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Approach Temperature

When you calculate subcooling, you measure the liquid line temperature and then substract it from the condenser saturation temperature to obtain subcooling, right?

Well, when you use the approach temperature method, you are charging with respect to outside ambient temperature and liquid line temperature.

Check this out:

Let's say you are dealing with a standard efficeincy system and you are using the 30-degree rule of thumb as the difference between condenser saturation temperature and the outside ambient. For example, if the outside ambient temperature is 85 degrees, the refrigerant should be condensing at a temperature of about 115 degrees. Now, let's say you want 25 degrees of subcooling. The liquid line temperature should be about 90 degrees.

The difference between the outside ambient (85 degrees) and the liquid line temperature (90 degrees) is 5 degrees. THe "approach" temperature is therefore 5 degrees.

Lennox is basically eliminating the calculations for you.

For a higher efficiency unit, the delta-t between the outdoor ambient and the condenser saturation temperature may be 25 degrees and the system may be operating with 21 degrees of subcooling. In this case the approach temperature would be 4 degrees.

Cool, huh?


  • John Starcher_4
    John Starcher_4 Member Posts: 794
    I'm just curious..... to why some manufacturers (Lennox comes to mind) tell you to check your charge using approach temperature rather than subcooling.

    Just worked on a unit yesterday where we had to relocate the condenser due to an addition being put on the house. It's a Lennox 2.5 ton unit with txv at the evaporator coil.

    Lennox specifies a 5 degree +/ 1 approach temperature. I was able to achieve it, but why not use subcooling?

  • John Starcher_4
    John Starcher_4 Member Posts: 794
    Thanks, Prof!

    I figured there was a connection there somehow, but couldn't see it in my mind's eye!

    Thanks for the explanation!!!

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    No problem!

    Always a pleasure to help.
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