Understanding HVAC Coils - 10 Easy to Understand Facts

With all the types and sizes available, replacing a HVAC coil can be intimidating. Keep in mind that 95% of the overall HVAC coil cost is the finned surface area, number of rows and fins per inch. Casing dimensions, coil depth, connection sizes and header sizes only account for 5% of the overall cost.

10 Simple HVAC coil facts:

1. If you eliminated the fins and only used the tubes, you would only get about 30% of the performance compared to the same HVAC coils with fins. The fins impact heat transfer much more than the tubes.


2. Because steam is erosive, steam coils are often constructed differently then hot water coils. Water and steam coils cannot always be used interchangeably. Some duct booster coils can be used with hot water or steam. Be aware that the steam should NOT be modulated through this type of coil if it will be exposed to below freezing air temperatures. As the valve modulates down, there is not enough pressure to push the condensate through this type of coil. If you are using steam in a hot water booster coil, you should choose 2 position control (on/off). Modulating type steam coils, aka "non-freeze" steam coils should not be used with hot water.


3. The industry has arbitrarily named 1 and 2 row coils "hot water HVAC coils" and 3 - 10 row coils "chilled water HVAC coils". Hot water coils and chilled water coils are constructed the same. Hot water temperatures are much higher than the air temperature, resulting in a coil with fewer rows. Chilled water temperatures are very close to the air temperature, resulting in a much deeper coil.


4. Chilled water and DX (direct expansion) HVAC coils are identical except for the supply and return arrangement. You can't feed refrigerant through a M.P.T. (male pipe thread) connection and header arrangement. You have to replace the standard hydronic feed with a refrigerant distributer. The "spaghetti tubing" connected to the distributer feeds as many tubes as you want it to feed. Beyond that, chilled water and DX HVAC coils are identical.


5. Fins have corrugations that run the entire height and depth of each fin. Fins are not flat. Corrugations promote turbulence, increasing the surface area and coil efficiency. Corrugations also help in preventing moisture carryover which is very common with chilled water HVAC coils running at face velocities higher the 550 FPM.


6. When you look at HVAC coils, you can see that the fins are rippled. This is another way to mix up the air, causing more turbulence which creates more heat transfer, increasing coil efficiency.


7. Most HVAC coils last 15-20 years. HVAC coils can last as long as 30 years if they are properly maintained. Proper maintenance includes regular cleaning on the airside, and proper treatment of the fluid passing through the tubes.


8. 5/8" tube chilled water coils use more tubes per row than ½" tube chilled water coils. The ½ tube coils have more finned surface area, but the thicker 5/8" tubes take up more surface area. The performance of the two different HVAC coils is similar. People often choose the 5/8" tube coil due to its slightly lower water pressure drop and thicker tube wall thickness.


9. Since you pay for copper by the pound, you will pay more for a 5/8" tube coil than a 1/2" tube coil. This is because the wall thickness on a 5/8" tube coil is 10 - 15% more than a comparable 1/2" tube coil. Even though the performance is the same, you will pay more for a 5/8" tube coil. One could claim that a 5/8" tube coil will outlast a 1/2" tube coil.


10. 95% of the overall cost of HVAC coils is the finned surface area, number of rows and fins per inch. Casing dimensions, coil depth, connection sizes and header sizes only account for 5% of the overall cost. Changing the materials can have a huge impact on the overall cost. Stainless steel casings and copper fins are very common in corrosive environments. The material costs are higher, and the heat transfer properties of different materials will affect the size of the coil. If you change the fins to copper, you will lose about 15% of the capacity, which means you will need a bigger coil.

HVAC Coil templates specific to your application can be downloaded and printed. Bring these templates to your jobsite, mark them up, and fax them to your manufacturer. They will use your template to build an exact duplicate.