Capillary Tubes in 15 Minutes

Great video (with our old logo).

Sometimes you do not have the cap tube size recommended by the manufacturer. The following J/B conversion chart shows how much longer or shorter the replacement cap tube must be.

  • Assume 9 feet of .040 cap tube is needed.
  • How long would a .036 tube need to be to give the same amount of refrigerant and pressure drop?
  • What if all you had was a .042 cap tube?


For a cap tube to work properly:

  • It must be sized properly.
  • The evaporator load must be at designed conditions.
  • The refrigerant charge must be correct.
  • There must be a full column of liquid to the cap tube.

Causes of evaporator flooding in a cap tube system:

  • Too much high side pressure
    • Dirty condenser
    • Refrigerant overcharge
  • Low evaporator load
    • Box temperature too low
    • Blocked evaporator (dirt or ice)
    • Fan motor bad
  • Incorrectly sized cap tube (low pressure drop)
  • Inside diameter (I.D.) too large
  • Cap tube length too short

Causes of evaporator starving in a cap tube system:

  • High side pressure too low.
    • Low on refrigerant.
    • Low ambient.
  • High evaporator load (quickly vaporizing refrigerant)
    • Door open or not sealing
    • High product load
      • Initial start up
      • Hot product in the box
  • Incorrectly sized cap tube (high pressure drop).
  • Inside diameter (I.D.) too small
  • Cap tube length too long.

Cause for plugged cap tubes:

  • Dirt or ice blocking the cap tube
  • Filter desiccant can also plug cap tube


  • Cut off some cap tube
  • Replace filter drier
About The Author

Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.