Demystifying Superheat

Q: I attended a training by Carrier Corporation that convinced me that refrigerant charge should be checked using superheat for most common air conditioners. However, sometimes when I use the Carrier Required Superheat Calculator, it doesn't specify a required superheat. In these cases it just says, "Do not attempt to adjust charge at [these] conditions." What am I supposed to do?

Superheated in Phoenix

HVAC expert John Proctor responds:

A: First, you have made a good decision to use a system's superheat measurement--the temperature difference between the refrigerant returning to the compressor and the saturated refrigerant in the indoor coil--to determine charge, since almost all major AC manufacturers specify the superheat method as the preferred technique to determine refrigerant charge on non-TXV systems. (Air conditioners generally use fixed metering devices to control the refrigerant flow into the evaporator coil. These metering devices are generically known as non-TXVs to distinguish them from thermostatic expansion valves (TXVs), which automatically adjust the metering opening to the operating conditions. Most TXV systems use subcooling to indicate correct charge.)

While superheat is the standard, it is used infrequently. Indeed, a recent EPA-funded study showed that 72% of air conditioners are incorrectly charged. Incorrectly charged air conditioners lead directly to early compressor failure, reduced cooling capacity, reduced comfort, and high utility bills. The average energy savings from using proper charging methods have been estimated at 13%, with many units saving more than 25%.

For those who are unfamiliar with superheat, let me summarize how it can be used:

When an air conditioner has the correct amount of charge, the measured superheat will closely match (within 5°F) the required superheat.

When the air conditioner has too much refrigerant, the measured superheat will be significantly lower than the required superheat. The recommendation would be to remove refrigerant. When the air conditioner has too little refrigerant, the measured superheat will be significantly higher than the required superheat. The recommendation would be to add refrigerant.

Finally, the required superheat changes with the operating conditions (the temperature of the air entering the outdoor coil, and the wet-bulb temperature of the air entering the indoor coil). This required superheat is specified on charts, tables, and computer programs provided by A/C manufacturers and others.

All A/C manufacturers want to prevent HVAC technicians from adding too much refrigerant, because excess liquid refrigerant slugs the compressor. When it is hot out and the air entering the indoor coil is cool and dry (as on a hot day in Phoenix), the required superheat is less than 5°F. These are the conditions under which some calculators discourage you from adjusting charge. Don't despair. You can tell if a unit is substantially undercharged under these conditions, because the measured superheat will exceed 10°F.

Follow these guidelines for correctly charging a system when the required superheat is less than 5°F:

• If the actual superheat is more than 10°F, add refrigerant to bring it to between 5°F and 9°F.
• If the actual superheat is less than 5°F, the unit may be correctly charged, or it may be overcharged; there are no easy, fast ways to tell which. Often the most practical course of action is to check the temperature split--the dry-bulb temperature difference between the air entering the indoor coil and the air leaving the indoor coil. If this is within the correct range for the conditions (using the Carrier Proper Air Flow Range Calculator or equivalent, not a rule of thumb), then leave the unit as it is. If the temperature split is low, there is a chance that the unit is overcharged. You should retest the unit under more favorable conditions. Of course, there is always the possibility of recharging by weight with corrections for line set length and diameter.

Avoiding the following mistakes will greatly increase the accuracy of your charging decisions:

• Don't use uncalibrated test equipment. Your superheat numbers are only as good as the calibration of your equipment. Check the calibration of your gauges and digital thermometer at least once a month.
• Don't use old rules of thumb for pressures or temperature split. They are wrong as often as they are right.
• Don't buy an expensive hygrometer (Rh meter). They are difficult to keep calibrated. A wet cotton shoelace over a thermocouple works better.
• Watch out for poor contact between the temperature probe and the refrigerant line when measuring the returning refrigerant (suction line) temperature.
• Don't assume that the temperatures entering the inside coil are the same as the temperatures at the return grille.

I hope the superheat information helps.

As I said earlier, the superheat method is the preferred technique to determine refrigerant charge on non-TXV systems. To judge the correct charge in TXV systems, a similar measurement, subcooling, is used--and that's a whole different column.

This column states the views of the author, not necessarily those of Home Energy magazine. John Proctor, P.E., is the developer of the CheckMe! programs. Send your comments or questions about HVAC systems and their interactions with the house as a whole to proctor@homeenergy.org.

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