AHU & Coil Cleaning
By Don Dorff, ASCS
Air Quality Controllers

For many duct cleaning companies, there is the age-old question, “What is the best way to effectively clean an air handling unit?”  As an ASCS Technician and owner of Air Quality Controllers, I found that there are no specific guidelines for cleaning the interior components of AHU’s.  Over the last three years, I have developed a process that will restore the coil pressure drop within 10% of the original pressure drop from when the unit was first installed.

The first step to take in completing this task is to take accurate measurements of the existing pressure drop.  I use a Magnehelic to take two measurements on the unit before I start the cleaning process.  The first would be to measure the pressure drop on the upstream/return side of the coil.  Then, measure the pressure drop on the downstream/supply side of the coil.  Record your data so that a quantitative comparison can be made when cleaning is complete.  Now you can shut the unit off using Lock Out/Tag Out procedures.  Next, remove the filters and mark the positions of the outside air and the return dampers.  Close these dampers.  If there is a damper for the supply, mark it and close it.  If there is not a damper, use a bladder to protect the supply ductwork.

Now you are ready to start the actual cleaning.  Always wear a respirator mask and contact clean both sides of the coil to remove the larger debris and accumulation.  The upstream side will appear dirtier than the downstream side.  Be very careful not to apply too much pressure.  The coil fins can be bent and damaged very easily. Wearing a respirator mask, eye protection and ear protection, blow out the coil from the downstream side to the upstream side of the coil.  Visually inspect both sides of the coil by shining a light between the fins.  The technician should be looking for any remaining debris.  If necessary, air wash the coil a second time and inspect it.

In many cases, it is just as difficult to gain access to the blower fan as it is to clean it.  Depending on the unit, the technician will have to remove panels, safety grates and/or belt guards to accomplish this.  Typically the accumulation on a blower fan wheel and housing is “Caked On,” I have found that a wire brush and a scraper are adequate for loosening the accumulation.  While brushing and scraping, have a vacuum hose in hand to remove the loosened debris.  This will help the technician see if he/she has overlooked any areas that still need to be cleaned.  All panels, safety grates, and/or belt guards should be cleaned before they are installed.

Once the coil has been air washed and the blower fan is clean, the technician can start cleaning the rest of the AHU.  This would include the interior walls, filter racks, and the floor.  I have found that the most efficient way of doing this is to start at the top/ceiling and work downward to the bottom/floor.

If the unit has a drain pan, I recommend that the technician applies a coil cleaner.  This will dissolve stubborn calcium deposits that may be remaining on the coil.  It is important to rinse the coil thoroughly with water to remove chemical residues.  During the rinse process, make sure the drain is operating properly.  If there is not a drain pan, DO NOT perform this last step.

With the cleaning portion of the process completed, the technician must set the dampers to their original position, remove bladders, install new filters and replace panels.  Next, remove the Lock Out/Tag Out, turn the unit on and test the unit.  Record the results and include them in the Post Project Report.

Depending on the size of the AHU, a thorough cleaning can take anywhere from 3 hours to 1.5 days.  With this in mind, make sure the client is aware that the AHU could possibly shut down for 24 hours or more.  This process has proven to be successful for our organization and it provides excellent results for out clients.

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