Low AC Refrigerant Charge – How to be SURE (Does it really need Freon?)
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Subcooling tells us how much liquid is stacking in the condenser, with higher numbers representing more liquid and lower numbers indicating less liquid. We need to feed the metering device with a full column of liquid, but we don’t want excessive subcooling; too much liquid in the condenser will raise the head pressure and the compression ratio. In refrigeration, there will be a sight glass to indicate that you have a full line of liquid going to the metering device, so connecting gauges to check the subcooling is less important.
Superheat indicates how much liquid we’re feeding the evaporator coil. High superheat indicates a starved evaporator, whereas low superheat may bring liquid back to the compressor. The superheat should be high enough to ensure that we won’t deal with compressor flooding, but it shouldn’t be high enough to indicate a starved evaporator coil.
A visual inspection is also critical; filter restrictions, dirty coils, and improper thermostat settings may cause your readings to be off. Once you’ve verified that there are no obvious airflow issues, you can check the superheat and subcooling to begin verifying the charge level. Subcooling will generally be the main indicator of the charge level for modern residential systems.
After subcooling, suction pressure warrants the most attention. You can figure out the suction pressure by figuring out the evaporator TD. You can then use that temperature to refer to a P-T chart to get the corresponding suction pressure. If the TD is lower than the design temperature difference (DTD), the system will have a lower suction pressure than normal. If the suction pressure is low, double-check to make sure that the coil isn’t frozen and that there are no airflow restrictions.
We can also look at the condensing temperature over ambient (CTOA), which is usually more variable than the suction pressure. The CTOA refers to the difference between the saturation temperature at the condenser and the ambient temperature. In cases with a 15-degree design CTOA, the condensing temperature would be 15 degrees higher than the ambient temperature (105 degrees on a 90-degree day). We can then use the condensing temperature with a P-T chart or app to figure out the head pressure.
A low delta T may also be an indicator of low charge. The telltale signs are a low condensing temperature, low evaporator temperature, low subcooling, and high superheat. To get the most accurate readings, you should ensure that there are NO airflow restrictions and that the Schrader cores have been depressed if you take your readings with gauges.