
  • Location of access doors/windows 
  • Fan type – forward curve, housed airfloil, plenum fan, fan array etc?
  • Drive type- belt or direct drive (VFD yes or no)
  • Fan wheel size – diameter and width
  • Supply fan motor HP rating 
  • Physical dimensions of each coil (Most important measurements are coil face length x width, and number of rows)
  • Supply & return duct layout (wrong fan arrangement could lead to system effect)
  • Measure each doorway or access point that you will be using to move the air handler into place 

Step #2:

Next, gather the all design information, which may be a done through a combination of field measure and discussion with owner 

  • cfm & external static pressure 
  • Entering & leaving summer DB/WB
  • Entering & leaving winter DB/WB
  • Entering & leaving CW / HW temperatures if applicable
  • Glycol type and percentage (none, propylene, ethylene, percentage 0-50%)
  • Steam pressure if applicable 
  • Capacity required
  • How should system be controlled 

If cfm is unknown, there are a couple of ways to estimate this value. First, identifying the motor HP from step 1, along with fan type and wheel size will help us out a bunch. Also, if we know the existing DX or chilled water face area, we can assume 500 FPM (ish) face velocity to determine an approximate cfm. We may also need to look at the duct system and possibly even do a load calculation. 

Step #3:

Run performance and check work

The final step is to run a selection to double check that we are able to achieve the required capacities and leaving air temperatures, along with a fan selection we are happy with. A few key things to look for:

  • OV (outlet velocity) – there is no magic number here, keep to a minimum
  • Fan RPM – in general the lower the better. High RPM = high bearing lube frequency 
  • Fan curve selection point – don’t pick a point too far to the left, especially with a forward curve fan or you could experience surging and noise issues
  • DX coil suction temperature – a good rule of thumb is 45-50 degF at design. If you start much lower than this, you might run into freeze conditions in low load times
  • If your total static pressure is more than about 2.5″ TSP, probably best to avoid a forward curve fan. 

Are you working on custom air unit project? Contact a cfm sales engineer today for a selection and quote!

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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.
Brad joined the cfm team in 2006, and now as Vice President of Sales, Applied Systems Group he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size HVAC project. When he’s not at work, Brad enjoys reading, running and spending time with his family.
Brad Telker
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