SizingKitLiquid side
Subcooling calculator
Subcooling is how far below its condensing temperature the liquid leaving the outdoor unit has been cooled, and this free page reports it both in Fahrenheit degrees and in Celsius degrees, for R-410A, R-22 or R-134a, from a liquid line pressure and a clamp reading. It compares against the figure printed on your equipment, prefilled at 10 °F and editable, and it names what a high or a low result points at. On a piston or capillary-tube system it hands back the degrees and declines to read a charge into them, because there is nothing there for the number to mean.
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- TXV and fixed-orifice
- 8-12 °F band
- Condensing temp shown
The red gauge and the liquid line
Both taken at the liquid line service valve on the outdoor unit, with the condenser coil clean and the fan running. What the answer is worth afterwards depends entirely on the valve at the far end.
The standard residential refrigerant since the R-22 phase-out. Being phased down in turn under the AIM Act on its GWP of 2088; new residential equipment is moving to R-454B and R-32.
The valve backs liquid up in the condenser, so this measurement tracks the charge.
Discharge pressure read at the liquid valve, which is the condenser outlet less the small drop through the coil.
On the small line, downstream of the valve, away from the condenser’s own discharge air.
Default 10 °F — The figure most residential manufacturers print for target subcooling on a TXV system, in the middle of the 8–12 °F band. Manufacturer-dependent — this is the commonest specified value, not a standard. Use the number on the unit.
Default 0 ft — Sea level. One standard atmosphere of 14.696 psia is the only condition under which psig plus 14.696 is psia. Wherever the condensing unit actually sits. The high side is the forgiving end: at 5,280 ft a 340 psig R-410A reading condenses at 104.3 °F against 104.8 °F at sea level, a 0.5 °F shift, where the same correction on the suction side is worth two or three times that.
Subcooling and condensing temperature
SUBCOOLING °F
9.8 °F
SUBCOOLING °C
5.5 °C
- Condensing temperature
- 104.8 °F / 40.5 °C
- Target in both scales
- 10.0 °F / 5.6 °C
- Off target by
- −0.2 °F
Within 2 °F of the figure you set. 9.8 °F against 10 °F. Two degrees is roughly what a clamp thermometer on a warm line and a manifold read at arm’s length are worth together, so a result this close is a match rather than a small error — chasing the last degree with the charge is how a correct system gets ruined.
The condensing temperature above is not looked up: R-410A is held here as published saturation pressures every 20 °F from 0 to 140 °F, psig at sea level, and anything between two of them is interpolated. Past either end this page stops, because a saturation curve extended beyond its last point returns a figure that looks exactly as confident as the ones inside it. On the high side that interpolation costs less than it does on the suction side: near 300 psig a psi of R-410A is worth about 0.22 °F of condensing temperature, against 0.63 °F at 65 psig, so gauge error is a third as expensive up here. The thermometer is not: a clamp a degree out is a degree of subcooling out either way, which on a 10 °F target is a tenth of the whole measurement.
Which measurement charges which system
Four rows, and the two that say “not a charging measurement” are the ones worth taping inside the van door. Reading the wrong one is the commonest way a correctly charged machine gets refrigerant added to it.
| Metering device | Measurement | Target °F / °C | Taken at |
|---|---|---|---|
| Fixed orifice / piston / cap tube | Superheat | no single figure | Suction line at the outdoor unit's service valve |
| Fixed orifice / piston / cap tube | Subcooling | no single figure | Liquid line at the service valve |
| TXV or EEV | Superheat | 8-12 / 4.4-6.7 | Evaporator outlet, before the suction line leaves the coil |
| TXV or EEV | Subcooling | 8-12 / 4.4-6.7 | Liquid line at the outdoor unit's service valve |
The bands the trade works to for a TXV system, and the reason a fixed-orifice system has no band. Every figure is subordinate to the manufacturer's own charging chart.
Manufacturer-dependent throughout. Equipment with a factory-specified subcooling of 5 °F and equipment specifying 14 °F both exist, and a system charged to a generic 10 will be wrong on both. Weighing the charge in on an evacuated system, against the nameplate charge plus the line-set adjustment, is the accurate method; superheat and subcooling are how a charge is verified and how an existing system is diagnosed.
How to read subcooling at a service valve
Two instruments, one port, and one question about the indoor coil that decides whether the answer is worth anything.
Find out what is metering the refrigerant
Look at the indoor coil, not the outdoor unit. A thermostatic or electronic expansion valve is a brass body with a sensing bulb strapped to the suction line leaving the coil; a piston is a small brass fitting inside the liquid line connection with nothing attached to it. Cased coils usually carry a label. This is the first step rather than an afterthought because it decides whether subcooling is the charging measurement or an interesting but idle number, and the tool below changes what it reports on the strength of it.
Put the gauge and the clamp on the small line
The red gauge goes on the liquid line service valve and the clamp goes on the same line just downstream of it, out of the path of the air the condenser fan is throwing. The liquid line is the thin one and it runs warm to the touch, so a clamp reading it in a 100 °F discharge stream will read high and shrink the answer. Give the condenser a clean coil and a working fan first: neither is a detail here, because both raise the condensing temperature and eat the result without a single ounce having left the system.
Compare against the number on the unit, not against ten
Type the target subcooling the manufacturer prints on the outdoor unit's service panel into the target field, replacing the prefilled 10 °F. Equipment specifying 5 °F and equipment specifying 14 °F both exist, and a system charged to a generic figure is wrong on both of them. The tool then reports the gap and what that direction usually means, and if you have not yet weighed the charge in it will be worth checking the line-set adjustment before chasing the reading with a hose.
Technical specifications
| Metering devices covered | TXV or EEV, and fixed orifice, piston or capillary tube. The fixed-orifice path returns the degrees and stops, because a piston system holds no liquid seal for the reading to describe. |
|---|---|
| Target subcooling | 10 °F prefilled — the commonest figure residential manufacturers print, in the middle of the 8-12 °F band — and editable, because it is a manufacturer's specification and not a standard. |
| High-side resolution | One psi of R-410A near 300 psig is worth about 0.22 °F of condensing temperature, against 0.63 °F at 65 psig. Gauge error costs roughly a third as much on this side as on the suction side. |
| Thermometer resolution | Unchanged by the pressure: a clamp one degree out is one degree of subcooling out, which against a 10 °F target is a tenth of the whole measurement. Up here the thermometer is the instrument that limits the answer. |
| Elevation correction | 0 to 15,000 ft. R-410A at 340 psig condenses at 104.8 °F at sea level and 104.3 °F at 5,280 ft — half a degree, where the same correction on the low side is worth two or three times that. |
| Upper limit of the tables | R-134a's anchors end at 229.0 psig, R-22's at 337.2 and R-410A's at 540.0, all of them a 140 °F condensing temperature. Above that the page says the table stops rather than extending the curve. |
| Result conversion | Through the temperature-difference conversion, so 10 °F of subcooling reads 5.6 °C. Running it through an ordinary thermometer conversion would report −12.2 °C. |
| What leaves the tab | Nothing. The readings, the target and the elevation are computed by JavaScript in the page and never transmitted. |
Frequently asked questions
Why is subcooling not a charging measurement on a fixed-orifice system?
Because a piston system does not hold a liquid seal at the condenser outlet the way a valve system does, so its subcooling drifts with outdoor temperature and load whatever the charge is. A thermostatic expansion valve restricts flow to hold its own superheat, and one consequence of that restriction is a column of liquid backed up in the bottom of the condenser — more refrigerant in the machine makes that column taller and the liquid colder as it leaves. Remove the valve and nothing is holding the column, so the reading tells you about the weather rather than about the charge.
What should a residential TXV system read?
Whatever the outdoor unit's service panel says, which is usually a single figure near 10 °F with a tolerance of a couple of degrees. The 8-12 °F band this page falls back to is the range the trade works to across equipment generally, not a specification for your machine: manufacturers publish anything from 5 to 14 °F depending on how the condenser and the receiver are designed. If the panel is unreadable, the installation instructions carry the same number, and the model number will find them.
Subcooling is high and superheat is high at the same time. What is that?
That combination points at a restriction between the condenser and the evaporator rather than at the charge. Liquid stacks up behind the blockage, which raises subcooling, and the evaporator is starved on the other side of it, which raises superheat — a plugged filter drier, a kinked liquid line or a valve that is not opening. An overcharge raises subcooling too, but it pushes superheat down rather than up, so the two readings taken together separate the cases in a way that neither does alone.
Should I measure at the service valve or at the condenser outlet?
At the service valve, which is what the manufacturer's target figure is written against. The condenser outlet is a different point: between it and the valve sits the last pass of the coil and any drop through the tubing, so a reading taken there is a few degrees of subcooling that the specification was not talking about. Consistency matters more than which point is theoretically purer — take it where the specification says and the number is comparable to what the plate expects.
I am reading zero subcooling. What does that mean?
It means the liquid line is not full of liquid, and something is boiling in it. The refrigerant leaving the condenser is right at its saturation temperature with no margin, so any pressure drop down the line — a rise, a filter drier, a long horizontal run — flashes some of it to vapor and the metering device is being fed a mixture instead of liquid. A serious undercharge does this, and so does a condenser that cannot reject heat. It is worth confirming with a sight glass if one is fitted: bubbles at zero subcooling is the same story told twice.
Is the condensing temperature just the outdoor temperature plus a bit?
It is related to it but the gap is not a constant, and treating it as one is how people talk themselves out of a real fault. The condensing temperature is whatever it has to be for the coil to reject the heat it is being given, so it climbs with a dirty coil, a slow fan, an overcharge or a high load, and it is reported beside the answer here for exactly that reason. Use the measured figure rather than an ambient plus a rule of thumb: on a machine with a fouled condenser the rule of thumb still produces a comfortable-looking number while the measurement shows you the fault.
Why does 10 °F of subcooling show as 5.6 °C rather than −12.2 °C?
Because subcooling is a gap between two temperatures and not a temperature reading, and a gap converts with the size of the degree alone. The 32-degree offset between the Fahrenheit and Celsius zeros is carried by both ends of the measurement and cancels when you subtract them, so ten Fahrenheit degrees of difference is ten times five ninths, which is 5.6 Celsius degrees. Any converter that returns −12.2 has treated your subcooling as a thermometer reading, and that is the single most common bug in this corner of HVAC software.
About subcooling, and the inversion that catches people
The condenser has three jobs in sequence along its length. First it takes the superheated discharge vapor down to its saturation temperature; then it condenses it, holding that temperature steady while the heat of vaporization leaves; and then, in whatever length of coil is left over, it goes on cooling liquid that has nowhere else to go. Those last degrees are the subcooling, and the reason they exist at all is that the expansion device downstream is holding refrigerant back. How much coil is left over depends on how much liquid is stacked in the bottom of it, and how tall that stack gets is set by the total quantity the system was charged with. That chain is the whole basis of charging a valve system by this measurement.
Here is the inversion, and it is the one sentence in this corner of the trade worth memorising: the metering device decides which of the two measurements reports on the charge, and it decides it by taking the other one away. A thermostatic expansion valve actively holds evaporator superheat at its set point, so superheat on a valve system stays in band while the charge is anywhere near right and tells you almost nothing about it — which is exactly what makes the superheat calculator a valve check rather than a charge check on that equipment. Take the valve away and the roles swap: on a piston there is nothing holding superheat but the charge itself, so superheat becomes the charging measurement and subcooling becomes the number that wanders. Two readings, two systems, and a straight exchange of jobs between them.
None of it replaces the scale. This reading checks a charge and points at a fault on a machine already running; putting the right weight into an evacuated one is a different operation, and the refrigerant charge calculator is where the line-set correction that goes on top of the nameplate figure comes from. Two smaller things are worth knowing before you trust a reading. The pressure side of this measurement is forgiving — near 300 psig a whole psi is worth only about a fifth of a degree — but the temperature side is not, so the clamp is where accuracy is bought. And a liquid line that sweats in humid weather is sitting below the air's dew point rather than doing anything wrong; the humidity calculator gives the surface temperature at which that starts.
What happens to the numbers off your manifold
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The metering device you pick and the target you type are held in the page for as long as the tab is open and are gone when it closes, so nothing about the equipment you are standing at survives the call.