Target superheat for fixed-orifice systems
A piston does not adjust, so on a fixed-orifice system you charge to a target superheat that moves with the weather. Here is how to find that target from indoor wet-bulb and outdoor dry-bulb — and charge to it.
A fixed-orifice metering device — a piston or capillary tube — has no moving parts. It cannot open up when the load climbs or pinch down when it drops, so unlike a TXV it does not hold superheat steady for you. That means superheat is the right charging number on a piston system, but the target is not a single value — it slides with indoor humidity and outdoor temperature. Charge to a fixed number and you will be wrong most of the year.
Why the target moves
Because the orifice is fixed, the only things that change how much refrigerant boils in the coil are the conditions across it. Two readings capture that:
- Indoor wet-bulb — a stand-in for the total heat load on the evaporator, sensible plus latent. Higher indoor wet-bulb means more heat to boil off refrigerant, which drives the correct target superheat up.
- Outdoor dry-bulb — drives condensing pressure and how hard the system is working. Higher outdoor temperature pushes the correct target superheat down.
This is exactly why the old shop rule of "charge it to 10 degrees of superheat" gets techs in trouble. Ten degrees might be dead-on for a hot, dry afternoon and badly overcharged on a mild, humid morning. The target follows a charging chart, and the chart follows the conditions.
Finding your target
The manufacturer's fixed-orifice charging chart cross-references indoor wet-bulb against outdoor dry-bulb to give a target superheat. Where you do not have the printed chart, the relationship is well-approximated by a linear fit to the ACCA / industry tables. To anchor your intuition, three industry-typical points on that curve:
Anchor points
70°F WB / 95°F ODB → about 9°F. 75°F WB / 95°F ODB → about 13°F. 75°F WB / 85°F ODB → about 22°F. Notice how a 10°F drop in outdoor temperature, at the same indoor wet-bulb, swings the target by nearly 9°F — that is how much the conditions matter.
Once you have the target, aim to land actual superheat within roughly 3°F of it. That window is the acceptable range; closer to the number is better, but you do not need to chase a perfect match on a piston system.
Charging to target superheat
- Stabilize first. Run the system 10 to 15 minutes. The target-superheat method assumes a steady state, indoor temperature in a normal comfort range, and correct airflow across the coil.
- Take the conditions. Measure indoor return-air wet-bulb and outdoor dry-bulb, then look up (or calculate) your target superheat for those two numbers.
- Measure actual superheat at the suction line and compare to target.
- Adjust the right direction. Actual superheat above target means the coil is starved — add refrigerant to bring it down. Actual below target means it is flooding — recover to bring it up. Move in small steps and re-stabilize between readings.
Two systems, two methods
This is the fixed-orifice method. If the unit has a TXV or EEV, the valve holds superheat by design — charge those to target subcool instead. And always defer to the charging chart printed on the unit when one is present. Refrigerant work requires EPA 608 certification.
Try the tool
Get your target with the Target Superheat calculator
Enter indoor wet-bulb and outdoor dry-bulb and get the target superheat for the current conditions, plus the acceptable range to charge within — methodology shown, works offline on the truck.
Open the Target Superheat tool