Hot Water Booster Coil Selection & Special Situations

Hot Water Booster Coil Selection & Special Situations

If you’re in the HVAC industry, then you’ve been involved in projects and installations that have duct mounted booster heating coils.  How did the term “booster” originate?  Most heating coil applications have a main heating coil that takes air from the lowest entering air temperature to an intermediate air temperature.  Each individual space has its own heat load, and the duct mounted booster coil is the solution that allows for individual space control.  For example, one space may have a south or western exposure that may increase the heat load. Placing the “booster” coil in the duct feeding that space will take it from an intermediate temperature to its final temperature.


Most ducts are sized for 800 to 1200 feet per minute (FPM) face velocity.  This is about 9 to 13 miles per hour (MPH) and gives you an idea of the speed of air as it moves through the air duct.  Duct coils usually need to be sized between 500 to 750 FPM and require a “duct transition” in order to have the air slowed down to go through this larger face area coil.  Why is this necessary?  There is always a supply fan in the system.  This fan produces the entire system air volume (usually measured in cubic feet per minute (CFM).  If you size coils at the duct size (at the higher velocity), then the total static pressure (resistance) will drastically increase the working brake horsepower to produce the required volume of air.  A coil sized at 1200 FPM will have 2-1/2 times the static pressure of a 600 FPM coil.  When sizing a coil at a higher velocity, you automatically have less face area and thus increase the rows and/or the fins per inch.  If you increase the rows in the direction of the air flow, then another negative is created.  You now have to pump the water through 2 rows instead of one row, and you have increased the resistance against the pump, and it will require more brake horsepower to distribute the water volume in the system.


The following are the specific requirements needed to calculate a coil:


  1. The maximum height and length of the coil

Many times ducts are above a ceiling and can be in between joists.  The coil engineer needs to know which way he can expand the coil to keep the air velocity at a reasonable speed.

  1. Required CFM volume of air
  2. Entering air temperature and either desired leaving air temperature or desired BTUH load required
  3. Entering water temperature and either desired leaving water temperature or desired GPM water volume

There’s a lot of difference between a coil selection at 180 degrees entering water and 140 degrees entering water temperature.  There’s also a large difference when the leaving water is based on a 20 degree temperature difference (Example:  180 degrees in and 160 degrees out) and a 40 degree temperature difference (Example:  180 degrees in and 140 degrees out).

  1. Required maximum air resistance (inches of water)
  2. Required maximum water resistance (feet of water)
  3. Flange or slip and drive type mounting
  4. Coil construction

This will include fin thickness if you want to wash and clean coils and whether you want brazed copper sweat connections or threaded MPT connections.


There are also special situations with booster heating coils.  Here are some of them and what you will need to alleviate problems.


Low water flow


Low water flow happens on many coils under 500 CFM, because the calculated water volume (GPM) is so low that the coil actually develops “laminar flow”.  This is nothing more than a flow of water so low that it destroys heat transfer.  Picture a coil where the actual water velocity falls below .75 feet per second (FPS) and the result is a coil without enough water along the inside tube wall.  This problem can reduce a coil capacity by 75 to 80 percent.  Simply allow the coil engineer to use a smaller diameter tube and the water velocity can usually be raised above 1 FPS, and the laminar flow condition disappears.


Air stratification


Unequal air flow across a coil is created when the upstream side of a coil (entering air side) is too close to a fan or a bend in the ductwork.  A coil’s efficiency is based on using the entire effective area of the coil.  Air is produced downstream of a fan in a wavy pattern, and if too close to the outlet, then you can effectively blow the air through the middle and reduce the heat transfer out and onto the perimeter of the coil.  When coming around a duct bend, a baffle or turning vanes may be required to have the coil receive air properly across the effective area.


Lack of access


Many hot water coils mounted in ducts are locked into the duct so tightly that there is no access to the fins and tube surface.  We have replaced thousands of booster coils over the years, because the fin surface has never been cleaned.  It’s smart to provide at least a hand hole access door on one side or the other to clean the coil at least every 4 to 5 years.  Dirty coils reduce the heat transfer of any coil and certainly increase the resistance and operating expense.  While you are at it – specify heavier fins so the fins won’t bend during cleaning.



USA Coil & Air builds thousands and thousands of booster coils annually.  Whether it’s a large project with hundreds of coils or one coil required for a small tonnage system, we are here to assist you with proper selection, very competitive pricing, and the shortest lead time in the industry.  TRY US ON YOUR NEXT PROJECT.  You won’t regret it!

USA Coil