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The “Approach” Method of Charging and Condenser Efficiency
We have been discussing many methods for checking a refrigerant charge without connecting gauges over the last few months. This got me thinking about the “approach” method of charging that many Lennox systems require.
The approach is simply how many degrees warmer the liquid line leaving the condenser is than the air entering the condenser. The approach method does not require gauges connected to the system, but it does require a good temperature reading on the liquid line and suction line (shown using the Testo 115i clamp and 605i thermo-hygrometer smart probes).
When taking an approach reading, make sure to take the air temperature in the shade entering the coil and ensure that you have good contact between your other sensor and the liquid line.
The difference in temperature between the liquid line and the outdoor temperature can help illustrate the amount of refrigerant in a system and the efficiency of the condenser coil. A coil that rejects more heat will have a leaving temperature that is lower and therefore closer to the outdoor temperature. The liquid line exiting condenser should never be colder than the outdoor air, nor can it be without a refrigerant restriction before the measurement point.
Here is an approach method chart for an older 11 SEER Lennox system showing the designed approach levels:
While most manufacturers don't publish an approach value, you can estimate the approach by finding the CTOA (condensing temperature over ambient) for the system you are servicing and subtracting the design subcooling.
6 – 10 SEER Equipment (Older than 1991) = 30°F CTOA
10 -12 SEER Equipment (1992 – 2005) = 25°F CTOA
13 – 15 SEER Equipment (2006 – Present) = 20°F CTOA
16 SEER+ Equipment (2006 – Present) = 15°F CTOA
I did this test on a Carrier 14 SEER system at my office. So, the CTOA would be approximately 20°.
Then, find the design subcooling. In this case, it is 13°F.
Subtract 13°F from 20°F, and my estimated approach is 7°F +/- 3°F. I used the Testo 115i to take the liquid line temperature and the 605i to take the outdoor temperature using the Testo Smart Probes App, and I got an approach of 4.1°F, as shown below.
More than anything else, you can use the approach method in conjunction with other readings to show the effectiveness of the condenser at rejecting heat.
If the system superheat and subcooling are in range, but the approach is high (liquid line temperature high in relation to the outdoor air), it is an indication that the condenser should be looked at for condition, cleanliness, condenser fan size and operation, and fan blade positioning. If the approach is low, it can indicate refrigerant restriction when combined with low suction, high superheat, and normal to high subcooling.
If the approach value is low with normal to low superheat and normal to high suction pressure and high subcooling, it is an indication of overcharge.
The approach method is only highly useful by itself (without gauges) on a system that has been previously benchmarked or commissioned, and the CTOA and subcooling or the approach previously marked, or on systems (like Lennox) that provide a target approach specific to the model.
P.S. – TruTech Tools has a guide to recharging A/C units, and it includes the Lennox approach method (#5 on the list). If you're interested in their charging tips, click the link to check it out.