Why a fixed-orifice system has no single correct superheat
A piston or capillary tube is a hole. It has no moving parts, no sensing bulb, and no way to respond to what the evaporator is doing. The amount of refrigerant it passes is set by the pressure difference across it and nothing else, so the superheat leaving the coil is whatever the load happens to make it.
That has one useful consequence and one awkward one. The useful one: because the orifice cannot compensate, superheat responds directly to how much refrigerant is in the system, which makes it the charging variable. The awkward one: superheat also responds to the load, so "correct" superheat on a hot dry afternoon and "correct" superheat on a cool humid morning are very different numbers on the same, perfectly charged system.
The two conditions that matter are the indoor wet bulb, which sets how much total heat the evaporator has to absorb, and the outdoor dry bulb, which sets the condensing pressure and therefore how hard refrigerant is pushed through the orifice. Higher indoor wet bulb means more heat to boil off and higher superheat; higher outdoor temperature means more refrigerant forced through the hole and lower superheat. That is exactly the shape of the expression above — the wet-bulb term is positive, the outdoor term is negative.
Contrast that with a TXV system, where the valve throttles specifically to hold superheat at its setpoint. There, superheat carries no charge information and you charge by subcooling instead.
The expression and where it comes from
Target SH = (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2
This is a straight-line fit to the target-superheat charging charts manufacturers publish on the panel of fixed-orifice condensing units. Those charts are a grid: indoor wet bulb across one axis, outdoor dry bulb down the other, target superheat in the cells. The expression reproduces the middle of that grid closely enough for field work and, unlike the printed chart, does not need you to interpolate between rows.
Read the coefficients. Each degree of indoor wet bulb raises the target by 1.5 °F; each degree of outdoor dry bulb lowers it by 0.5 °F. So indoor humidity moves the target three times as fast as outdoor temperature does, which is why measuring wet bulb properly matters more than getting the outdoor reading exactly right. A wet bulb read two degrees high sends the target three degrees high, and you will chase that error with a refrigerant cylinder.
The expression has a domain, and it is not marked on it. It is fitted over roughly 50–72 °F indoor wet bulb and 55–115 °F outdoors. Push far outside that and it returns targets below 5 °F and eventually negative ones, which is the arithmetic telling you the method has run out — not that the system should run flooded. Dry indoor air on a very hot day is the classic case: at 55 °F wet bulb and 110 °F outdoors it returns −12.5 °F. That system cannot be charged by superheat at all, and the honest procedure is to recover, evacuate and weigh in the nameplate charge with the line length adjustment added.
Worked example: charging an R-410A piston system
An 85 °F afternoon. You sling a wet bulb of 63.0 °F at the return grille. Gauges on the suction port read 118.2 psig with a clamp probe on the line showing 55.0 °F.
- Three times the wet bulb. 3 × 63.0 = 189.0.
- Subtract 80 and the outdoor temperature. 189.0 − 80 − 85.0 = 24.0.
- Halve it. 24.0 ÷ 2 = 12.0 °F target superheat.
- Convert suction pressure. R-410A at 118.2 psig is saturated at 40.0 °F.
- Measured superheat. 55.0 − 40.0 = 15.0 °F.
- Deviation. 15.0 − 12.0 = +3.0 °F.
Three degrees over target is inside the ±5 °F band this method is normally accepted within, so the correct action is no action. If it had read 15 degrees over, the system would be short of refrigerant — but the first move is still to confirm airflow, because a coil starved of air produces high superheat with a perfectly correct charge, and adding refrigerant to fix it leaves you overcharged the moment somebody changes the filter.
Work the same example an hour later at 95 °F outdoors with the wet bulb unchanged: the target falls to (189 − 80 − 95) ÷ 2 = 7.0 °F. Nothing about the system changed. The target did.
Which way to move the charge
On a fixed orifice the relationship is direct: adding refrigerant lowers superheat, recovering refrigerant raises it. More refrigerant in the system means more liquid pushed through the orifice, which takes longer to boil off and leaves less coil length available to superheat the vapour.
Measured above target — the coil is starved. Add refrigerant in small increments, waiting several minutes between them for the system to settle, and re-measure both the wet bulb and the pressures each time. The target itself will move as the house cools down, so recompute rather than working to the first figure you got.
Measured below target — the coil is over-fed. Recover refrigerant in small amounts. Do not vent it; recovery is a legal requirement, not a preference.
Within ±5 °F — leave it alone. Two field instruments, a wet bulb reading, a pressure reading and a system that is not perfectly steady add up to several degrees of honest uncertainty. Chasing the last two degrees does more harm than good.
Three checks belong before any of that. Confirm airflow is at design, because low airflow raises superheat by starving the coil of heat. Confirm the liquid line and filter drier are not restricted, because a restriction raises superheat with the correct charge in the system. And confirm the system has been running at least 15 minutes, because the first few minutes after a start are refrigerant migration, not operating charge.
Target superheat by indoor wet bulb and outdoor temperature
| Indoor wet bulb | 75 °F outdoors | 85 °F outdoors | 95 °F outdoors | 105 °F outdoors |
|---|---|---|---|---|
| 57 °F | 8.0 | 3.0 | n/a | n/a |
| 61 °F | 14.0 | 9.0 | 4.0 | n/a |
| 63 °F | 17.0 | 12.0 | 7.0 | 2.0 |
| 65 °F | 20.0 | 15.0 | 10.0 | 5.0 |
| 67 °F | 23.0 | 18.0 | 13.0 | 8.0 |
| 71 °F | 29.0 | 24.0 | 19.0 | 14.0 |
Any cell below about 5 °F sits at the edge of what superheat charging can resolve — the target is smaller than the measurement uncertainty. Weigh the charge in for those conditions instead of adjusting to a number you cannot measure.
What goes wrong with superheat charging
- Using it on a TXV system. The valve holds superheat at its setpoint whatever the charge, so a TXV system reading textbook superheat can still be badly overcharged. Charge those by subcooling.
- A bad wet-bulb reading. The target moves 1.5 °F for every degree of wet bulb. A dry sock on a sling psychrometer, or a probe held in a supply draught rather than the return, is the most common source of a wrong target.
- Charging before checking airflow. A dirty filter, a fouled evaporator or a collapsed flex duct raises superheat exactly the way an undercharge does. Measure static pressure first.
- Charging in mild weather. Under about 55 °F outdoors, head pressure falls, the orifice passes less refrigerant, and superheat stops tracking charge. Weigh the charge in.
- Chasing a moving target. As the house pulls down, the indoor wet bulb falls and the target falls with it. Recompute the target every time you re-measure, rather than working to the number you got when you arrived.
- Adding refrigerant to a system with a leak. A system that was correct last year and is low now is leaking. Topping it up without a leak search is poor practice and, for most refrigerants, a violation of federal refrigerant management rules.
The chart on the equipment outranks this page
The expression here reproduces the middle of a typical manufacturer charging chart, and it is a useful field shortcut. It is not a standard and it is not equipment-specific. Where the condensing unit still carries its charging label, use the label. Where the label is gone, the installation instructions for the model number are the next authority. Use this calculator when neither is available, and treat its answer as a target to verify rather than a specification to hit.
How this fits with the other charging methods
There are three ways to establish refrigerant charge, and they are not equally good.
Weigh-in is the most accurate and the only one that works in any weather. Recover the existing charge, pull a deep vacuum, and weigh in the nameplate charge plus the adjustment for line set length beyond the factory allowance. Every manufacturer publishes both numbers. This is the correct method on a new installation and on any system where the weather rules the other methods out.
Target superheat is the field method for fixed-orifice equipment, which is what this page computes. It needs a warm day, a stable system, an accurate wet bulb and known-good airflow.
Subcooling is the field method for TXV and EEV equipment. It needs the same conditions, and the target comes from the equipment label rather than from the weather.
Whichever you use, the air side has to agree. Check the supply-to-return split with the delta-T calculator once the charge is set: refrigerant readings that look correct alongside a split that does not is the signature of an airflow problem that the charge was quietly compensating for. And if you are unsure of the wet bulb you measured, the wet bulb calculator converts a dry bulb and relative humidity reading into the wet bulb this method needs.