Effects of Water Removal and Water Carryover in HVAC Coil Sections

swcoil_superadminCoil Moisture, DX Coils, Finned Tube Coils


Southwest Coil has replaced thousands and thousands of chilled water and DX cooling coils over the last 4+ decades.  Many of these coils have literally fallen apart in their casings and support.  An  air handler process always includes  filtration, cooling, heating,  and the distribution of air to the space through a supply fan.  It is in the cooling coil area that the process of dehumidification creates large volumes of water.  Let’s look at the best ways to capture and remove the moisture as it’s produced.

Coil Moisture

All dehumidifying coils do not create the same amount of moisture.  If a system is partly or fully cooling only fresh air, there will be a lot more moisture than a system cooling return air from a space.  The volume of water also depends on the ambient temperature and the percentage of humidity each day.

Coil moisture cascades down through the finned tube pack towards the closest drain pan. Not every coil system has perfect air flow across the face, so this cascading process can be different in some parts of the coil versus other parts.  The rule is that a drain pan be located no more than 38” from the top of the coil fin surface.  Why?  Because water left on the surface increases resistance to the air as it moves down the coil finned pack, and the air can be significantly reduced by this added resistance.  Many people do not realize that a dehumidifying cooling coil can have 40% to 50% more resistance than a dry coil. This can be increased further if moisture isn’t removed quickly on the coil surface.

Intermediate Drain Pans

Many cooling systems have stacked coils (one on top of the other and some side to side). Based on the 38” criteria to remove moisture, the stacked coil system needs intermediate drain pans between the coils.  These pans must be full size pans that fit fully under the coil from entrance rows to exit rows plus 2” to 4” beyond the leaving face of the coil.  It is important that the coil being drained of its moisture has holes in the bottom casing allowing the moisture to drop into the intermediate pan below.  Less moisture in the coil in the direction of air flow improves the overall efficiency of the cooling capacity.  These intermediate pans need a drain line to the main bottom drain pan and may also have 2 drain lines to the main bottom pan if it’s a long system.

Coil and Drain Pan Construction

Many systems have galvanized steel coil casings and drain pans as well as coil section frames/safe offs that hold the coils in place.  Galvanizing is a zinc coating that wears away over a short period of time – especially in cooling sections that have tremendous amounts of water flying around in the air flow.  Always have this material fabricated out of 304 stainless steel.  It will last forever, and you won’t have rust and impurities in the air flow from delipidated coil section parts.

New air handlers are all double wall construction – always specify that the coil sections have an inner wall of 304 stainless steel versus galvanized.  This means everything inside the coil section is stainless steel.  The cost is well worth the number of rust-free years you will receive from this addition to your specifications.

Water Carryover

The term “water carryover” simply means that dehumidified moisture moves past the leaving face of the coil and beyond any drain pan to remove it.  There are a lot of reasons why a system may have water carryover.  The major ones are listed below.

  • Total system air velocity is beyond 550 FPM (feet per minute) velocity through the coil.  The higher the velocity, the more potential water carryover.
  • Plugged coils from dirt and debris in the finned pack do not load evenly.   Air travels to the path of least resistance; and if one part is plugged and the other not as plugged, air will rush to the area that has the least resistance.
  • In a draw thru system, the fan housing may be too close to the leaving face of the coil, thereby blocking part of the coil surface and producing high velocity in those areas not affected by the housing.
  • System diffusers on blow thru systems may not be adequate to distribute air evenly across the entire effective area of the coil.  Sometimes the fan can pour large amounts of air through the middle portion of the downstream coil, while the perimeter area receives much less air.
  • Unequal air flow if air travels around a corner, and the air flow doesn’t have a chance to distribute evenly across the coil face. One side could be 75% of the air flow and the other sides only 25%.

Water carryover can be very destructive since a fan can move the water across fans and motors and even down supply ducts.  A simple air velocity test is required to assess the severity of the velocity problems.  Corrective action must take place to alleviate the enormous damage water carryover can have on an HVAC system.

Southwest Coil exclusively represents USA Coil & Air in the Southwest Region.  We can help you with your cooling coil needs.  Over our 4+ decades, we have replaced thousands of problem coils with new designs and construction that has created more efficient cooling and greater longevity.  We have expedited shipments for our coils, and we have the burst proof Sentry Guard™ Coil Line for coils that are prone to freeze damage.  In many cases, we can replace the entire coil section with new stainless-steel lined walls and internal parts that will last forever.  Take advantage of our decades of knowledge and experience.