In a recent post, we covered how to accurately measure a DX evaporator coil for pricing purposes. Now, let's take it one step further. Once you've secured the job and have the replacement coil in hand, the next challenge is making sure it fits perfectly—no shortcuts, no guesswork.

This is where precise measurements of the casing depth, headers, distributor locations, and return connections become essential. These details can mean the difference between a smooth installation and a costly delay.

The casing depth is one of the most critical measurements. It refers to the distance from the outside of the casing on the air-in side to the outside on the air-out side. This is important because the casing often uses the full depth of the unit, and there’s little room for error. I always double-check this measurement from both the top and the end of the coil.

Measuring casing depth

I also like to measure the total available depth inside the coil housing just to be safe. This helps if the customer later wants to adjust the system’s capacity or change the number of rows used.

The casing height is another key dimension. I check it in two ways: by measuring from the top edge of the top flange to the bottom edge of the bottom flange, and by adding the height of each flange to the finned section. Be careful—some flanges are not standard (like inverted or knife-edged ones), so make sure to note the actual height if they’re non-standard.

Next up is the casing length, which is measured from the edge of the vertical flange near the supply and return connections to the opposite flange that covers the copper U-bends.

Measuring casing length

The overall length (OAL) is just as important. To get an accurate OAL, take three specific measurements:

1. C Dimension: Measure from the end of the fin pack to the end of the return header.
2. Fin Length: Measure at the top or bottom of the coil, near the flanges—not in the middle. The tape should start and end inside the end flanges.
3. Add Fin Length to MAX Dimension: That’s the distance from the end of the fin pack to the outside of the U-bend on the other end of the coil.

The sum of these three measurements gives you the overall length of the coil—including headers, but not including the return connection, which may stick out of the unit. I always verify the available space in the unit to make sure the OAL fits well and to see if there's extra room for adjustments.

When it comes to circuiting, every detail matters. Be sure to record:

• The number of circuits
• The number of feeds per circuit
• The number of capillary tubes per circuit

Also, count the number of tubes from top to bottom in one row. This can help when verifying the finned height, especially if things don’t add up at first.

While some coils may be re-piped during installation, it's best to keep the original return connection and distributor locations intact. Always document:

• The distance from the bottom of the casing to the center of each connection point
• The distance from either face of the coil to the connection points
• The distance in or out from the end of the fin pack

On a dual-circuited coil, the center-to-center distance between connections can help confirm their placement. Also, measure the diameter of the sweat return connection.

Once all construction details are verified, the coil’s performance and structure should match the required specifications. In some cases, fins per inch, number of feeds, or circuits might be adjusted to fine-tune capacity without changing the construction. At this point, it’s about choosing what works best for the customer’s needs.

If this all sounds like a lot of work that should be handled by a pro, that’s exactly what it is! Let Nationwide Coils handle the tough measurements so you can focus on the job at hand.