Pressure-temperature chart questions look like table lookups, and candidates treat them that way: find the pressure, read the temperature, pick the answer. The questions are actually testing whether you know which pressure, which temperature, and which column, and then whether you can subtract in the right direction. This guide walks through what a P-T chart is, the exact steps for superheat and subcooling questions, and original practice problems with the arithmetic shown.
What a P-T chart tells you
A pressure-temperature chart lists, for one refrigerant, the temperature at which the refrigerant boils or condenses at each pressure. That pairing is called the saturation temperature. For a single-component refrigerant or an azeotrope, there is one saturation temperature for each pressure, so a traditional chart has one temperature column. For a zeotropic blend, the liquid and the vapor that coexist at a given pressure are at different temperatures (the blend's composition shifts as it boils), so the chart carries two columns, bubble point and dew point, and the difference between them is the glide (refrigerant reference charts from suppliers such as Honeywell and Refrigerants Inc. show both forms). Inside the evaporator and the condenser, where liquid and vapor exist together, the refrigerant is at saturation, so a pressure reading there also tells you a temperature. Everywhere else (the suction line after the evaporator, the liquid line after the condenser) the refrigerant is fully vapor or fully liquid, and its temperature can drift away from saturation. The difference between the measured temperature and the saturation temperature for the pressure at that same point is what the questions ask you to compute.
Three details on the chart itself decide whether your lookup is right:
- The refrigerant. Every refrigerant has its own column or its own chart. Using the R-22 column for an R-410A system gives a wrong saturation temperature by a wide margin.
- Gauge or absolute pressure. Service gauges read psig (gauge pressure). Most technician charts are in psig too, but some tables are in psia (absolute). At sea level, psia is roughly psig plus 14.7. If the question gives psia and the chart is psig, or the reverse, convert before you look anything up.
- Blends have two columns. Refrigerants that are zeotropic blends (many 400-series refrigerants) do not boil at a single temperature at a given pressure; they have a bubble point (saturated liquid, where liquid starts to boil) and a dew point (saturated vapor, where the last drop of liquid evaporates). Manufacturer charts for these refrigerants show both, and the manufacturer procedure is to use the dew point for superheat and the bubble point for subcooling (Honeywell's pressure-temperature tech tool states both procedures explicitly). If the chart in the question shows two columns, the question expects you to pick the right one.
- Pressure and temperature must come from the same point. Superheat pairs the pressure and the temperature at the evaporator outlet; subcooling pairs the pressure and the temperature at the condenser outlet. A stem that gives a pressure from one place and a temperature from another is either testing whether you notice or expecting you to use a stated assumption (for example, "ignore line pressure drop"). Read for that.
Superheat: the low side
Superheat is how far the vapor leaving the evaporator has warmed above its saturation temperature. It tells you whether the evaporator is boiling off all its liquid with margin to spare.
- Read the suction (low-side) pressure the question gives.
- On the chart, find the saturation temperature for that pressure for the named refrigerant (dew point column for a blend).
- Read the measured suction line temperature the question gives.
- Superheat = measured suction line temperature minus saturation temperature.
On a normally operating system the result is positive. If your arithmetic gives a negative superheat, do not flip the sign and move on. Re-check, in this order: the units (F versus C), the refrigerant column, bubble versus dew, whether the pressure and temperature came from the same point, the numbers as copied from the stem, and the assumption that the line actually carried vapor. A negative result can be an arithmetic slip, a wrong column, or a stem describing liquid where you assumed vapor; only the re-check tells you which.
Subcooling: the high side
Subcooling is how far the liquid leaving the condenser has cooled below its saturation temperature. It tells you whether the condenser is fully condensing the refrigerant and delivering solid liquid to the metering device.
- Read the liquid (high-side) pressure the question gives.
- Find the saturation temperature for that pressure (bubble point column for a blend).
- Read the measured liquid line temperature.
- Subcooling = saturation temperature minus measured liquid line temperature.
Notice the subtraction is reversed compared with superheat. Superheat is "measured minus saturation"; subcooling is "saturation minus measured." Both come out positive on a normally operating system. A common exam trap is a question that reports the temperature of the wrong line, for example a suction line temperature next to a high-side pressure; if the numbers do not fit the side of the system, re-read which line was measured and at which point.
Original practice problems
The training excerpt below was made up for this guide so the arithmetic can be checked without a real chart. The values are not real refrigerant data and must not be used for service work. On the exam, use only the chart the question supplies or references.
| Pressure (psig) | Saturation temperature (F) |
|---|---|
| 110 | 36 |
| 118 | 40 |
| 126 | 44 |
| 340 | 101 |
| 365 | 106 |
| 390 | 111 |
Problem 1 (superheat). A T-1 system has a suction pressure of 118 psig, and the suction line temperature measured near the evaporator outlet is 52 F. What is the superheat?
Saturation at 118 psig = 40 F (from the excerpt). Superheat = 52 minus 40 = 12 F. Check: positive, and the measured temperature is above saturation, as it should be for vapor leaving an evaporator.
Problem 2 (subcooling). The same system shows a liquid line pressure of 365 psig and a liquid line temperature of 95 F at the condenser outlet. What is the subcooling?
Saturation at 365 psig = 106 F. Subcooling = 106 minus 95 = 11 F. Check: positive, and the liquid is cooler than saturation, as it should be leaving a condenser.
Problem 3 (the negative number). A candidate computes subcooling for Problem 2 as 95 minus 106 = negative 11 F and, seeing a negative number, concludes the system has "no subcooling." What should happen next?
A re-check, not a conclusion. Going down the list: units are both F; the column is the single column of this excerpt; the pressure (365 psig) and temperature (95 F) are both stated at the condenser outlet; the numbers match the stem. That leaves the formula, and subcooling is saturation minus measured, so the correct value is 106 minus 95 = 11 F. In this case the sign error was the cause, but the candidate only knows that because the other checks came up clean. On a multiple-choice exam the choices would likely include both "11 F" and a distractor such as "0 F."
Problem 4 (gauge versus absolute). A question states the suction pressure as 132.7 psia and provides the T-1 excerpt above in psig. What saturation temperature applies, assuming sea level?
Convert first: 132.7 psia minus 14.7 = 118 psig. Saturation at 118 psig = 40 F. Reading 132.7 as if it were psig would put you between 126 psig (44 F) and the next row, and every downstream answer would be off.
Problem 5 (bubble or dew). A second fictional refrigerant, "T-2," is a zeotropic blend. The training excerpt for it (made up for this guide, not real data) lists at 118 psig a bubble point of 36 F and a dew point of 42 F, and at 365 psig a bubble point of 104 F and a dew point of 108 F. The suction line at the evaporator outlet reads 118 psig and 52 F; the liquid line at the condenser outlet reads 365 psig and 95 F. Find superheat and subcooling.
Superheat uses the dew point: 52 minus 42 = 10 F. Subcooling uses the bubble point: 104 minus 95 = 9 F. Using the wrong column gives 16 F and 13 F, both of which would appear as distractors. The glide here is 6 F on the low side and 4 F on the high side; a question may ask for that too.
Problem 6 (interpolating). The measured suction pressure is 122 psig, halfway between the 118 and 126 rows. What saturation temperature should you use?
Halfway between 40 F and 44 F is 42 F. Real charts are printed in fine steps and rarely need interpolation, but exam excerpts sometimes do, and the expected method is straight-line interpolation between the two nearest rows.
What the numbers mean on the exam
Questions often add a second step: "what does this reading suggest?" The table lists explanations that training materials commonly associate with each reading. They are possibilities to weigh against the conditions the question states (metering device type, airflow, ambient, what else was measured), not a one-reading diagnosis; on an exam, the stem will include the detail that makes one choice fit better than the others.
| Reading | Possible explanations | Typically the primary charging indicator on |
|---|---|---|
| Superheat well above the target | Evaporator starved: low charge or a restriction upstream of the evaporator | Fixed-orifice (piston, capillary tube) systems |
| Superheat near zero | Evaporator flooded: overcharge, low evaporator airflow, or a metering device passing too much | Fixed-orifice systems |
| Subcooling well above the target | Excess liquid backed up in the condenser: overcharge or a restriction downstream | Thermostatic expansion valve (TXV) systems |
| Subcooling near zero | Not enough liquid: undercharge, or the condenser is not fully condensing | TXV systems |
Target values come from the equipment manufacturer's charging chart or the question stem, never from memory. If a question gives a target superheat, your job is the subtraction and the comparison, not guessing what the target should be.
Practice next
The Refrigeration and Air Conditioning topic covers the cycle these readings come from and has checkpoint questions. Then take the free HVAC practice test; for any chart question you miss, write down which of the three chart details (refrigerant, psig versus psia, bubble versus dew) or which subtraction direction caused it. The HVAC study guide puts this material in order with the rest of the exam.
Sources and verification notes
The training excerpts in this guide use fictional refrigerants with made-up values so the arithmetic can be checked; they are not real refrigerant data and must not be used for service work. The superheat and subcooling procedures (actual temperature minus dew temperature at the evaporator outlet; bubble temperature minus actual temperature at the condenser outlet) and the single-component / azeotrope / zeotropic-blend distinction were checked on September 18, 2026 against Honeywell's pressure-temperature tech tool and Refrigerants Inc.'s two-column chart explainer. The diagnostic table lists possibilities commonly taught in training materials, not a verified diagnostic standard; equipment manufacturer charging charts are the authority for targets. Problems are original practice examples, not official exam questions.
- Honeywell Refrigerants, Refrigeration and A/C Pressure-Temperature Charts tech tool (single component, azeotrope and blend charts; superheat = actual minus dew temperature; subcooling = bubble minus actual temperature)Copy retrieved September 18, 2026; SHA-256 eda8a90c2f9e6402b4bdd62fb0076c77ebf247993c899206414017942fe0570b.Checked September 18, 2026
- Refrigerants Inc., How to Use a Two-Column Pressure-Temperature Chart (single-component and azeotropic refrigerants vs. zeotropic blends, bubble point, dew point, glide)Checked September 18, 2026
Written by the ExamsLib editorial team. Practice examples in this guide are original and are not official exam questions. Exam rules change; the candidate bulletin from your licensing authority is the final word. Found an error? Contact us.
