Need to Add Cooling to an Existing HVAC System?

Duct Booster Coil

Need to Add Cooling to an Existing HVAC System?

There are many existing systems that require the addition of cooling to a heating-only unit. Systems need to keep pace with changing requirements for spaces that include a mandatory requirement for cooling. Existing systems can already have cooling, but a booster will have to be placed into the system to add additional cooling.


When cooling is added, it must be decided whether the tonnage can be met with an existing chilled water system or do their needs require an added split DX system. Either way, the existing system must overcome some obstacles before it can run successfully with the cooling addition.


If you are adding cooling to an existing heating-only unit, first you must find the space. You can mount a multi-row cooling coil within the unit or in an area adjacent to the unit. Adjacent, meaning either on the incoming air side or the discharge side of the unit. Almost all areas of the country dehumidify during the cooling process; therefore, a drain pan will be required. It’s usually difficult to find room in a heating-only unit that can incorporate both a cooling coil and its drain pan.


Most times, the added cooling will be placed in a factory fabricated coil section outside of the unit. If this is located on the incoming side of the unit, this coil section will also require a filter section, since the cooling coil is now located before the filter in the existing unit. If it’s located on the downstream exit side of the system, it can be a coil section, only because the filter in the unit will suffice for that application.


The first obstacle for an added cooling coil section is finding enough height x length to have the interior coil in the section be at or below 550 feet per minute velocity. Why?  With velocities over 550 FPM, you can have a terrible problem known as “water carryover”. Water is picked off the finned-tube surface and carried down the ductwork. Most incoming and outgoing ducts are sized for 1,000 to 1,200 FPM (feet per minute) velocity. This means that the new coil section must be expanded out with a transition to be of a size below 550 FPM. The sheet metal transition from the smaller duct to and from this higher and longer section will also take up more room in the direction of air flow. You must have an incoming transition followed by the coil section, followed by the leaving side transition back down to the original duct size. We can provide coil sections with eliminators, which help reduce the size of the transitions required (see coils and sections available below).


The next obstacle is the existing fan and drive must be able to overcome the extra static pressure of a cooling coil now in the airstream. Remember that a dehumidifying coil has about 40% more resistance to the air because of the water forming on the finned tube surface. You will have to go to the fan manufacturer and find out if the fan can handle another .75” to 1.00” of pressure. After that review, you will almost always have to speed the drive up to overcome a cooling coil resistance.


Coils and Coil Sections Available


USA Coil & Air has any cooling coil that would be required as an add on inside an existing unit. We have chilled water and single or multi-circuit DX coils to perform as required, and we can make the casing depth a minimum to fit in an allotted space. When involved in a new coil section, we have insulated single wall or double wall sections that include integral drain pans and safe-offs to seal the coil inside the section. Our double wall sections can be mounted indoors or outdoors. Accessories can include access doors, moisture eliminators for velocities above 550 FPM, stainless drain pans, and any other normal coil addition as well. Double wall units can also be painted outside and can have stainless steel inner walls.


Extra or added cooling is a very normal occurrence in our industry. Some systems may not be able to have cooling added. Most systems can add cooling but there are times when many boxes need to be checked before it can become a reality.

USA Coil