By Randy Carstens, Senior Sales Engineer Colmac, Coil Mfg
Circuiting coils can be intimidating to the novice, but it is not as difficult as it may seem. It is essential to understand
circuiting because coil performance and life can be dramatically affected by it. Feeds, passes, and dropped tubes are often used to describe circuiting. Just what does it all mean's
Circuits and feeds are frequently used interchangeably at coil manufacturers. In water coils, the #feeds are the number of tubes that leave the supply header and go into the coil.

Many coil companies also call these feeds “circuits.” Colmac Coil Mfg. Prefers to reserve the term “circuits” to mean fluids that circulate through a standard coil but will never come into contact with each other in the coil or the rest of the system. In a direct expansion coil, each compressor is on a separate circuit, so if one compressor “burns up,” it doesn't destroy the other compressor because the two refrigerant loops are on separate “circuits.”

How many times each feed travels up and down the length of the coil is called “passes.” The number of feeds times the number of passes has to equal the total number of tubes in the entire coil. This is true only if you “drop” tubes in your coil. Why would you want to drop tubes? Generally, dropping tubes is not desirable. However, sometimes, due to the physical size of the coil or due to the number of tubes high and rows deep, no ordinary integers exist to express the circuiting, so tubes must be dropped. For example: (1-row deep coil, 13-tubes high, same end connections, 1feed-12 passes, with one tube dropped.) Coil circuiting, as a rule, should be drainable.

A drainable circuit can keep the customer out of trouble if he does not follow up with a glycol flush. It also helps return refrigerant oil to the compressor at “low-load” conditions. Circuiting coils ensures the correct fluid velocities in the coil tubes. Fluid velocities that are too low result in poor heat transfer, and tube velocities that are too high can lead to premature failures from velocity erosion.

Below are a few of the “Rules of thumb” for coil circuiting. Remember, if you have any questions about coil circuiting, call us—we would be more than happy to help!


#1: If you want connections on the same end of the coil, you must have an even number of passes.

#2: If you wish to connect on the opposite ends of the coil, you must have an odd number of passes.

#3: The number of tubes high times the rows deep must equal the number of feeds times passes (plus any dropped tubes)

  • The double circuit is 1/2 the passes as rows;
  • A complete circuit is the same number of passes as rows;
  • 1/2 circuit is twice the number of passes as rows;
  • 1/4 circuit is four times the number of passes as rows;
  • 1/4 circuit must be in increments of 4 tubes high;
  • 1/2 circuit must be in increments of 2 tubes high.