Fundamentals: Charging A Capillary Tube System

August 23, 2002
The work: Replacement, start-up, and check of an existing air conditioning system condensing unit.
The apprentice: Allen Charles Edwards (ACE) — first-year mechanic.
The technician: N. Arthur Thomas Egan (NATE) — 15-year HVACR veteran.

Nate and Ace are replacing another condensing unit during the cooling season. The equipment they are installing has been uncrated and installed according to the manufacturer’s installation instructions. They are about to charge the system.

NATE: “Ace, are we going to charge this system the same way as we charged the system yesterday?”

ACE: “Nate, I catch on pretty quick, and after the last unit I know that I have to make sure I use the charging chart that comes with the equipment. The chart for this system, which uses a capillary tube as the metering device, is in the installation instructions. I’ll get the instructions and review them.”


NATE: “Great. What is the charging method we need to use for charging a capillary tube metering device system?”

a) Superheat.
b) Subcooling.
c) Sight glass.
d) All the methods are acceptable.


NATE: “Right! Let’s look at a couple of different sets of circumstances to make sure we are using the chart correctly and then we’ll charge the system.”

Data Information:

System 1: Metering – Capillary tube; Superheat – 8 degrees F; Indoor WB temperature – 56 degrees F; Indoor DB Temperature – 70 degrees F; Outdoor Air Temperature – 75 degrees F.

System 2: Metering – Capillary tube; Superheat – 17 degrees F; Indoor WB temperature – 60 degrees F; Indoor DB Temperature – 75 degrees F; Outdoor Air Temperature – 90 degrees F.

System 3: Metering – Capillary tube; Superheat – 0 degrees F; Indoor WB temperature – 56 degrees F; Indoor DB Temperature – 80 degrees F; Outdoor Air Temperature – 105 degrees F.

Answer: System 1? System 2? System 3?

Please remember no question appearing in these articles is on a NATE exam. These questions and dialogue are my creation alone. The NATE Technical Committee does not review the article content, and the committee has the final decision for the use of a question on the tests.

Interpretation of codes, regulations, and standards comes from my experience as a technician and a contractor. Different jurisdictions have varying interpretations. The particular area a job is being done in will dictate which viewpoint is to be properly used.

I learn a lot from your commentary and try to incorporate information into the next article, so please continue to comment.

Patrick L. Murphy
Director of Technical Development

Answers: 1) a; 2) System 1: Properly charged; System 2: Undercharged; System 3: Overcharged.

Murphy is director of technical development, North American Technician Excellence (NATE). If you have any further questions or comments on this Fundamentals quiz, contact Murphy at (e-mail).

Publication date: 08/26/2002