Direct Expansion (DX) coils are an integral part of air handling units (AHUs), playing a key role in delivering efficient cooling through direct refrigerant expansion. Proper installation is critical for maximizing system performance, reducing energy consumption, and preventing future failures. Whether it’s a new HVAC project or a retrofit, following installation best practices ensures that your DX coils work reliably from day one.
Here’s a step-by-step guide highlighting key considerations during DX coil installation inside AHUs.
Confirm Proper Coil Orientation and Airflow Direction
The first step is ensuring the coil is correctly oriented with respect to airflow. Most DX coils are designed for air to flow from the finned side to the return bend side. Reversing airflow can reduce heat transfer efficiency and cause condensate carryover.
Tip: Check for directional arrows or labeling on the coil casing before mounting.
Use Correct Support and Secure Mounting
DX coils must be mounted on a rigid, level platform to prevent sagging or movement. Use anti-vibration pads or isolation supports to reduce mechanical stress during operation. Misalignment can lead to refrigerant line fatigue and drain pan failures.
Ensure coil is level for proper condensate drainage.
Seal All Coil Perimeters to Prevent Air Bypass
Any air leakage around the coil reduces its cooling effectiveness. Use closed-cell foam gaskets or neoprene strips to seal between the coil casing and AHU framework. Avoid using materials that may degrade with moisture or refrigerant exposure.
Gaps allow unconditioned air to bypass the coil, causing inefficiency and uneven cooling.
Ensure Proper Refrigerant Piping and Brazing Techniques
Use copper piping compatible with the coil’s connection type (sweat or flare fittings). Always braze joints using nitrogen purging to prevent oxide formation inside the pipes, which can clog TXVs and capillaries.
- Avoid sharp bends and excessive pipe lengths.
- Include a p-trap on vertical suction lines if needed.
- Use line size reducers or risers as per manufacturer guidelines.
Install and Adjust the Expansion Valve Properly
A Thermal Expansion Valve (TXV) is usually installed to control refrigerant flow into the coil. Ensure:
- Sensing bulb is tightly clamped to the suction line (at 4 or 8 o’clock position).
- The valve is externally equalized if required.
- Superheat is adjusted during commissioning.
Improper valve selection or adjustment can cause coil icing, short cycling, or poor cooling.
Confirm Drain Pan Position and Trap Installation
The drain pan under the coil must be sloped toward the outlet and free from debris. Install a condensate trap that matches static pressure to prevent air from blowing back into the drainage line.
- Use corrosion-resistant pans (stainless steel or coated) in harsh environments.
- Always test for water drainage before system startup.
Insulate Suction Lines and Avoid Condensation Issues
Suction lines exiting the coil must be well-insulated to prevent condensation and energy loss. Use closed-cell rubber insulation and seal all joints. Check for gaps or tears, especially in humid environments.
Moisture on exposed lines can lead to water damage or mold in the AHU.